One document matched: draft-ietf-wts-shttp-03.txt
Differences from draft-ietf-wts-shttp-02.txt
The Secure HyperText Transfer Protocol
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
This document describes S-HTTP version 1.2. Previous versions of S-
HTTP numbered 1.0 and 1.1 have also been released as Internet-Drafts.
A companion draft, draft-ietf-wts-shtml-01.txt, describes extensions
to HTML to bind S-HTTP negotiation options to HTML anchors.
Abstract
This memo describes a syntax for securing messages sent using the
Hypertext Transfer Protocol (HTTP), which forms the basis for the
World Wide Web. Secure HTTP (S-HTTP) provides independently applica-
ble security services for transaction confidentiality, authenticity/
integrity and non-repudiability of origin.
The protocol emphasizes maximum flexibility in choice of key manage-
ment mechanisms, security policies and cryptographic algorithms by
supporting option negotiation between parties for each transaction.
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1. Introduction
The World Wide Web (WWW) is a distributed hypermedia system which has
gained widespread acceptance among Internet users. Although WWW
browsers support other, preexisting Internet application protocols,
the native and primary protocol used between WWW clients and servers
is the HyperText Transfer Protocol (HTTP) [BERN95b]. The ease of use
of the Web has prompted widespread interest in its employment as a
client/server architecture for many applications. Many such applica-
tions require the client and server to be able to authenticate each
other and exchange sensitive information confidentially. The original
HTTP specification had only modest support for the cryptographic
mechanisms appropriate for such transactions.
Secure HTTP (S-HTTP) provides secure communication mechanisms between
an HTTP client-server pair in order to enable spontaneous commercial
transactions for a wide range of applications. Our design intent is
to provide a flexible protocol that supports multiple orthogonal
operation modes, key management mechanisms, trust models, crypto-
graphic algorithms and encapsulation formats through option negotia-
tion between parties for each transaction.
1.1. Summary of Features
Secure HTTP is a secure message-oriented communications protocol
designed for use in conjunction with HTTP. It is designed to coexist
with HTTP's messaging model and to be easily integrated with HTTP
applications. Consequently, it mimics much of HTTP's style and syn-
tax.
Secure HTTP provides a variety of security mechanisms to HTTP clients
and servers, providing the security service options appropriate to
the wide range of potential end uses possible for the World-Wide Web.
The protocol provides symmetric capabilities to both client and
server (in that equal treatment is given to both requests and
replies, as well as for the preferences of both parties) while
preserving the transaction model and implementation characteristics
of HTTP.
Several cryptographic message format standards may be incorporated
into S-HTTP clients and servers, particularly, but in principle not
limited to, [PKCS-7] and [MOSS]. S-HTTP supports interoperation among
a variety of implementations, and is compatible with HTTP. S-HTTP
aware clients can communicate with S-HTTP oblivious servers and
vice-versa, although such transactions obviously would not use S-HTTP
security features.
S-HTTP does not require client-side public key certificates (or
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public keys), as it supports symmetric key-only operation modes. This
is significant because it means that spontaneous private transactions
can occur without requiring individual users to have an established
public key. While S-HTTP is able to take advantage of ubiquitous
certification infrastructures, its deployment does not require it.
S-HTTP supports end-to-end secure transactions, in contrast with the
original HTTP authorization mechanisms which require the client to
attempt access and be denied before the security mechanism is
employed. Clients may be "primed" to initiate a secure transaction
(typically using information supplied in message headers); this may
be used to support encryption of fill-out forms, for example. With
S-HTTP, no sensitive data need ever be sent over the network in the
clear.
S-HTTP provides full flexibility of cryptographic algorithms, modes
and parameters. Option negotiation is used to allow clients and
servers to agree on transaction modes (e.g., should the request be
signed or encrypted or both -- similarly for the reply?); crypto-
graphic algorithms (RSA vs. DSA for signing, DES vs. RC2 for encrypt-
ing, etc.); and certificate selection (please sign with your "Block-
buster Video certificate").
S-HTTP attempts to avoid presuming a particular trust model, although
its designers admit to a conscious effort to facilitate multiply-
rooted hierarchical trust, and anticipate that principals may have
many public key certificates.
1.2. Changes
This document describes S-HTTP/1.2. It differs from the previous
draft in coalescing all option negotiation headers onto a single
header line and in replacing PEM with MOSS. Also, the relationship
between S-HTTP and HTTP has been clarified.
1.3. Processing Model
1.3.1. Message Preparation
The creation of an S-HTTP message can be thought of as a a function
with three inputs:
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1. The cleartext message. This is either an HTTP message or some
other data object.
2. The receiver's cryptographic preferences and keying material.
This is either explicitly specified by the receiver or subject
to some default set of preferences.
3. The sender's cryptographic preferences and keying material.
This input to the function can be thought of as implicit
since it exists only in the memory of the sender.
In order to create an S-HTTP message, then, the sender integrates the
sender's preferences with the receiver's preferences. The result of
this is a list of cryptographic enhancements to be applied and keying
material to be used to apply them. This may require some user inter-
vention. For instance, there might be multiple keys available to sign
the message. (See Section 3.2.4.9.3 for more on this topic.) Using
this data, the sender applies the enhancements to the message clear-
text to create the S-HTTP message.
The processing steps required to transform the cleartext message into
the S-HTTP message are described in Sections 2 and 3. The processing
steps required to merge the sender's and receiver's preferences are
described in Sections 3.2.
1.3.2. Message Recovery
The recovery of an S-HTTP message can be thought of as a function of
four distinct inputs:
1. The S-HTTP message.
2. The receiver's stated cryptographic preferences and keying
material. The receiver has the opportunity to remember what
cryptographic preferences it provided in order for this document
to be dereferenced.
3. The receiver's current cryptographic preferences and keying
material.
4. The sender's previously stated cryptographic options.
The sender may have stated that he would perform certain
cryptographic operations in this message. (Again, see sections
4 and 5 for details on how to do this.)
In order to recover an S-HTTP message, the receiver needs to read the
headers to discover which cryptographic transformations were per-
formed on the message, then remove the transformations using some
combination of the sender's and receiver's keying material, while
taking note of which enhancements were applied.
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The receiver may also choose to verify that the applied enhancements
match both the enhancements that the sender said he would apply
(input 4 above) and that the receiver requested (input 2 above) as
well as the current preferences to see if the S-HTTP message was
appropriately transformed. This process may require interaction with
the user to verify that the enhancements are acceptable to the user.
(See Section 6.4 for more on this topic.)
1.4. Modes of Operation
Message protection may be provided on three orthogonal axes: signa-
ture, authentication, and encryption. Any message may be signed,
authenticated, encrypted, or any combination of these (including no
protection).
Multiple key management mechanisms are supported, including
password-style manually shared secrets, public-key key exchange and
Kerberos [RFC-1510] ticket distribution. In particular, provision
has been made for prearranged (in an earlier transaction or out of
band) symmetric session keys in order to send confidential messages
to those who have no public key pair.
Additionally, a challenge-response (``nonce'') mechanism is provided
to allow parties to assure themselves of transaction freshness.
1.4.1. Signature
If the digital signature enhancement is applied, an appropriate cer-
tificate may either be attached to the message (possibly along with a
certificate chain) or the sender may expect the recipient to obtain
the required certificate (chain) independently.
1.4.2. Key Exchange and Encryption
In support of bulk encryption, S-HTTP defines two key transfer
mechanisms, one using public-key enveloped key exchange and another
with externally arranged keys.
In the former case, the symmetric-key cryptosystem parameter is
passed encrypted under the receiver's public key.
In the latter mode, we encrypt the content using a prearranged ses-
sion key, with key identification information specified on one of the
header lines. Keys may also be extracted from Kerberos tickets.
1.4.3. Message Integrity and Sender Authentication
Secure HTTP provides a means to verify message integrity and sender
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authenticity for a message via the computation of a Message Authenti-
cation Code (MAC), computed as a keyed hash over the document using a
shared secret -- which could potentially have been arranged in a
number of ways, e.g.: manual arrangement or Kerberos. This technique
requires neither the use of public key cryptography nor encryption.
This mechanism is also useful for cases where it is appropriate to
allow parties to identify each other reliably in a transaction
without providing (third-party) non-repudiability for the transac-
tions themselves. The provision of this mechanism is motivated by our
bias that the action of "signing" a transaction should be explicit
and conscious for the user, whereas many authentication needs (i.e.,
access control) can be met with a lighter-weight mechanism that
retains the scalability advantages of public-key cryptography for key
exchange.
1.4.4. Freshness
The protocol provides a simple challenge-response mechanism, allowing
both parties to insure the freshness of transmissions. Additionally,
the integrity protection provided to HTTP headers permits implementa-
tions to consider the Date: header allowable in HTTP messages as a
freshness indicator, where appropriate (although this requires imple-
mentations to make allowances for maximum clock skew between parties,
which we choose not to specify).
1.5. Implementation Options
In order to encourage widespread adoption of secure documents for the
World-Wide Web in the face of the broad scope of application require-
ments, variability of user sophistication, and disparate implementa-
tion constraints, Secure HTTP deliberately caters to a variety of
implementation options. See Section 8 for implementation recommenda-
tions and requirements.
2. Message Format
Secure HTTP syntax deliberately mimics HTTP syntax in an effort to
ease integration with systems that already process HTTP. In addition,
certain HTTP headers are promoted to be Secure HTTP headers because
they provide useful functionality that has security implications.
A Secure HTTP message consists of a request or status line (as in
HTTP) followed by a series of RFC-822 style headers followed by
encapsulated content. Once the content has been recovered, it should
either be another Secure HTTP message, an HTTP message, or simple
data.
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For the purposes of compatibility with existing HTTP implementations,
we distinguish S-HTTP transaction requests and replies with a dis-
tinct protocol designator ('Secure-HTTP/1.2').
2.1. The Request Line
The S-HTTP request line format is similar to that of HTTP. However,
all S-HTTP request use the method, 'Secure'. All S-HTTP requests
(using this version of the protocol) should read:
Secure * Secure-HTTP/1.2
All case variations should be accepted. The asterisk shown here is a
placeholder and should be ignored by servers; proxy-aware clients
should substitute the URL (and must provide at least the host+port
portion) of the request when communicating via proxy, as is the
current HTTP convention; (e.g. http://www.terisa.com/*); proxies
should remove the appropriate amount of this information to minimize
the threat of traffic analysis. See Section 7.2.2.1 for a situation
where providing more information is appropriate.
2.2. The Status Line
For server responses, the first line should be:
Secure-HTTP/1.2 200 OK
whether the request succeeded or failed. This prevents analysis of
success or failure for any request, which the correct recipient can
determine from the encapsulated data. All case variations should be
accepted.
2.3. Secure HTTP Header Lines
The header lines described in this section go in the header of a
Secure HTTP message. All except 'Content-Type' and 'Content-Privacy-
Domain' are optional. The message body shall be separated from the
header block by two successive CRLFs.
All data and fields in header lines should be treated as case insen-
sitive unless otherwise specified. Linear whitespace [RFC-822] should
be used only as a token separator unless otherwise quoted. Long
header lines may be line folded in the style of [RFC-822].
This document refers to the header block following the S-HTTP
request/response line and preceding the successive CRLFs collectively
as "S-HTTP headers".
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2.3.1. Content-Privacy-Domain
The two values defined by this document are 'MOSS' and 'PKCS-7'.
PKCS-7 [PKCS-7] refers to the privacy enhancement specified in sec-
tion 3. MOSS refers to the format defined in [RFC-1847] and [RFC-
1848].
2.3.2. Content-Transfer-Encoding
The PKCS-7 message format is designed for an 8-bit clear channel, but
may be passed over other channels using base-64 encoding (see [RFC-
1421] for a description of base-64).
For 'Content-Privacy-Domain: PKCS-7', acceptable values for this
field are 'BASE64','8BIT', or 'BINARY'. Unless such a line is
included, the rest of the message is assumed to be 'BINARY'. (Note
that the difference between 'BINARY' and '8BIT' has to do with line
length. See [RFC-1521] for details)
For 'Content-Privacy-Domain: MOSS' all content transfer encodings are
permitted.
2.3.3. Content-Type for PKCS7
Under normal conditions, the terminal encapsulated content (after all
privacy enhancements have been removed) would be an HTTP message. In
this case, there shall be a Content-Type line reading:
Content-Type: application/http
If the inner message is an S-HTTP message, then the content type
shall be 'application/s-http'.
It is intended that these types be registered with IANA as MIME con-
tent types.
The terminal content may be of some other type provided that the type
is properly indicated by the use of an appropriate Content-Type
header line. In this case, the header fields for the encapsulation of
the terminal content apply to the terminal content (the 'final
headers'). But in any case, final headers should themselves always be
S-HTTP encapsulated, so that the applicable S-HTTP/HTTP headers are
never passed unenhanced.
S-HTTP encapsulation of non-HTTP data is a useful mechanism for pass-
ing pre-enhanced data (especially presigned data) without requiring
that the HTTP headers themselves be pre-enhanced.
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2.3.4. Content-Type for MOSS
The Content-Type for MOSS shall be an acceptable MIME content type
describing the cryptographic processing applied. (e.g.
multipart/signed). The content type of the inner content is described
in the content type line corresponding to that inner content, and for
HTTP messages shall be 'application/http'.
2.3.5. Prearranged-Key-Info
This header line is intended to convey information about a key which
has been arranged outside of the internal cryptographic format. One
use of this is to permit in-band communication of session keys for
return encryption in the case where one of the parties does not have
a key pair. However, this should also be useful in the event that the
parties choose to use some other mechanism, for instance, a one-time
key list.
This specification defines three methods for exchanging named keys,
Inband, Kerberos and Outband. Inband and Kerberos indicates that the
session key was exchanged previously, using a Key-Assign header of
the corresponding method. Outband arrangements imply that agents
have external access to key materials corresponding to a given name,
presumably via database access or perhaps supplied immediately by a
user from keyboard input. The syntax for the header line is:
Prearranged-Key-Info: <Hdr-Cipher>','<CoveredDEK>','<CoverKey-ID>
<CoverKey-ID> := <method>':'<key-name>
<CoveredDEK> := <hex-digits>
<method> := 'inband' | 'krb-'<kv> | 'outband'
<kv> := '4' | '5'
While chaining ciphers require an Initialization Vector (IV) [FIPS-
81] to start off the chaining, that information is not carried by
this field. Rather, it should be passed internal to the cryptographic
format being used. Likewise, the bulk cipher used is specified in
this fashion.
<Hdr-Cipher> should be the name of the block cipher used to encrypt
the session key (see section 3.2.4.7)
<CoveredDEK> is the protected Data Encryption Key (a.k.a. transaction
key) under which the encapsulated message was encrypted. It should be
appropriately (randomly) generated by the sending agent, then
encrypted under the cover of the negotiated key (a.k.a. session key)
using the indicated header cipher, and then converted into hex.
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In order to avoid name collisions, cover key namespaces must be main-
tained separately by host and port.
Note that some Content-Privacy-Domains, notably likely future revi-
sions of MOSS and PKCS-7 may have support for symmetric key manage-
ment. The Prearranged-Key-Info field need not be used in such cir-
cumstances. Rather, the native syntax is preferred. Keys exchanged
with Key-Assign, however, may be used in this situation.
2.3.6. MAC-Info
This header is used to supply a Message Authenticity Check, providing
both message authentication and integrity, computed from the message
text, the time (optional -- to prevent replay attack), and a shared
secret between client and server. The MAC should be computed over the
encapsulated content of the S-HTTP message. S-HTTP/1.1 defined that
MACs should be computed using the following algorithm ('||' means
concatenation):
MAC = hex(H(Message||[<time>]||<shared key>))
The time should be represented as an unsigned 32 bit quantity
representing seconds since 00:00:00 GMT January 1, 1970 (the UNIX
epoch), in network byte order. The shared key format is a local
matter.
Recent research [VANO95] has demonstrated some weaknesses in this
approach, and this draft introduces a new construction, derived from
[KRAW96a]. In the name of backwards compatibility, we retain the pre-
vious constructions with the same names as before. However, we also
introduce a new series of names (See Section 3.2.4.8 for the names)
that obey a different (hopefully stronger) construction. (^ means
bitwise XOR)
HMAC = hex(H(K' ^ pad2 || H(K' ^ pad1 ||[<time>]|| Message)))
pad1 = the byte 0x36 repeated enough times to fill out a
hash input block. (I.e. 64 times for both MD5 and SHA-1)
pad2 = the byte 0x5c repeated enough times to fill out a
hash input block.
K' = H(<shared key>)
The original HMAC construction is for the use of a key with length
equal to the length of the hash output. Although it is considered
safe to use a key of a different length (Note that strength cannot be
increased past the length of the hash function itself, but can be
reduced by using a shorter key.) [KRAW96b] we hash the original key
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to permit the use of shared keys (e.g. passphrases) longer than the
length of the hash. It is noteworthy (though obvious) that this tech-
nique does not increase the strength of short keys.
The format of the MAC-Info line is:
MAC-Info: [hex(<time>)],<hash-alg>, hex(<hash-data>),<key-spec>
<time> := "unsigned seconds since Unix epoch"
<hash-alg> := "hash algorithms from section 3.2.4.8"
<hash-data> := "computation as described above"
<Key-Spec> := 'null' | 'dek' | <Key-ID>
Key-Ids can refer either to keys bound using the Key-Assign header
line or those bound in the same fashion as the Outband method
described later. The use of a 'Null' key-spec implies that a zero
length key was used, and therefore that the MAC merely represents a
hash of the message text and (optionally) the time. The special
key-spec 'DEK' refers to the Data Exchange Key used to encrypt the
following message body (it is an error to use the DEK key-spec in
situations where the following message body is unencrypted).
If the time is omitted from the MAC-Info line, it should simply not
be included in the hash.
Note that this header line can be used to provide a more advanced
equivalent of the original HTTP Basic authentication mode in that the
user can be asked to provide a username and password. However, the
password remains private and message integrity can be assured. More-
over, this can be accomplished without encryption of any kind.
In addition, MAC-Info permits fast message integrity verification (at
the loss of non-repudiability) for messages, provided that the parti-
cipants share a key (possibly passed using Key-Assign in a previous
message).
Note that some Content-Privacy-Domains, notably likely future revi-
sions of MOSS and PKCS-7 may have support for symmetric integrity
protection The MAC-Info field need not be used in such circumstances.
Rather, the native syntax is preferred. Keys exchanged with Key-
Assign, however, may be used in this situation.
2.4. Content
The content of the message is largely dependent upon the values of
the Content-Privacy-Domain and Content-Transfer-Encoding fields.
For a PKCS-7 message, with '8BIT' Content-Transfer-Encoding, the
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content should simply be the PKCS-7 message itself.
If the Content-Transfer-Encoding is 'BASE64', the content should be
preceded by a line that reads:
-----BEGIN PRIVACY-ENHANCED MESSAGE-----
and followed by a line that reads
-----END PRIVACY-ENHANCED MESSAGE-----
(see RFC1421) with the content simply being the base-64 representa-
tion of original content. If the inner (protected) content is itself
a PKCS-7 message, then the ContentType of the outer content should be
set appropriately; else, the ContentType should be represented as
'Data'.
If the Content-Privacy-Domain is MOSS, the content should consist of
a MOSS Security Multipart as described in RFC1847.
It is expected that once the privacy enhancements have been removed,
the resulting (possibly protected) contents will be a normal HTTP
request. Alternately, the content may be another Secure-HTTP message,
in which case privacy enhancements should be unwrapped until clear
content is obtained or privacy enhancements can no longer be removed.
(This permits embedding of enhancements, such as sequential Signed
and Enveloped enhancements.) Provided that all enhancements can be
removed, the final de-enhanced content should be a valid HTTP request
(or response) unless otherwise specified by the Content-Type line.
Note that this recursive encapsulation of messages potentially per-
mits security enhancements to be applied (or removed) for the benefit
of intermediaries who may be a party to the transaction between a
client and server (e.g., a proxy requiring client authentication).
How such intermediaries should indicate such processing is described
in Section 7.2.1.
2.5. Encapsulation Format Options
2.5.1. Content-Privacy-Domain: PKCS-7
Content-Privacy-Domain 'PKCS-7' follows the form of the PKCS-7 stan-
dard (see Appendix).
Message protection may proceed on two orthogonal axes: signature and
encryption. Any message may be either signed, encrypted, both, or
neither. Note that the 'auth' protection mode of S-HTTP is provided
independently of PKCS-7 coding via the MAC-Info header of section
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2.3.6 since PKCS-7 does not support a 'KeyDigestedData' type,
although it does support a 'DigestedData' type.
2.5.1.1. Signature
This enhancement uses the 'SignedData' (or 'SignedAndEnvelopedData')
type of PKCS-7. When digital signatures are used, an appropriate
certificate may either be attached to the message (possibly along
with a certificate chain) as specified in PKCS-7 or the sender may
expect the recipient to obtain its certificate (and/or chain)
independently. Note that an explicitly allowed instance of this is a
certificate signed with the private component corresponding to the
public component being attested to. This shall be referred to as a
self-signed certificate. What, if any, weight to give to such a cer-
tificate is a purely local matter. In either case, a purely signed
message is precisely PKCS-7 compliant.
2.5.1.2. Encryption
2.5.1.2.1. Encryption -- normal, public key
This enhancement is performed precisely as enveloping (using either
'EnvelopedData' or 'SignedAndEnvelopedData' types) under PKCS-7. A
message encrypted in this fashion, signed or otherwise, is PKCS-7
compliant.
2.5.1.2.2. Encryption -- prearranged key
This uses the 'EncryptedData' type of PKCS-7. In this mode, we
encrypt the content using a DEK encrypted under cover of a prear-
ranged session key (how this key may be exchanged is discussed
later), with key identification information specified on one of the
header lines. The IV is in the EncryptedContentInfo type of the
EncryptedData element. To generate signed, encrypted data, it is
necessary to generate the 'SignedData' production and then encrypt it
(since PKCS-7 does not support a 'SignedAndEncryptedData' type -- see
Appendix A for a description of PKCS-7 operational modes.).
2.5.2. Content-Privacy-Domain: MOSS
The body of the message should be a MIME compliant message with con-
tent type that matches the Content-Type line in the S-HTTP headers.
Encrypted messages should use multipart/encrypted. Signed messages
should use multipart/signed. However, since multipart/signed does not
convey keying material, is is acceptable to use multipart/mixed where
the first part is application/mosskey-data and the second part is
multipart/mixed in order to convey certificates for use in verifying
the signature.
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Implementation Note: When both encryption and signature are applied
by the same agent, signature should in general be applied before
encryption.
2.5.3. Imported HTTP headers
2.5.3.1. Overview
Some HTTP facilities, particularly those involved with caching and
proxies, require special consideration when S-HTTP processing has
been applied. Secure HTTP makes special accomodation for these
features by copying the relevant HTTP header lines into Secure HTTP
header syntax as well.
The behavior of these headers is intended to be in line with HTTP
practice. This text is largely paraphrased from HTTP/1.0. [BERN95b].
Note that since the semantics of some of these headers may vary
across HTTP versions, this document is the definitive reference for
the meaning of these headers when present in S-HTTP headers.
2.5.3.2. Connection: Keep-Alive
The 'Connection: Keep-Alive' header is designed to permit persistent
connections between client/proxy and proxy/server pairs. A client or
proxy which desires persistent connections should send the header
'Connection: Keep-Alive'. A server which agrees should respond with
'Connection: Keep-Alive'.
The persistent connection ends when either side closes the connection
or after the receipt of a response which lacks the "keep-alive" key-
word. The server may close the connection immediately after respond-
ing to a request without a "keep-alive" keyword. A client can tell if
the connection will be closed by looking for a "keep-alive" in the
response.
Proxies and gateways should remove the Keep-Alive header, though, of
course, they may optionally regenerate it if they desire a persistent
connection with the next hop. HTTP clients not using gateways and
desiring a persistent connection with the server should not use this
mechanism, but rather should use whatever mechanisms HTTP provides.
2.5.3.3. If-Modified-Since
This may be used by the proxy to indicate that the document may be in
its cache and that it is prepared to serve the document to the
current requestor. Servers receiving this header and deciding not to
resend the document should respond using the 320 response code as
described in Section 5.2.5.
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This header should only be placed in S-HTTP headers by proxies.
Clients wanting to use If-Modified-Since should place it in the HTTP
headers of the inner content.
2.5.3.4. Content-MD5
Servers may generate a Content-MD5 header to enable proxies to detect
when valid cache hits have occurred. Note that the Content-MD5 header
provides the possibility of traffic analysis. Servers using this
should bear that risk in mind.
The Content-MD5 is exactly as described in [RFC-1864] except that it
is computed on the inner content rather than on the ciphertext.
3. Cryptographic Parameters
3.1. Options Headers
As described in Section 1.3.2, every S-HTTP request is (at least con-
ceptually) preconditioned by the negotiation options provided by the
potential receiver. The two primary locations for these options are
1. In the headers of an HTTP Request/Response.
2. In the HTML which contains the anchor being dereferenced.
There are two kinds of cryptographic options which may be provided:
Negotiation options, as discussed in Section 3.2 convey a potential
message recipient's cryptographic preferences. Keying options, as
discussed in Section 3.3 provide keying material (or pointers to key-
ing material) which may be of use to the sender when enhancing a mes-
sage.
Binding cryptographic options to anchors using HTML extensions is the
topic of the companion document draft-ietf-wts-shtml-01.txt and will
not be treated here.
3.2. Negotiation Options
3.2.1. Negotiation Overview
Both parties are able to express their requirements and preferences
regarding what cryptographic enhancements they will permit/require
the other party to provide. The appropriate option choices depend on
implementation capabilities and the requirements of particular appli-
cations.
Rescorla, Schiffman [Page 15] Internet-Draft Secure HTTP
A negotiation header is a sequence of specifications each conforming
to a four-part schema detailing:
Property -- the option being negotiated, such as bulk
encryption algorithm.
Value -- the value being discussed for the property, such
as DES-CBC
Direction -- the direction which is to be affected, namely:
during reception or origination (from the perspective of
the originator).
Strength -- strength of preference, namely: required,
optional, refused
As an example, the header line:
SHTTP-Symmetric-Content-Algorithms: recv-optional=DES-CBC,RC2
could be thought to say: ``You are free to use DES-CBC or RC2 for
bulk encryption for encrypting messages to me.''
We define new headers (to be used in the encapsulated HTTP header,
not in the S-HTTP header) to permit negotiation of these matters.
3.2.2. Negotiation Option Format
The general format for negotiation options is:
<Option> := <Field> ':' <Key-val>(';'<Key-val>)*
<Key-val> := <Key> '=' <Value>(','<Value>)*
<Key> := <Mode>'-'<Action>
<Mode> := 'orig'|'recv'
<Action> := 'optional'|'required'|'refused'
The <Mode> value indicates whether this <Key-val> refers to what the
agent's actions are upon sending privacy enhanced messages as opposed
to upon receiving them. For any given mode-action pair, the interpre-
tation to be placed on the enhancements (<Value>s) listed is:
'recv-optional:' The agent will process the enhancement if
the other party uses it, but will also gladly process mes-
sages without the enhancement.
'recv-required:' The agent will not process messages
without this enhancement.
Rescorla, Schiffman [Page 16] Internet-Draft Secure HTTP
'recv-refused:' The agent will not process messages with
this enhancement.
'orig-optional:' When encountering an agent which refuses
this enhancement, the agent will not provide it, and when
encountering an agent which requires it, this agent will
provide it.
'orig-required:' The agent will always generate the
enhancement.
'orig-refused:' The agent will never generate the enhance-
ment.
The behavior of agents which discover that they are communicating
with an incompatible agent is at the discretion of the agents. It is
inappropriate to blindly persist in a behavior that is known to be
unacceptable to the other party. Plausible responses include simply
terminating the connection, or, in the case of a server response,
returning 'Not implemented 501'.
Optional values are considered to be listed in decreasing order of
preference. Agents are free to choose any member of the intersection
of the optional lists (or none) however.
If any <Key-Val> is left undefined, it should be assumed to be set to
the default. Any key which is specified by an agent shall override
any appearance of that key in any <Key-Val> in the default for that
field.
3.2.3. Parametrization for Variable-length Key Ciphers
For ciphers with variable key lengths, values may be parametrized
using the syntax <cipher>'['<length>']'
For example, 'RSA[1024]' represents a 1024 bit key for RSA. Ranges
may be represented as
<cipher>'['<bound1>'-'<bound2>']'
For purposes of preferences, this notation should be treated as if it
read (assuming x and y are integers)
<cipher>[x], <cipher>[x+1],...<cipher>[y] (if x<y)
and
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<cipher>[x], <cipher>[x-1],...<cipher>[y] (if x>y)
The special value 'inf' may be used to denote infinite length.
Using simply <cipher> for such a cipher shall be read as the maximum
range possible with the given cipher.
3.2.4. Negotiation Syntax
3.2.4.1. SHTTP-Privacy-Domains
This header refers to the Content-Privacy-Domain type of section
2.3.1. Acceptable values are as listed there. For instance,
SHTTP-Privacy-Domains: orig-required=pkcs-7;
recv-optional=pkcs-7,MOSS
would indicate that the agent always generates PKCS-7 compliant mes-
sages, but can read PKCS-7 or MOSS (or, unenhanced messages).
3.2.4.2. SHTTP-Certificate-Types
This indicates what sort of Public Key certificates the agent will
accept. Currently defined values are 'X.509' and 'X.509v3'.
3.2.4.3. SHTTP-Key-Exchange-Algorithms
This header indicates which algorithms may be used for key exchange.
Defined values are 'RSA', 'Outband', 'Inband', and 'Krb-'<kv>. RSA
refers to RSA enveloping. Outband refers to some sort of external key
agreement. Inband and Kerberos refer to the protocols of sections
3.3.3.1 and 3.3.3.2 respectively.
The expected common configuration of clients having no certificates
and servers having certificates would look like this (in a message
sent by the server):
SHTTP-Key-Exchange-Algorithms: orig-optional=Inband, RSA;
recv-required=RSA
3.2.4.4. SHTTP-Signature-Algorithms
This header indicates what Digital Signature algorithms may be used.
Defined values are 'RSA' [PKCS-1] and 'NIST-DSS' [FIPS-186] Since
NIST-DSS and RSA use variable length moduli the parametrization syn-
tax of section 3.2.3 should be used. Note that a key length
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specification may interact with the acceptability of a given certifi-
cate, since keys (and their lengths) are specified in public-key cer-
tificates.
3.2.4.5. SHTTP-Message-Digest-Algorithms
This indicates what message digest algorithms may be used. Previ-
ously defined values are 'RSA-MD2' [RFC-1319], 'RSA-MD5' [RFC-1321],
3.2.4.6. SHTTP-Symmetric-Content-Algorithms
This header specifies the symmetric-key bulk cipher used to encrypt
message content. Defined values are:
DES-CBC -- DES in Cipher Block Chaining (CBC) mode [FIPS-81]
DES-EDE-CBC -- 2 Key 3DES using Encrypt-Decrypt-Encrypt in outer CBC mode
DES-EDE3-CBC -- 3 Key 3DES using Encrypt-Decrypt-Encrypt in outer CBC mode
DESX-CBC -- RSA's DESX in CBC mode
IDEA-CBC -- IDEA in CBC mode [XXXX]
RC2-CBC -- RSA's RC2 in CBC mode
CDMF-CBC -- IBM's CDMF (weakened key DES) [JOHN93] in CBC mode
Since RC2 keys are variable length, the syntax of section 3.2.3
should be used.
3.2.4.7. SHTTP-Symmetric-Header-Algorithms
This header specifies the symmetric-key cipher used to encrypt mes-
sage headers.
DES-ECB -- DES in Electronic Codebook (ECB) mode [FIPS-81]
DES-EDE-ECB -- 2 Key 3DES using Encrypt-Decrypt-Encrypt in ECB mode
DES-EDE3-ECB -- 3 Key 3DES using Encrypt-Decrypt-Encrypt in ECB mode
DESX-ECB -- RSA's DESX in ECB mode
IDEA-ECB -- IDEA
RC2-ECB -- RSA's RC2 in ECB mode
CDMF-ECB -- IBM's CDMF in ECB mode
Since RC2 is variable length, the syntax of section 3.2.3 should be
used.
3.2.4.8. SHTTP-MAC-Algorithms
This header indicates what algorithms are acceptable for use in pro-
viding a symmetric key MAC. 'RSA-MD2', 'RSA-MD5' and 'NIST-SHS' per-
sist from S-HTTP/1.1 using the old MAC construction. The tokens
'RSA-MD2-HMAC', 'RSA-MD5-HMAC' and 'NIST-SHS-HMAC' indicate the new
Rescorla, Schiffman [Page 19] Internet-Draft Secure HTTP
HMAC construction of 2.3.6 with the MD2, MD5, and SHA-1 algorithms
respectively.
3.2.4.9. SHTTP-Privacy-Enhancements
This header indicates security enhancements to apply. Possible
values are 'sign', 'encrypt' and 'auth' indicating whether messages
are signed, encrypted, or authenticated (i.e., provided with a MAC),
respectively.
3.2.4.10. Your-Key-Pattern
This is a generalized pattern match syntax to describe identifiers
for a large number of types of keying material. The general syntax
is:
Your-Key-Pattern : <key-use>','<pattern-info>
<key-use> := 'cover-key' | 'auth-key' | 'signing-key' | 'krbID-'<kv>
3.2.4.10.1. Cover Key Patterns
This header specifies desired values for key names used for encryp-
tion of transaction keys using the Prearranged-Key-Info syntax of
section 2.3.5. The pattern-info syntax consists of a series of comma
separated regular expressions. Commas should be escaped with
backslashes if they appear in the regexps. The first pattern should
be assumed to be the most preferred.
3.2.4.10.2. Auth key patterns
Auth-key patterns specify name forms desired for use for MAC authen-
ticators. The pattern-info syntax consists of a series of comma
separated regular expressions. Commas should be escaped with
backslashes if they appear in the regexps. The first pattern should
be assumed to be the most preferred.
3.2.4.10.3. Signing Key Pattern
This parameter describes a pattern or patterns for what keys are
acceptable for signing for the digital signature enhancement. The
pattern-info syntax for signing-key is:
<pattern-info> := <name-domain>','<pattern-data>
The only currently defined name-domain is 'DN-1485'. This parameter
specifies desired values for fields of Distinguished Names. DNs are
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considered to be represented as specified in RFC1485, the order of
fields and whitespace between fields is not significant.
All RFC1485 values should use ',' as a separator rather than ';',
since ';' is used as a statement separator in S-HTTP.
Pattern-data is a modified RFC1485 string, with regular expressions
permitted as field values. Pattern match is performed field-wise,
unspecified fields match any value (and therefore leaving the DN-
Pattern entirely unspecified allows for any DN). Certificate chains
may be matched as well (to allow for certificates without name subor-
dination). DN chains are considered to be ordered left-to-right with
the issuer of a given certificate on its immediate right, although
issuers need not be specified. A trailing '.' indicates that the
sequence of DNs is absolute. I.e. that the one furthest to the right
is a root.
The syntax for the pattern values is,
<Value> := <DN-spec> (','<Dn-spec>)*[.]
<Dn-spec> := '/'<Field-spec>*'/'
<Field-spec> := <Attr>'='<Pattern>
<Attr> := 'CN' | 'L' | 'ST' | 'O' |
'OU' | 'C' | "or as appropriate"
<Pattern> := "POSIX 1003.2 regular expressions"
For example, to request that the other agent sign with a key certi-
fied by the RSA Persona CA (which uses name subordination) one could
use the expression below. Note the use of RFC1485 quoting to protect
the comma (an RFC1485 field separator) and the POSIX 1003.2 quoting
to protect the dot (a regular expression metacharacter).
Your-Key-Pattern: signing-key, DN-1485,
/OU=Persona Certificate, O="RSA Data Security, Inc\."/
3.2.4.11. Example
A representative header block for a server follows.
SHTTP-Privacy-Domains: recv-optional=MOSS, PKCS-7;
orig-required=PKCS-7
SHTTP-Certificate-Types: recv-optional=X.509;
orig-required=X.509
SHTTP-Key-Exchange-Algorithms: recv-required=RSA;
orig-optional=Inband,RSA
SHTTP-Signature-Algorithms: orig-required=RSA; recv-required=RSA
SHTTP-Privacy-Enhancements: orig-required=sign;
orig-optional=encrypt
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3.2.4.12. Defaults
Explicit negotiation parameters take precedence over default values.
For a given negotiation option type, defaults for a given mode-action
pair (such as 'orig-required') are implicitly merged unless expli-
citly overridden.
The default values (these may be negotiated downward or upward) are:
SHTTP-Privacy-Domains: orig-optional=PKCS-7, MOSS;
recv-optional=PKCS-7, MOSS
SHTTP-Certificate-Types: orig-optional=X.509;
recv-optional=X.509
SHTTP-Key-Exchange-Algorithms: orig-optional=RSA,Inband,Outband;
recv-optional=RSA,Inband,Outband
SHTTP-Signature-Algorithms: orig-optional=RSA; recv-optional=RSA
SHTTP-Message-Digest-Algorithms: orig-optional=RSA-MD5;
recv-optional=RSA-MD5
SHTTP-Symmetric-Content-Algorithms: orig-optional=DES-CBC;
recv-optional=DES-CBC
SHTTP-Symmetric-Header-Algorithms: orig-optional=DES-ECB;
recv-optional=DES-ECB
SHTTP-Privacy-Enhancements: orig-optional=sign,encrypt, auth;
recv-required=encrypt;
recv-optional=sign, auth
3.3. Non-Negotiation Headers
There are a number of options that are used to communicate or iden-
tify the potential recipient's keying material.
3.3.1. Encryption-Identity
This header identifies a potential principal for whom the message
described by these options could be encrypted; Note that this expli-
citly permits return encryption under (say) public key without the
other agent signing first (or under a different key than that of the
signature). Or, in the Kerberos case, provides information as the
agent's Kerberos identity. The syntax of the Encryption-Identity
line is:
Encryption-Identity: <name-class>,<key-sel>,<name-arg>
<name-class> := 'DN-1485' | MOSS name forms
The name-class is an ASCII string representing the domain within
which the name is to be interpreted, in the spirit of the new MOSS
drafts. In addition to the MOSS name forms of RFC1848, we add the
Rescorla, Schiffman [Page 22] Internet-Draft Secure HTTP
DN-1485 name form to represent a more convenient form of dis-
tinguished name.
Note: The Kerberos name forms of previous drafts are subsumed by the
MOSS email string form.
3.3.1.1. DN-1485 Name Class
The argument is an RFC-1485 encoded DN.
3.3.2. Certificate-Info
In order to permit public key operations on DNs specified by
Encryption-Identity headers without explicit certificate fetches by
the receiver, the sender may include certification information in the
Certificate-Info option. The format of this option is:
Certificate-Info: <Cert-Fmt>','<Cert-Group>
<Cert-Fmt> should be the type of <Cert-Group> being presented.
Defined values are 'PEM' and 'PKCS-7'. PKCS-7 certificate groups are
provided as a base-64 encoded PKCS-7 SignedData message containing
sequences of certificates with or without the SignerInfo field. A PEM
format certificate group is a list of comma-separated base64-encoded
PEM certificates.
Multiple Certificate-Info lines may be defined.
3.3.3. Key-Assign
This option serves to indicate that the agent wishes to bind a key to
a symbolic name for (presumably) later reference.
The general syntax of the key-assign header is:
Key-Assign: <Method>,<Key-Name>,<Lifetime>,<Ciphers>;<Method-args>
<Key-name> := <string>
<Lifetime> := 'this' | 'reply' | ''
<Method> :='inband' | 'krb-'<kv>
<Ciphers> := 'null' | <Cipher>+
<Cipher> := "Header cipher from section 3.2.4.7"
<kv> := '4' | '5'
Key-Name is the symbolic name to which this key is to be bound.
Ciphers is a list of ciphers for which this key is potentially appli-
cable (see the list of header ciphers in section 3.2.4.7). The
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keyword 'null' should be used to indicate that it is inappropriate
for use with ANY cipher. This is potentially useful for exchanging
keys for MAC computation.
Lifetime is a representation of the longest period of time during
which the recipient of this message can expect the sender to accept
that key. 'this' indicates that it is likely to be valid only for
reading this transmission. 'reply' indicates that it is useful for a
reply to this message. If a Key-Assign with the reply lifetime
appears in a CRYPTOPTS block, it indicates that it is good for at
least one (but perhaps only one) dereference of this anchor. An
unspecified lifetime implies that this key may be reused for an inde-
finite number of transactions.
Method should be one of a number of key exchange methods. The
currently defined values are 'inband', 'krb-4' and 'krb-5', referring
respectively to Inband keys (i.e., direct assignment) and Kerberos
versions 4 and 5 respectively. Method-args will depend on methods.
This header line may appear either in an unencapsulated header or in
an encapsulated message, though when an uncovered key is being
directly assigned, it may only appear in an encrypted encapsulated
content. Assigning to a key that already exists causes that key to be
overwritten.
Keys defined by this header are referred to elsewhere in this specif-
ication as Key-IDs, which have the syntax:
<Key-ID> := <method>':'<key-name>
Key-Assign may also be used as a header line in the S-HTTP headers if
the data it is carrying does not need to be secured itself, e.g. with
Kerberos.
3.3.3.1. Inband Key Assignment
This refers to the direct assignment of an uncovered key to a sym-
bolic name. Method-args should be just the desired session key
encoded in hexidecimal as in:
Key-Assign: inband,akey,reply,DES-ECB;0123456789abcdef
Short keys should be derived from long keys by reading bits from left
to right.
Note that inband key assignment is especially important in order to
Rescorla, Schiffman [Page 24] Internet-Draft Secure HTTP
permit confidential spontaneous communication between agents where
one (but not both) of the agents have key pairs. However, this
mechanism is also useful to permit key changes without public key
computations. The key information is carried in this header line must
be in the inner secured HTTP request, therefore use in unencrypted
messages is not permitted.
3.3.3.2. Kerberos Key Assignment
This permits the binding of the shared secret derived from a Kerberos
ticket/authenticator pair to a symbolic keyname. In this case,
method-args should be the ticket/authenticator pair (each base64-
encoded), comma separated. For example:
Key-Assign: krb-4,akerbkey,reply,DES-ECB;<krb-ticket>,<krb-auth>
3.3.4. Nonces
Nonces are opaque, transient, session-oriented identifiers which may
be used to provide demonstrations of freshness. Nonce values are a
local matter, although they are might well be simply random numbers
generated by the originator. The value is supplied simply to be
returned by the recipient.
3.3.4.1. Nonce
This header is used by an originator to specify what value is to be
returned in the reply. The field may be any value. Multiple nonces
may be supplied, each to be echoed independently.
The Nonce should be returned in a Nonce-Echo header line. See section
4.1.1.
3.4. Grouping Headers With SHTTP-Cryptopts
In order for servers to bind a group of headers to an HTML anchor, it
is possible to combine a number of headers on a single S-HTTP Cryp-
topts header line. The names of the anchors to which these headers
apply is indicated with a 'scope' parameter.
3.4.1. SHTTP-Cryptopts
This option provides a set of cryptopts and a list of references to
which it applies. (For HTML, these references would be named using
the NAME tag). The names are provided in the scope attribute as a
comma separated list and separated from the next header line by a
semicolon. The format for the SHTTP-Cryptopts line is:
Rescorla, Schiffman [Page 25] Internet-Draft Secure HTTP
SHTTP-Cryptopts: <scope>';'<cryptopt-list>
<scope> := 'scope='<tag-spec>
<tag-spec> := <tag>(','<tag>)* | <null>
<cryptopt-list> := <cryptopt>(';'<cryptopt>)*
<cryptopt> := "S-HTTP cryptopt lines described below"
<tag> := "value used in HTML anchor NAME attribute"
For example:
SHTTP-Cryptopts: scope=tag1,tag2;
SHTTP-Privacy-Domains:
orig-required=pkcs-7; recv-optional=pkcs-7,MOSS
If a message contains both S-HTTP negotiation headers and headers
grouped on SHTTP-Cryptopts line(s), the other headers shall be taken
to apply to all anchors not bound on the SHTTP-Cryptopts line(s).
Note that this is an all-or-nothing proposition. That is, if a
SHTTP-Cryptopts header binds options to a reference, then none of
these global options apply, even if some of the options headers do
not appear in the bound options. Rather, the S-HTTP defaults found in
Section 3.2.4.11 apply.
4. New Header Lines for HTTP
Two non-negotiation header lines for HTTP are defined here.
4.1. Security-Scheme
All S-HTTP compliant agents must generate the Security-Scheme header
in the headers of all HTTP messages they generate. This header per-
mits other agents to detect that they are communicating with an S-
HTTP compliant agent and generate the appropriate cryptographic
options headers.
For implementations compliant with this specification, the value must
be 'S-HTTP/1.2'.
4.1.1. Nonce-Echo
The header is used to return the value provided in a previously
received Nonce: field. This has to go in the encapsulated headers so
that it an be cryptographically protected.
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5. (Retriable) Server Status Error Reports
We describe here the special processing appropriate for client
retries in the face of servers returning an error status.
5.1. Retry for Option (Re)Negotiation
A server may respond to a client request with an error code that
indicates that the request has not completely failed but rather that
the client may possibly achieve satisfaction through another request.
HTTP already has this concept with the 3XX redirection codes.
In the case of S-HTTP, it is conceivable (and indeed likely) that the
server expects the client to retry his request using another set of
cryptographic options. E.g., the document which contains the anchor
that the client is dereferencing is old and did not require digital
signature for the request in question, but the server now has a pol-
icy requiring signature for dereferencing this URL. These options
should be carried in the header of the encapsulated HTTP message,
precisely as client options are carried.
The general idea is that the client will perform the retry in the
manner indicated by the combination of the original request and the
precise nature of the error and the cryptographic enhancements
depending on the options carried in the server response.
The guiding principle in client response to these errors should be to
provide the user with the same sort of informed choice with regard to
dereference of these anchors as with normal anchor dereference. For
instance, in the case above, it would be inappropriate for the client
to sign the request without requesting permission for the action.
5.2. Specific Retry Behavior
5.2.1. Unauthorized 401, PaymentRequired 402
The HTTP errors 'Unauthorized 401', 'PaymentRequired 402' represent
failures of HTTP style authentication and payment schemes. While S-
HTTP has no explicit support for these mechanisms, they can be per-
formed under S-HTTP while taking advantage of the privacy services
offered by S-HTTP. (There are other errors for S-HTTP specific
authentication errors.)
5.2.2. 420 SecurityRetry
This server status reply is provided so that the server may inform
the client that although the current request is rejected, a retried
request with different cryptographic enhancements is worth
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attempting. This header shall also be used in the case where an HTTP
request has been made but an S-HTTP request should have been made.
Obviously, this serves no useful purpose other than signalling an
error if the original request should have been encrypted, but in
other situations (e.g. access control) may be useful.
5.2.2.1. SecurityRetries for S-HTTP Requests
In the case of a request that was made as an SHTTP request, it indi-
cates that for some reason the cryptographic enhancements applied to
the request were unsatisfactory and that the request should be
repeated with the options found in the response header. Note that
this can be used as a way to force a new public key negotiation if
the session key in use has expired or to supply a unique nonce for
the purposes of ensuring request freshness.
5.2.2.2. SecurityRetries for HTTP Requests
If the 420 code is returned in response to an HTTP request, it indi-
cates that the request should be retried using S-HTTP and the crypto-
graphic options indicated in the response header.
5.2.3. 421 BogusHeader
This error code indicates that something about the S-HTTP request was
bad. The error code is to be followed by an appropriate explanation,
e.g.:
421 BogusHeader Content-Privacy-Domain must be specified
5.2.4. 422 SHTTP Proxy Authentication Required
This response is analagous to the 420 response except that the
options in the message refer to enhancements that the client must
perform in order to satisfy the proxy.
5.2.5. 320 SHTTP Not Modifed
This response code is specifically for use with proxy-server interac-
tion where the proxy has placed the If-Modified-Since header in the
S-HTTP headers of its request. This response indicates that the fol-
lowing S-HTTP message contains sufficient keying material for the
proxy to forward the cached document for the new requestor.
In general, this takes the form of an S-HTTP message where the actual
enhanced content is missing, but all the headers and keying material
are retained. (I.e. the optional content section of the PKCS7 message
Rescorla, Schiffman [Page 28] Internet-Draft Secure HTTP
has been removed.) So, if the original response was encrypted, the
response contains the original DEK re-covered for the new recipient.
(Notice that the server performs the same processing as it would have
in the server side caching case of 7.1 except that the message body
is elided.)
5.2.6. Redirection 3XX
These headers are again internal to HTTP, but may contain S-HTTP
negotiation options of significance to S-HTTP. The request should be
redirected in the sense of HTTP, with appropriate cryptographic pre-
cautions being observed.
5.3. Limitations On Automatic Retries
Permitting automatic client retry in response to this sort of server
response permits several forms of attack. Consider for the moment
the simple credit card case:
The user views a document which requires his credit card.
The user verifies that the DN of the intended recipient is
acceptable and that the request will be encrypted and
dereferences the anchor. The attacker intercepts the
server's reply and responds with a message encrypted under
the client's public key containing the Moved 301 header. If
the client were to automatically perform this redirect it
would allow compromise of the user's credit card.
5.3.1. Automatic Encryption Retry
This shows one possible danger of automatic retries -- potential
compromise of encrypted information. While it is impossible to con-
sider all possible cases, clients should never automatically reen-
crypt data unless the server requesting the retry proves that he
already has the data. So, situations in which it would be acceptable
to reencrypt would be if:
1. The retry response was returned encrypted under an inband key
freshly generated for the original request.
2. The retry response was signed by the intended recipient of the
original request.
3. The original request used an outband key and the response is
encrypted under that key.
This is not an exhaustive list, however the browser author would be
well advised to consider carefully before implementing automatic
reencryption in other cases. Note that an appropriate behavior in
Rescorla, Schiffman [Page 29] Internet-Draft Secure HTTP
cases where automatic reencryption is not appropriate is to query the
user for permission.
5.3.2. Automatic Signature Retry
Since we discourage automatic (without user confirmation) signing in
even the usual case, and given the dangers described above, it is
prohibited to automatically retry signature enchancement.
5.3.3. Automatic MAC Authentication Retry
Assuming that all the other conditions are followed, it is permissi-
ble to automatically retry MAC authentication.
6. Other Issues
6.1. Compatibility of Servers with Old Clients
Servers which receive requests in the clear which should be secured
should return 'SecurityRetry 420' with header lines set to indicate
the required privacy enhancements.
6.2. URL Protocol Type
We define a new URL protocol designator, 'shttp'. Use of this desig-
nator as part of an anchor URL implies that the target server is S-
HTTP capable, and that a dereference of this URL should undergo S-
HTTP processing.
Note that S-HTTP oblivious agents should not be willing to derefer-
ence a URL with an unknown protocol specifier, and hence sensitive
data will not be accidentally sent in the clear by users of non-
secure clients.
6.3. Server Conventions
6.3.1. Certificate Requests
We define the convention that issuing a normal HTTP request:
GET /SERVER-CERTIFICATE[<B64-DN>] <http-version>
shall cause the server to return the corresponding certificate.
<B64-DN> is the base-64 encoding (to protect whitespace) of the
fully-specified canonical ASCII form for the DN of the requested cer-
tificate (as in RFC 1485). If no DN is specified, then the server
shall choose whatever certificate it deems most appropriate. The
server should sign the response with the key corresponding to the DN
Rescorla, Schiffman [Page 30] Internet-Draft Secure HTTP
supplied.
6.3.2. Policy Requests
Servers should (but not must) store the policies of the Policy Cer-
tification Authorities, if available, corresponding to their various
certificates. The convention for retrieving such policies via HTTP is
the request:
GET /POLICY-<B64-DN> <http-version>
Again, <B64-DN> is the DN (encoded as per section 6.3.1) of the cer-
tificate corresponding to the requested policy. It is recommended
that this document be (pre-) signed by the PCA.
6.3.3. CRL Requests
Servers should (but not must) store the CRLs of the PCAs correspond-
ing to their various certificates. The convention for retrieving such
CRLs is:
GET /CRL-<B64-DN> <http-version>
Again, <B64-DN> is the DN (encoded as per section 6.3.1) of the cer-
tificate corresponding to the requested CRL.
6.4. Browser Presentation
6.4.1. Transaction Security Status
While preparing a secure message, the browser should provide a visual
indication of the security of the transaction, as well as an indica-
tion of the party who will be able to read the message. While reading
a signed and/or enveloped message, the browser should indicate this
and (if applicable) the identity of the signer. Self-signed certifi-
cates should be clearly differentiated from those validated by a cer-
tification hierarchy.
6.4.2. Failure Reporting
Failure to authenticate or decrypt an S-HTTP message should be
presented differently from a failure to retrieve the document. Com-
pliant clients may at their option display unverifiable documents but
must clearly indicate that they were unverifiable in a way clearly
distinct from the manner in which they display documents which pos-
sessed no digital signatures or documents with verifiable signatures.
Rescorla, Schiffman [Page 31] Internet-Draft Secure HTTP
6.4.3. Certificate Management
Clients shall provide a method for determining that HTTP requests are
to be signed and for determining which (assuming there are many) cer-
tificate is to be used for signature. It is suggested that users be
presented with some sort of selection list from which they may choose
a default. No signing should be performed without some sort of expli-
cit user interface action, though such action may take the form of a
persistent setting via a user preferences mechanism (although this is
discouraged.)
6.4.4. Anchor Dereference
Clients shall provide a method to display the DN and certificate
chain associated with a given anchor to be dereferenced so that users
may determine for whom their data is being encrypted. This should be
distinct from the method for displaying who has signed the document
containing the anchor since these are orthogonal pieces of encryption
information.
7. Implementation Notes
7.1. Preenhanced Data
While S-HTTP has always supported preenhanced documents, in previous
versions it was never made clear how to actually implement them.
This section describes two methods for doing so: preenhancing the
HTTP request/response and preenhancing the underlying data.
7.1.1. Motivation
The two primary motivations for preenhanced documents are security
and performance. These advantages primarily accrue to signing but may
also under special circumstances apply to confidentiality or repudi-
able (MAC-based) authentication.
Consider the case of a server which repeatedly serves the same con-
tent to multiple clients. One such example would be a server which
serves catalogs or price lists. Clearly, customers would like to be
able to verify that these are actual prices. However, since the
prices are typically the same to all comers, confidentiality is not
an issue. (Note: see Section 7.1.5 below for how to deal with this
case as well).
Consequently, the server might wish to sign the document once and
simply send the cached signed document out when a client makes a new
request, avoiding the overhead of a private key operation each time.
Note that conceivably, the signed document might have been generated
Rescorla, Schiffman [Page 32] Internet-Draft Secure HTTP
by a third party and placed in the server's cache. The server might
not even have the signing key! This illustrates the security benefit
of presigning: Untrusted servers can serve authenticated data without
risk even if the server is compromised.
7.1.2. Presigned Requests/Responses
The obvious implementation is simply to take a single
request/response, cache it, and send it out in situations where a new
message would otherwise be generated.
7.1.3. Presigned Documents
It is also possible using S-HTTP to sign the underlying data and send
it as an S-HTTP messsage. In order to do this, one would take the
signed document (a PKCS-7 or MOSS message) and attach both S-HTTP
headers (e.g. the S-HTTP request/response line, the Content-Privacy-
Domain) and the necessary HTTP headers (including a Content-Type
that reflects the inner content).
SECURE * Secure-HTTP/1.2
Content-Type: text/html
Content-Privacy-Domain: PKCS-7
Content-Transfer-Encoding: base64
-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Random signed message here...
-----END PRIVACY-ENHANCED MESSAGE-----
This message itself cannot be sent, but needs to be recursively
encapsulated, as described in the next section.
7.1.4. Recursive Encapsulation
As required by Section 7.3, the result above needs to be itself
encapsulated to protect the HTTP headers. the obvious case [and the
one illustrated here] is when confidentiality is required, but the
auth enhancement or even the null transform might be applied instead.
That is, the message shown above can be used as the inner content of
a new S-HTTP message, like so:
Rescorla, Schiffman [Page 33] Internet-Draft Secure HTTP
SECURE * Secure-HTTP/1.2
Content-Type: application/s-http
Content-Privacy-Domain: PKCS-7
Content-Transfer-Encoding: base64
-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Encrypted version of the message above...
-----END PRIVACY-ENHANCED MESSAGE-----
To unfold this, the receiver would decode the outer S-HTTP message,
reenter the (S-)HTTP parsing loop to process the new message, see
that that too was S-HTTP, decode that, and recover the inner content.
Note that this approach can also be used to provide freshness of
server activity (though not of the document itself) while still pro-
viding nonrepudiation of the document data if a NONCE is included in
the request.
7.1.5. Preencrypted Messages
Although preenhancement works best with signature, it can also be
used with encryption under certain conditions. Consider the situation
where the same confidential document is to be sent out repeatedly.
The time spent to encrypt can be saved by caching the ciphertext and
simply generating a new key exchange block for each recipient. [Note
that this is logically equivalent to a multi- recipient message as
defined in both MOSS and PKCS-7 and so care must be taken to use
proper PKCS-1 padding if RSA is being used since otherwise, one may
be open to a low encryption exponent attack.[HAST96]
7.2. Proxy Interaction
The use of S-HTTP presents implementation issues to the use of HTTP
proxies. While simply having the proxy blindly forward responses is
straightforward, it would be preferable if S-HTTP aware proxies were
still able to cache responses in at least some circumstances. In
addition, S-HTTP services should be usable to protect client-proxy
authentication. This section describes how to achieve those goals
using the mechanisms described above.
7.2.1. Client-Proxy Authentication
When an S-HTTP aware proxy receives a request (HTTP or S-HTTP) that
(by whatever access control rules it uses) it requires to be S-HTTP
authenticated (and if it isn't already so), it should return the 422
response code (5.7.4).
Rescorla, Schiffman [Page 34] Internet-Draft Secure HTTP
When the client receives the 422 response code, it should read the
cryptographic options that the proxy sent and determine whether or
not it is willing to apply that enhancement to the message. If the
client is willing to meet these requirements, it should recursively
encapsulate the request it previously sent using the appropriate
options. (Note that since the enhancement is recursively applied,
even clients which are unwilling to send requests to servers in the
clear may be willing to send the already encrypted message to the
proxy without further encryption.) (See Section 7.1 for another exam-
ple of a recursively encapsulated message)
When the proxy receives such a message, it should strip the outer
encapsulation to recover the message which should be sent to the
server.
7.2.2. Proxy Caching of S-HTTP Messages
Although it is often considered that security in general and confi-
dentiality in specific obviate caching, this is only true under cer-
tain circumstances. For example, when confidentiality is being used
to restrict access to some class of documents to a broad class of
users, and those users are behind a single proxy, it is obviously
advantageous if that proxy can cache such documents. S-HTTP's message
orientation makes this a fairly straightforward proposition, provided
that the parties cooperate.
7.2.2.1. Client Behavior
All the client needs to do is to provide enough URL information to
the proxy to enable the proxy to detect when potentially cached data
is being requested. In order to do this, the client simply provides
the whole URL HTTP style instead of the URI-less URL described in
Section 2.1. Note that this provides the proxy with the URI. Conse-
quently, clients which don't trust their proxy to receive that infor-
mation or are worried about traffic analysis by the proxy should not
enable caching in this way. (An insecure channel to the proxy can be
defended against using a recursive encapsulation.)
7.2.2.2. Proxy Behavior
When forwarding requests, the proxy merely needs to recognize URLs
that are in it's cache and add the If-Modified-Since header as it
does for HTTP.
When forwarding responses, the proxy needs to detect the 320 response
and reassemble a valid S-HTTP response from the cached data and the
new keying material provided by the server. The proxy should check
the Content-MD5 header if supplied to ensure that a valid cache hit
Rescorla, Schiffman [Page 35] Internet-Draft Secure HTTP
has occurred and retry the request minus the If-Modified-Since header
if the Content-MD5s do not match.
7.2.2.3. Server Behavior
The server needs to detect the If-Modified-Since header provided by
the proxy and generate the content-less message described in 5.2.5,
if the document has not been modified since the time the header.
8. Implementation Recommendations and Requirements
All S-HTTP agents must support the MD5 message digest and MAC authen-
tication. As of S-HTTP/1.2, all agents must also support the RSA-
MD5-HMAC construction.
All S-HTTP agents must support Outband key exchange.
Support for encryption is recommended; agents which implement encryp-
tion must support the in-band key exchange method and one of the fol-
lowing three cryptosystems (in ECB and CBC modes): DES, RC2[40] and
CDMF.
Agents are recommended to support signature verification; server sup-
port of signature generation is additionally recommended. Agents
which implement either signing or verification should support the RSA
algorithm.
Note that conformant implementations of the protocol (although not
recommended ones) can avoid the use of public key cryptography
entirely.
9. Protocol Syntax Summary
We present below a summary of the main syntactic features of S-
HTTP/1.2, excluding message encapsulation proper.
9.1. S-HTTP (Unencapsulated) Headers
Content-Privacy-Domain: ('PKCS-7' | 'MOSS')
Content-Transfer-Encoding: ('8BIT' | '7BIT' | 'BASE64')
Prearranged-Key-Info: <Hdr-Cipher>,<Key>,<Key-ID>
Content-Type: 'application/http'
MAC-Info: [hex(timeofday)',']<hash-alg>','hex(<hash-data>)','
<key-spec>
9.2. HTTP (Encapsulated) Non-negotiation Options
Key-Assign: <Method>','<Key-Name>','<Lifetime>','
Rescorla, Schiffman [Page 36] Internet-Draft Secure HTTP
<Ciphers>';'<Method-args>
Encryption-Identity: <name-class>','<key-sel>','<name-args>
Certificate-Info: <Cert-Fmt>','<Cert-Group>
Nonce: <string>
Nonce-Echo: <string>
9.3. Encapsulated Negotiation Options
SHTTP-Cryptopts: <scope>';'<string>(,<string>)*
SHTTP-Privacy-Domains: ('PKCS-7' | 'MOSS')
SHTTP-Certificate-Types: ('X.509')
SHTTP-Key-Exchange-Algorithms: ('RSA' | 'KRB-'<kv>)
SHTTP-Signature-Algorithms: ('RSA' | 'NIST-DSS')
SHTTP-Message-Digest-Algorithms: ('RSA-MD2' | 'RSA-MD5' | 'NIST-SHS'
'RSA-MD2-HMAC', 'RSA-MD5-HMAC', 'NIST-SHS-HMAC')
SHTTP-Symmetric-Content-Algorithms: ('DES-CBC' | 'DES-EDE-CBC' |
'DES-EDE3-CBC' | 'DESX-CBC' | 'CDMF-CBC' | 'IDEA-CBC' |
'RC2-CBC' )
SHTTP-Symmetric-Header-Algorithms: ('DES-ECB' | 'DES-EDE-ECB' |
'DES-EDE3-EBC' | 'DESX-ECB' | 'CDMF-ECB' |
'IDEA-ECB' | 'RC2-ECB')
SHTTP-Privacy-Enhancements: ('sign' | 'encrypt' | 'auth')
Your-Key-Pattern: <key-use>','<pattern-info>
9.4. HTTP Methods
Secure * Secure-HTTP/1.2
9.5. Server Status Reports
Secure-HTTP/1.2 200 OK
SecurityRetry 420
BogusHeader 421 <reason>
9.6. Server Conventions
GET SERVER-CERTIFICATE-<B64-DN> <http-version>
GET POLICY-<B64-DN> <http-version>
GET CRL-<B64-DN> <http-version>
10. An Extended Example
We provide here a contrived example of a series of S-HTTP requests
and replies. Rows of equal signs are used to set off the narrative
from sample message traces. Note that, since we use base-64 encoding
here for expository purposes, the example messages have the otherwise
unnecessary MOSS-style "BEGIN/END PRIVACY-ENHANCED MESSAGE" delim-
iters.
Rescorla, Schiffman [Page 37] Internet-Draft Secure HTTP
10.1. A request using RSA key exchange with Inband key reply
Alice, using an S-HTTP-capable client, begins by making an HTTP
request which yields the following response page:
============================================================
200 OK HTTP/1.0
Server-Name: Navaho-0.1.2.3alpha
Certificate-Info: PKCS7,MIAGCSqGSIb3DQEHAqCAMIACAQExADCABgkqh
kiG9w0BBwEAAKCAM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Encryption-Identity: DN-1485, null, CN=Setec Astronomy, OU=Persona
Certificate,O="RSA Data Security, Inc.", C=US;
SHTTP-Privacy-Enhancements: recv-required=encrypt
<A name=tag1 HREF="shttp://www.setec.com/secret">
Don't read this. </A>
============================================================
An appropriate HTTP request to dereference this URL would be:
============================================================
GET /secret HTTP/1.0
Security-Scheme: S-HTTP/1.2
User-Agent: Web-O-Vision 1.2beta
Accept: *.*
Key-Assign: Inband,1,reply,des-ecb;7878787878787878
============================================================
Rescorla, Schiffman [Page 38] Internet-Draft Secure HTTP
The added Key-Assign line that would not have been in an ordinary
HTTP request permits Bob (the server) to encrypt his reply to Alice,
even though Alice does not have a public key, since they would share
a key after the request is received by Bob. This request has the
following S-HTTP encapsulation:
============================================================
Secure * Secure-HTTP/1.2
Content-Transfer-Encoding: base64
Content-Type: application/http
Content-Privacy-Domain: PKCS-7
-----BEGIN PRIVACY-ENHANCED MESSAGE-----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-----END PRIVACY-ENHANCED MESSAGE-----
============================================================
The data between the delimiters is a PKCS-7 message, RSA enveloped
for Setec Astronomy.
Bob decrypts the request, finds the document in question, and is
ready to serve it back to Alice.
An appropriate HTTP server response would be:
============================================================
HTTP/1.0 200 OK
Security-Scheme: S-HTTP/1.2
Content-Type: text/html
Congratulations, you've won.
<A href="/prize.html"
CRYPTOPTS="Key-Assign: Inband,alice1,reply,des-ecb;020406080a0c0e0f;
SHTTP-Privacy-Enhancements: recv-required=auth">Click here to
claim your prize</A>
============================================================
This HTTP response, encapsulated as an S-HTTP message becomes:
Rescorla, Schiffman [Page 39] Internet-Draft Secure HTTP
============================================================
Secure * Secure-HTTP/1.2
Content-Transfer-Encoding: base64
Content-Type: application/http
Prearranged-Key-Info: des-ecb,697fa820df8a6e53,inband:1
Content-Privacy-Domain: PKCS-7
-----BEGIN PRIVACY-ENHANCED MESSAGE-----
MIAGCSqGSIb3DQEHBqCAMIACAQAwgAYJKoZIhvcNAQcBMBEGBSsOAwIHBAifqtdy
x6uIMYCCARgvFzJtOZBn773DtmXlx037ck3giqnV0WC0QAx5f+fesAiGaxMqWcir
r9XvT0nT0LgSQ/8tiLCDBEKdyCNgdcJAduy3D0r2sb5sNTT0TyL9uydG3w55vTnW
aPbCPCWLudArI1UHDZbnoJICrVehxG/sYX069M8v6VO8PsJS7//hh1yM+0nekzQ5
l1p0j7uWKu4W0csrlGqhLvEJanj6dQAGSTNCOoH3jzEXGQXntgesk8poFPfHdtj0
5RH4MuJRajDmoEjlrNcnGl/BdHAd2JaCo6uZWGcnGAgVJ/TVfSVSwN5nlCK87tXl
nL7DJwaPRYwxb3mnPKNq7ATiJPf5u162MbwxrddmiE7e3sST7naSN+GS0ateY5X7
AAAAAAAAAAA=
-----END PRIVACY-ENHANCED MESSAGE-----
============================================================
The data between the delimiters is a PKCS7 message encrypted under a
randomly-chosen DEK which can be recovered by computing:
DES-DECRYPT(inband:1,697fa820df8a6e53)
where 'inband:1' is the key exchanged in the Key-Assign line in the
original request.
10.2. A request using the auth enhancement
There is a link on the HTML page that was just returned, which Alice
dereferences, creating the HTTP message:
============================================================
GET /prize.html HTTP/1.0
Security-Scheme: S-HTTP/1.2
User-Agent: Web-O-Vision 1.1beta
Accept: *.*
============================================================
Which, when encapsulated as an S-HTTP message, becomes:
Rescorla, Schiffman [Page 40] Internet-Draft Secure HTTP
============================================================
Secure * Secure-HTTP/1.2
Content-Transfer-Encoding: base64
Content-Type: application/http
MAC-Info:31ff8122,rsa-md5,b3ca4575b841b5fc7553e69b0896c416,inband:alice1
Content-Privacy-Domain: PKCS-7
-----BEGIN PRIVACY-ENHANCED MESSAGE-----
MIAGCSqGSIb3DQEHAaCABGNHRVQgL3ByaXplLmh0bWwgSFRUUC8xLjAKU2VjdXJp
dHktU2NoZW1lOiBTLUhUVFAvMS4xClVzZXItQWdlbnQ6IFdlYi1PLVZpc2lvbiAx
LjFiZXRhCkFjY2VwdDogKi4qCgoAAAAA
-----END PRIVACY-ENHANCED MESSAGE-----
============================================================
The data between the delimiters is a PKCS-7 'Data' representation of
the request.
Rescorla, Schiffman [Page 41] Internet-Draft Secure HTTP
Appendix: A Review of PKCS-7
PKCS-7 ("Cryptographic Message Syntax Standard") is a cryptographic
message encapsulation format, similar to PEM, which was defined by
RSA Laboratories as part of a family of related standards. They
state: "The PKCS standards are offered by RSA Laboratories to
developers of computer systems employing public key cryptography. It
is RSA Laboratories' intention to improve and refine the standards in
conjunction with computer system developers, with the goal of produc-
ing standards that most if not all developers adopt."
PKCS-7 is only one of two encapsulation formats supported by S-HTTP,
but it is to be preferred since it permits the least restricted set
of negotiable options, and permits binary encoding. In the interest
of making this specification more self-contained, we summarize PKCS-7
here.
PKCS-7 is a superset of PEM, in that PEM messages can be converted to
PKCS-7 messages without any cryptographic operations, and vice-versa
(given PKCS-7 messages which are restricted to PEM facilities).
Additionally, PEM key management materials such as certificates and
certificate revocation lists are compatible with PKCS-7's.
PKCS-7 is defined in terms of OSI's Abstract Syntax Notation (ASN.1,
defined in X.208), and is concretely represented using ASN.1's Basic
Encoding Rules (BER, defined in X.209). A PKCS-7 message is a
sequence of typed content parts. There are six content types, recur-
sively composable:
Data -- Some bytes, with no enhancement.
SignedData -- A content part, with zero or more signature
blocks, and associated keying materials. Keying materials
can be transported via the degenerate case of no signature
blocks and no data.
EnvelopedData -- One or more (per recipient) key exchange
blocks and an encrypted content part.
SignedAndEnvelopedData -- The obvious combination of
SignedData and EnvelopedData for a single content part.
DigestedData -- A content part with a single digest block.
EncryptedData -- An encrypted content part, with key
materials externally provided.
Here we will dispense with convention for the sake of ASN.1-impaired
Rescorla, Schiffman [Page 42] Internet-Draft Secure HTTP
readers, and present a syntax for PKCS-7 in informal BNF (with much
gloss). In the actual encoding, most productions have explicit tag
and length fields.
<Message> := (<Content>)+
<Content> := <Data> | <SignedData> | <EnvelopedData> |
<SignedAndEnvelopedData> |
<DigestedData> | <EncryptedData>
<Data> := <Bytes>
<SignedData> := <DigestAlg>* <Content> <Certificates>*
<CRLs>* <SignerInfo>*
<EnvelopedData> := <RecipientInfo>+ <BulkCryptAlg>
Encrypted(<Content>)
<SignedAndEnvelopedData> := <RecipientInfo>* <DigestAlg>*
<EncryptedData> <Certificates>*
<CRLs>* <SignerInfos>*
<DigestedData> := <DigestAlg> <Content> <DigestBytes>
<EncryptedData> := <BulkCryptAlg> Encrypted(<Bytes>)
<SignerInfo> := <CertID> ... Encrypted(<DigestBytes>) ...
<RecipientInfo> := <CertID> <KeyCryptAlg> Encrypted(<DEK>)
Rescorla, Schiffman [Page 43] Internet-Draft Secure HTTP
Bibliography and References
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Message Authentication", Preprint.
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draft-ietf-html-spec-04, June 1995 (working draft)
[BERN95b] Berners-Lee, T., Fielding, R. T., Nielsen, H., "Hypertext
Transfer Protocol -- HTTP/1.0", draft-ietf-http-v10-spec-00,
March 1995. (working draft)
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[HAST86] Hastad, J., "On Using RSA With Low Exponents in a Public Key
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Springer-Verlag, 1986.
[JOHN93] Johnson, D.B., Matyas, S.M., Le, A.V., Wilkins, J.D., "Design of the
Commercial Data Masking Facility Data Privacy Algorithm," Proceedings
1st ACM Conference on Computer & Communications Security,
November 1993, Fairfax, VA., pp. 93-96.
[KRAW96a] Krawczyk, H., Bellare, M., Canetti, R., "HMAC-MD5: Keyed-MD5 for
Message Authentication", draft-ietf-ipsec-hmac-md5-00.txt, March 1996.
[KRAW96b] Krawczyk, H. personal communication.
[LAI92] Lai, X. "On the Design and Security of Block Ciphers," ETH Series in
Information Processing, v. 1, Konstanz: Hartung-Gorre Verlag, 1992.
[PKCS-6] RSA Data Security, Inc. "Extended Certificate Syntax Standard",
PKCS-6, Nov 1, 1993.
[PKCS-7] RSA Data Security, Inc. "Cryptographic Message Syntax Standard",
PKCS-7, Nov 1, 1993.
[RFC-822] Crocker, D. "Standard For The Format Of ARPA Internet Text Messages",
Rescorla, Schiffman [Page 44] Internet-Draft Secure HTTP
RFC822, August 1982.
[RFC-1319] Kaliski, B. "The MD2 Message-Digest Algorithm", RFC1319, April 1992
[RFC-1321] Rivest, R. "The MD5 Message-Digest Algorithm", RFC1321, April 1992
[RFC-1421] Linn J. "Privacy Enhancement for Internet Electronic Mail:
Part I: Message Encryption and Authentication Procedures",
RFC1421, Feb 1993.
[RFC-1422] Kent, S. "Privacy Enhancement for Internet Electronic Mail:
Part II: Certificate-Based Key Management", RFC1422, Feb 1993.
[RFC-1485] Hardcastle-Kille, S. "A String Representation of Distinguished
Names", RFC1485, July 1993.
[RFC-1510] Kohl, J., and Neuman, C., "The Kerberos Authentication Service
(V5)", RFC1510, September 1993.
[RFC-1521] Borenstein, N., Freed, N., "MIME (Multipurpose Internet
Mail Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC-1521, September 1993.
[RFC-1738] Berners-Lee, T. "Uniform Resource Locators (URLs)", RFC1738,
Dec 1994
[RFC-1847] Galvin, J., Murphy, S., Crocker, S., Freed, N.,
"Security Muliparts for MIME: Multipart/Signed and Multipart/Encrypted",
RFC-1847, October 1995.
[RFC-1848] Crocker, S., Freed, N., Galvin, J., Murphy, S.,
"MIME Object Security Services", RFC-1848, October 1995.
[RFC-1864] Myers, J. Rose, M. "The Content-MD5 Header Field", 10/24/1995.
RFC1864, October 1995.
[SHTML] Rescorla, E., Schiffman, A., "Security Extensions For HTML",
draft-ietf-wts-shtml-02.txt.
[VANO95] B. Prennel and P. van Oorschot, "On the security of two MAC
algorithms", to appear Eurocrypt'96.
[X509] CCITT Recommendation X.509 (1988), "The Directory -
Authentication Framework".
Security Considerations
Rescorla, Schiffman [Page 45] Internet-Draft Secure HTTP
This entire document is about security.
Acknowledgements
The authors wish to thank our colleagues at Terisa Systems, Enter-
prise Integration, Technologies, RSA Data Security, TIS, MCI, BBN, HP
Labs Bristol, NCSA, Spyglass, MIT, CERN, Open Market, Spry, Digital,
W3C and elsewhere for their review of earlier drafts. We also wish to
thank the many users who shared their experience with the S-HTTP
reference implementation distributed by the CommerceNet Consortium.
This work was initiated at Enterprise Integration Technologies Cor-
poration and funded in part by the ARPA MADE (Manufacturing Automa-
tion and Design Engineering) program, under contract management by
the USAF Wright Laboratory. In addition to the funding support, we
appreciate the administrative and intellectual resources of the spon-
sors and the research community they maintain.
Authors' Address
Eric Rescorla <ekr@terisa.com>
Terisa Systems, Inc.
4984 El Camino Real
Los Altos, CA 94022
Phone: (415) 919-1753
Allan M. Schiffman <ams@terisa.com>
Terisa Systems, Inc.
4984 El Camino Real
Los Altos, CA 94022
Phone: (415) 919-1755
Rescorla, Schiffman [Page 46] Internet-Draft Secure HTTP
Table of Contents
1. Introduction ................................................... 2
1.1. Summary of Features .......................................... 2
1.2. Changes ...................................................... 3
1.3. Processing Model ............................................. 3
1.4. Modes of Operation ........................................... 5
1.5. Implementation Options ....................................... 6
2. Message Format ................................................. 6
2.1. The Request Line ............................................. 7
2.2. The Status Line .............................................. 7
2.3. Secure HTTP Header Lines ..................................... 7
2.3.2. Content-Transfer-Encoding .................................. 8
2.4. Content ...................................................... 11
2.5. Encapsulation Format Options ................................. 12
2.5.1. Content-Privacy-Domain: PKCS-7 ............................. 12
2.5.2. Content-Privacy-Domain: MOSS ............................... 13
2.5.3. Imported HTTP headers ...................................... 14
2.5.3.2. Connection: Keep-Alive ................................... 14
2.5.3.3. If-Modified-Since ........................................ 14
2.5.3.4. Content-MD5 .............................................. 15
3. Cryptographic Parameters ....................................... 15
Rescorla, Schiffman [Page 47] Internet-Draft Secure HTTP
3.1. Options Headers .............................................. 15
3.2. Negotiation Options ....................................... 15
3.2.1. Negotiation Overview .................................... 15
3.2.2. Negotiation Option Format .................................. 16
3.2.3. Parametrization for Variable-length Key Ciphers ............ 17
3.2.4. Negotiation Syntax ......................................... 18
3.3. Non-Negotiation Headers ...................................... 22
3.3.1. Encryption-Identity ........................................ 22
3.3.2. Certificate-Info ........................................... 23
3.3.4. Nonces ..................................................... 25
3.4. Grouping Headers With SHTTP-Cryptopts ........................ 25
3.4.1. SHTTP-Cryptopts ............................................ 25
4. New Header Lines for HTTP ...................................... 26
4.1. Security-Scheme .............................................. 26
5. (Retriable) Server Status Error Reports ........................ 27
5.1. Retry for Option (Re)Negotiation ............................. 27
5.2. Specific Retry Behavior ...................................... 27
5.3. Limitations On Automatic Retries ............................. 29
6. Other Issues ................................................... 30
6.1. Compatibility of Servers with Old Clients .................... 30
6.2. URL Protocol Type ............................................ 30
6.3. Server Conventions ........................................... 30
6.4. Browser Presentation ......................................... 31
7. Implementation Notes ........................................... 32
Rescorla, Schiffman [Page 48] Internet-Draft Secure HTTP
7.1. Preenhanced Data ............................................. 32
7.2. Note:Proxy Interaction ....................................... 34
7.2.1. Client-Proxy Authentication ................................ 34
7.2.2. Proxy Caching of S-HTTP Message ............................ 35
8. Implementation Recommendations and Requirements ................ 36
9. Protocol Syntax Summary ........................................ 36
10. An Extended Example ........................................... 37
Appendix: A Review of PKCS-7 ...................................... 42
Bibliography and References ....................................... 44
Security Considerations ........................................... 45
Acknowledgements .................................................. 46
Authors' Address .................................................. 46
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