One document matched: draft-ietf-smime-cms-auth-enveloped-00.txt
S/MIME Working Group R. Housley
Internet-Draft Vigil Security
Updates: 3852 (if approved) January 2007
The CMS AuthEnvelopedData Content Type
<draft-ietf-smime-cms-auth-enveloped-00.txt>
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Abstract
This document describes an additional content type for the
Cryptographic Message Syntax (CMS). The AuthEnvelopedData content
type is intended for use with authenticated encryption modes. All of
the various key management techniques that are supported in the
EnvelopedData content type are also supported by the
AuthEnvelopedData content type.
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1. Introduction
This document describes an additional content type for the
Cryptographic Message Syntax (CMS) [CMS]. The AuthEnvelopedData
content type is intended for use with authenticated encryption modes,
where an arbitrary content is both authenticated and encrypted.
Also, some associated data, in the form of authenticated attributes
can also be authenticated. All of the various key management
techniques that are supported in the EnvelopedData content type are
also supported by the AuthEnvelopedData content type.
The AuthEnvelopedData content type, like all of the other CMS content
types, employs ASN.1 [X.208-88], and it uses both the Basic Encoding
Rules [X.209-88] and the Distinguished Encoding Rules (DER)
[X.509-88].
1.1 Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to be interpreted as
described in [STDWORDS].
1.2 Version Numbers
The major data structure (AuthEnvelopedData) includes a version
number as the first item in the data structure. The version number
is intended to avoid ASN.1 decode errors. Some implementations do
not check the version number prior to attempting a decode, and if a
decode error occurs, then the version number is checked as part of
the error handling routine. This is a reasonable approach; it places
error processing outside of the fast path. This approach is also
forgiving when an incorrect version number is used by the sender.
Whenever the structure is updated, a higher version number will be
assigned. However, to ensure maximum interoperability the higher
version number is only used when the new syntax feature is employed.
That is, the lowest version number that supports the generated syntax
is used.
2. Authenticated-enveloped-data Content Type
The authenticated-enveloped-data content type consists of an
authenticated and encrypted content of any type and encrypted
content-encryption keys for one or more recipients. The combination
of the authenticated and encrypted content and one encrypted content-
authenticated-encryption key for a recipient is a "digital envelope"
for that recipient. Any type of content can be enveloped for an
arbitrary number of recipients using any of the supported key
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management techniques for each recipient. In addition, authenticated
but not encrypted attributes may be provided by the originator.
The typical application of the authenticated-enveloped-data content
type will represent one or more recipients' digital envelopes on an
encapsulated content.
Authenticated-enveloped-data is constructed by the following steps:
1. A content-encryption key for a particular content-
authenticated-encryption algorithm is generated at random.
2. The content-authenticated-encryption key is encrypted for each
recipient. The details of this encryption depend on the key
management algorithm used, but four general techniques are
supported:
key transport: the content-authenticated-encryption key is
encrypted in the recipient's public key;
key agreement: the recipient's public key and the sender's
private key are used to generate a pairwise symmetric key,
then the content-authenticated-encryption key is encrypted
in the pairwise symmetric key;
symmetric key-encryption keys: the content-authenticated-
encryption key is encrypted in a previously distributed
symmetric key-encryption key; and
passwords: the content-authenticated-encryption key is
encrypted in a key-encryption key that is derived from a
password or other shared secret value.
3. For each recipient, the encrypted content-authenticated-
encryption key and other recipient-specific information are
collected into a RecipientInfo value, defined in Section 6.2 of
[CMS].
4. Any attributes that are to be authenticated but not encrypted
are collected in the authenticated attributes.
5. The attributes collected in step 4 are authenticated and the
content is authenticated and encrypted with the content-
authenticated-encryption key. If the authenticated encryption
algorithm requires the content to be padded to a multiple of some
block size, then the padding is added as described in Section 6.3
of [CMS].
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6. Any attributes that are to be provided without authentication
or encryption are collected in the unauthenticated attributes.
7. The RecipientInfo values for all the recipients, the
authenticated attributes, then unauthenticated attributes, and the
authenticated and encrypted content are collected together to form
an AuthEnvelopedData value as defined in Section 2.1.
A recipient opens the digital envelope by decrypting one of the
encrypted content-authenticated-encryption keys, and then using the
recovered key to decrypt and verify the integrity of the
authenticated and encrypted content as well as verifying the
integrity of the authenticated attributes.
This section is divided into three parts. The first part describes
the AuthEnvelopedData content type, the second part describes the
authentication and encryption process, and third part describes the
key encryption process.
2.1 AuthEnvelopedData Type
The following object identifier identifies the authenticated-
enveloped-data content type:
id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) ct(1) 23 }
The authenticated-enveloped-data content type MUST have ASN.1 type
AuthEnvelopedData:
AuthEnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
authAttrs [1] IMPLICIT AuthAttributes OPTIONAL,
authEncryptedContentInfo EncryptedContentInfo,
mac MessageAuthenticationCode,
unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL }
The fields of type AuthEnvelopedData have the following meanings:
version is the syntax version number. It MUST be set to 0.
originatorInfo optionally provides information about the
originator. It is present only if required by the key management
algorithm. It may contain certificates and CRLs, and the
OriginatorInfo type is defined in Section 6.1 of [CMS].
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recipientInfos is a collection of per-recipient information.
There MUST be at least one element in the collection. The
RecipientInfo type is defined in Section 6.2 of [CMS].
authAttrs optionally contains the authenticated attributes. The
AuthenticatedData content type uses the same type to carry
authenticated attributes. The AuthAttributes type is defined in
Section 9.1 of [CMS]. Useful attribute types are defined in
Section 11 of [CMS].
authEncryptedContentInfo is the authenticated and encrypted
content. The EnvelopedData content type uses the same type to
carry the encrypted content. The EncryptedContentInfo type is
defined in Section 6.1 of [CMS].
mac is the integrity check value (ICV) or message authentication
code (MAC) that is generated by the authenticated encryption
algorithm. The AuthenticatedData content type uses the same type
to carry a MAC. In this case, the MAC covers the authenticated
attributes and the content directly, and a digest algorithm is not
used. The MessageAuthenticationCode type is defined in Section 9.1
of [CMS].
unauthAttrs optionally contains the unauthenticated attributes.
The AuthenticatedData content type uses the same type to carry
unauthenticated attributes. The UnauthAttributes type is defined
in Section 9.1 of [CMS]. Useful attribute types are defined in
Section 11 of [CMS].
2.2. Authentication and Encryption Process
The content-authenticated-encryption key for the desired content-
authenticated-encryption algorithm is randomly generated.
If the authenticated encryption algorithm requires the content to be
padded to a multiple of some block size, then the padding MUST be
added as described in Section 6.3 of [CMS]. This padding method is
well defined if and only if number of octets in the block size is
less than 256.
If optional authenticated attributes are present, then they are DER
encoded. The result will be used as the authenticated associated
data (AAD) input to the authenticated encryption algorithm.
The inputs to the authenticated encryption algorithm are the content
(the data, which is padded if necessary), the DER-encoded
authenticated attributes (the AAD), and the content-authenticated-
encryption key. Under control of a content-authenticated-encryption
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key, the authenticated encryption operation maps an arbitrary string
of octets (the data) to another string of octets (the ciphertext) and
it computes an authentication tag over the AAD and the data. The
encrypted data is included in the AuthEnvelopedData
encryptedContentInfo encryptedContent OCTET STRING, and the
authentication tag is included in the AuthEnvelopedData mac.
2.3. Key Encryption Process
The input to the key encryption process -- the value supplied to the
recipient's key-encryption algorithm -- is just the "value" of the
content-authenticated-encryption key.
Any of the aforementioned key management techniques can be used for
each recipient of the same encrypted content.
3. Security Considerations
This specification defines and additional CMS content type. The
security considerations provided in [CMS] apply to this content type
as well.
Many authenticated encryption algorithms make use of a block cipher
in counter mode to provide encryption. When used properly, counter
mode provides strong confidentiality. Bellare, Desai, Jokipii,
Rogaway show in [BDJR] that the privacy guarantees provided by
counter mode are at least as strong as those for CBC mode when using
the same block cipher.
Unfortunately, it is easy to misuse counter mode. If counter block
values are ever used for more that one encryption operation with the
same key, then the same key stream will be used to encrypt both
plaintexts, and the confidentiality guarantees are voided.
Fortunately, the CMS AuthEnvelopedData provides all of the tools
needed to avoid misuse of counter mode. When using key transport or
key agreement, a fresh key should be generated for each content.
However, when using symmetric key-encryption keys or passwords, one
cannot assume that a fresh key is generated. Therefore,
authenticated encryption algorithms that make use of counter mode
must support the use of an unpredictable nonce value in the counter
block, and this unpredictable nonce value (sometimes called a "salt")
must be carried as an algorithm identifier parameter.
There are fairly generic precomputation attacks against all block
cipher modes that allow a meet-in-the-middle attack against the key.
These attacks require the creation and searching of huge tables of
ciphertext associated with known plaintext and known keys. Assuming
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that the memory and processor resources are available for a
precomputation attack, then the theoretical strength of any block
cipher mode is limited to 2^(n/2) bits, where n is the number of bits
in the key. The use of long keys is the best countermeasure to
precomputation attacks. Use of an unpredictable nonce value in the
counter block significantly increases the size of the table that the
attacker must compute to mount a successful precomputation attack.
Implementations must randomly generate content-authenticated-
encryption keys, padding, and unpredictable nonce values. Also, the
generation of public/private key pairs relies on a random numbers.
The use of inadequate pseudo-random number generators (PRNGs) to
generate cryptographic keys can result in little or no security. An
attacker may find it much easier to reproduce the PRNG environment
that produced the keys, searching the resulting small set of
possibilities, rather than brute force searching the whole key space.
The generation of quality random numbers is difficult. RFC 4086
[RANDOM] offers important guidance in this area.
4 ASN.1 Module
CMS-AuthEnvelopedData-2007
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) cms-authEnvelopedData(31) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS All
-- The types and values defined in this module are exported for use
-- in the other ASN.1 modules. Other applications may use them for
-- their own purposes.
IMPORTS
-- Imports from RFC 3852 [CMS], Section 12.1
AuthAttributes, CMSVersion, EncryptedContentInfo,
MessageAuthenticationCode, OriginatorInfo, RecipientInfos,
UnauthAttributes
FROM CryptographicMessageSyntax2004
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
cms-2004(24) } ;
id-ct-authEnvelopedData OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) ct(1) 23 }
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AuthEnvelopedData ::= SEQUENCE {
version CMSVersion,
originatorInfo [0] IMPLICIT OriginatorInfo OPTIONAL,
recipientInfos RecipientInfos,
authAttrs [1] IMPLICIT AuthAttributes OPTIONAL,
authEncryptedContentInfo EncryptedContentInfo,
mac MessageAuthenticationCode,
unauthAttrs [2] IMPLICIT UnauthAttributes OPTIONAL }
END -- of CMS-AuthEnvelopedData-2007
5. Normative References
[CMS] Housley, R., "Cryptographic Message Syntax",
RFC 3852, July 2004.
[STDWORDS] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[X.208-88] CCITT. Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1). 1988.
[X.209-88] CCITT. Recommendation X.209: Specification of Basic
Encoding Rules for Abstract Syntax Notation One (ASN.1).
1988.
[X.509-88] CCITT. Recommendation X.509: The Directory -
Authentication Framework. 1988.
6. Informative References
[BDJR] Bellare, M, Desai, A., Jokipii, E., and P. Rogaway,
"A Concrete Security Treatment of Symmetric Encryption:
Analysis of the DES Modes of Operation", Proceedings
38th Annual Symposium on Foundations of Computer
Science, 1997.
[RANDOM] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Recommendations for Security", RFC 4086, June 2005.
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7. Authors' Address
Russell Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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