One document matched: draft-ietf-sasl-rfc2831bis-08.txt
Differences from draft-ietf-sasl-rfc2831bis-07.txt
INTERNET-DRAFT P. Leach
Obsoletes: 2831 Microsoft
Intended category: Standards track C. Newman
Sun Microsystems
A. Melnikov
Isode Ltd.
March 2006
Using Digest Authentication as a SASL Mechanism
draft-ietf-sasl-rfc2831bis-08.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This specification defines how HTTP Digest Authentication [Digest]
can be used as a SASL [RFC 2222] mechanism for any protocol that has
a SASL profile. It is intended both as an improvement over CRAM-MD5
[RFC 2195] and as a convenient way to support a single authentication
mechanism for web, mail, LDAP, and other protocols.
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Table of Contents
1 INTRODUCTION.....................................................3
1.1 CONVENTIONS AND NOTATION......................................3
1.2 CHANNEL BINDINGS..............................................4
2 AUTHENTICATION...................................................5
2.1 INITIAL AUTHENTICATION........................................5
2.1.1 Step One...................................................5
2.1.2 Step Two...................................................9
2.1.3 Step Three................................................16
2.2 SUBSEQUENT AUTHENTICATION....................................17
2.2.1 Step one..................................................17
2.2.2 Step Two..................................................17
2.3 INTEGRITY PROTECTION.........................................18
2.4 CONFIDENTIALITY PROTECTION...................................18
3 SECURITY CONSIDERATIONS.........................................21
3.1 AUTHENTICATION OF CLIENTS USING DIGEST AUTHENTICATION........21
3.2 COMPARISON OF DIGEST WITH PLAINTEXT PASSWORDS................21
3.3 REPLAY ATTACKS...............................................21
3.4 ONLINE DICTIONARY ATTACKS....................................22
3.5 OFFLINE DICTIONARY ATTACKS...................................22
3.6 MAN IN THE MIDDLE............................................22
3.7 CHOSEN PLAINTEXT ATTACKS.....................................22
3.8 CBC MODE ATTACKS.............................................
3.9 SPOOFING BY COUNTERFEIT SERVERS..............................23
3.10 STORING PASSWORDS...........................................23
3.11 MULTIPLE REALMS.............................................24
3.12 SUMMARY.....................................................24
4 EXAMPLE.........................................................24
5 REFERENCES......................................................26
5.1 NORMATIVE REFERENCES.........................................26
5.2 INFORMATIVE REFERENCES.......................................27
6 IANA CONSIDERATIONS.............................................28
7 ABNF............................................................29
7.1 AUGMENTED BNF................................................29
7.2 BASIC RULES..................................................31
8 SAMPLE CODE.....................................................33
9 AUTHORS' ADDRESSES..............................................XX
10 ACKNOWLEDGEMENTS..............................................34
11 FULL COPYRIGHT STATEMENT.......................................35
Appendix A: Changes from 2831.....................................36
Appendix B: Open Issues...........................................37
<<Page numbers are all wrong, sorry.
Section ordering should be changed too>>
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1 Introduction
This specification describes the use of HTTP Digest Access
Authentication as a SASL mechanism. The authentication type
associated with the Digest SASL mechanism is "DIGEST-MD5".
This specification is intended to be upward compatible with the
"md5-sess" algorithm of HTTP/1.1 Digest Access Authentication
specified in [Digest]. The only difference in the "md5-sess"
algorithm is that some directives not needed in a SASL mechanism have
had their values defaulted.
There is one new feature for use as a SASL mechanism: integrity
protection on application protocol messages after an authentication
exchange.
Also, compared to CRAM-MD5, DIGEST-MD5 prevents chosen plaintext
attacks, and permits the use of third party authentication servers,
mutual authentication, and optimized reauthentication if a client has
recently authenticated to a server.
1.1 Conventions and Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
<<This specification uses the same ABNF notation and lexical
conventions as HTTP/1.1 specification; see section 7>>.
Let { a, b, ... } be the concatenation of the octet strings a, b, ...
Let ** denote the power operation.
Let H(s) be the 16 octet MD5 hash [RFC 1321] of the octet string s.
Let KD(k, s) be H({k, ":", s}), i.e., the 16 octet hash of the string
k, a colon and the string s.
Let HEX(n) be the representation of the 16 octet MD5 hash n as a
string of 32 hex digits (with alphabetic characters always in lower
case, since MD5 is case sensitive).
Let HMAC(k, s) be the 16 octet HMAC-MD5 [RFC 2104] of the octet
string s using the octet string k as a key.
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Let unq(X) be the value of the quoted-string X without the
surrounding quotes and with all escape characters "\\" removed. For
example for the quoted-string "Babylon" the value of unq("Babylon")
is Babylon; for the quoted string "ABC\"123\\" the value of
unq("ABC\"123\\") is ABC"123\.
The value of a quoted string constant as an octet string does not
include any terminating nul (0x00) character.
<<Other terms like "protocol profile" are defined in RFC2222.>>
1.2 Channel Bindings
"Channel binding" is a concept described in [RFC 2743] and which
refers to the act of cryptographically binding authentication at one
network layer to a secure channel at another layer and where the end-
points at both layers are the same entities. In the context of the
DIGEST-MD5 SASL mechanism this means ensuring that the challenge and
response messages include the "channel bindings" of any cryptographic
channel (e.g. TLS) over which the DIGEST-MD5 exchange is transported,
and that the inputs to the digest function include the same as well.
The "channel bindings" of a channel here refer to information which
securely identifies one instance of such a channel to both endpoints
such that MITM attacks are detectable. For TLS, the channel
bindings are the TLS client and server finished messages.
Channel bindings are herein added to DIGEST-MD5 by overloading the
nonce and cnonce fields of the digest-challenge and digest-response
messages, respectively. Because these nonces are treated as opaque
octet strings in previous versions of this mechanism such overloading
is backwards compatible. Because these nonces are used in the
construction of the response-value (i.e., as input to the digest
function) using these fields for carrying channel bindings data makes
the channel binding operation possible without requiring incompatible
changes to the message formats. The fact that the odds that older
implementations may select random nonces that resemble actual channel
bindings data are so low allows new implementations to detect old
peers and to decide whether to allow such peers or reject them
according to local policy.
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2 Authentication
DIGEST-MD5 can operate in two modes. Initial authentication (section
2.1) is usually used when a client authenticates to a server for the
first time. If protocol profile supports initial client response
(see "Protocol profile requirements" in [RFC 2222]) and the client
supports reauthentication and it has successfully authenticated to
the server before, the client may be able to use the more efficient
fast reauthentication mode as described in section 2.2.
The following sections describe these two modes in details.
2.1 Initial Authentication
If the client has not recently authenticated to the server, then it
must perform "initial authentication", as defined in this section. If
it has recently authenticated, then a more efficient form is
available, defined in the next section.
2.1.1 Step One
The server starts by sending a challenge. The data encoded in the
challenge is formatted according to the rules for the "digest-
challenge" defined as follows:
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digest-challenge =
1#( realm | nonce | qop-options | stale | server_maxbuf |
charset | algorithm | cipher-opts | auth-param )
realm = "realm" "=" realm-value
realm-value = quoted-string
nonce = "nonce" "=" nonce-value
nonce-value = quoted-string
;; contains data described by "nonce-data"
qop-options = "qop" "=" <"> qop-list <">
qop-list = 1#qop-value
qop-value = "auth" | "auth-int" | "auth-conf" |
qop-token
;; qop-token is reserved for identifying
;; future extensions to DIGEST-MD5
qop-token = token
stale = "stale" "=" "true"
server_maxbuf = "maxbuf" "=" maxbuf-value
maxbuf-value = 1*DIGIT
charset = "charset" "=" "utf-8"
algorithm = "algorithm" "=" "md5-sess"
cipher-opts = "cipher" "=" <"> 1#cipher-value <">
cipher-value = "rc4-40" | "rc4" | "rc4-56" |
"aes-ctr" | cipher-token
;; cipher-token is reserved for
;; new ciphersuites
cipher-token = token
auth-param = token "=" ( token | quoted-string )
nonce-data = new-nonce-data | obs-nonce-data
new-nonce-data = "CB-" channel-type ":" channel-bindings
":" nonce-octets
obs-nonce-data = nonce-octets
;; nonce value as defined in RFC 2831.
;; SHOULD be accepted. MUST NOT be
;; generated.
channel-type = "TLS" / channel-type-ext
channel-type-ext = 1*(ALPHA | DIGIT)
;; for future channel bindings
channel-bindings = 1*TEXTCHAR
;; channel binding data as defined by
;; the channel type
nonce-octets = 1*TEXTCHAR
The meanings of the values of the directives used above are as
follows:
realm
Mechanistically, a string which enables users to decide which
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username and password to use, in case they have different ones for
different servers. Conceptually, it is the name of a collection
of accounts that might include the user's account. This string
should contain the name of the host performing the authentication
and might additionally indicate the collection of users who might
have access. An example might be
"registered_users@gotham.news.example.com". Note that the server
MAY not advertise (hide) some or all realms it supports.
Other examples:
1) "dc=gotham, dc=news, dc=example, dc=com".
2) If there are two servers (e.g. server1.example.com and
server2.example.com) that share authentication database, they
both may advertise "example.com" as the realm.
A server implementation that uses a fixed string as the advertised
realm is compliant with this specification, however this is not
recommended. See also sections 3.10 "Storing passwords" and 3.11
"Multiple realms" for discussion.
The value of this directive is case-sensitive. This directive is
optional; if not present, the client SHOULD solicit it from the
user or be able to compute a default; a plausible default might be
the realm supplied by the user when they logged in to the client
system. Multiple realm directives are allowed, in which case the
user or client must choose one as the realm for which to supply
username and password.
Requirements on UIs: UIs MUST allow users to enter arbitrary user
names and realm names. In order to achieve this, UIs MAY present
two separate edit boxes. Alternatively, UIs MAY present a single
edit box and allow user to enter a special character that
separates user name from the realm name. In the latter case, UIs
MUST be able to escape the special character and they need to
present their escape rules to the user. UIs MUST also present the
list of realms advertised by the server.
nonce
A server-specified string erstwhile intended to add entropy to the
challenge. The nonce field may be used to exchange channel
binding data.
This directive is required and MUST appear exactly once; if not
present, or if multiple instances are present, the client should
abort the authentication exchange.
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Older implementations typically generate some random or pseudo-
random data and base64 [RFC 3548] or hexadecimally encode it.
When channel binding is not used the nonce string MUST be
different each time a digest-challenge is sent as part of initial
authentication. It is RECOMMENDED that the random data contain at
least 64 bits of entropy.
When channel binding is performed, the nonce must be generated
from the channel type, the bindings to the channel being bound to
and an actual nonce consisting of 64-bits or more of entropy and
base64-encoded, and formatted as follows:
"CB-" <channel type> ":" <channel bindings> ":" <nonce octets>
The only channel binding currently defined is to TLS channels.
The channel type for TLS is "TLS" and the channel bindings for TLS
channels consist of the TLS client and server finished messages
concatenated in that order and base64-encoded.
<<Do we need an IANA registry?>>
An actual nonce is included in order to allow for channel bindings
to possible future channels with channel bindings data which is
not necessarily unique for each instance.
When channel bindings are in use, clients MUST reject challenges
that contain server nonce values of this form and whose channel
bindings do not match those of the actual underlying channel as
observed by the client.
qop-options
A quoted string of one or more comma-separated tokens indicating
the "quality of protection" values supported by the server. The
value "auth" indicates authentication; the value "auth-int"
indicates authentication with integrity protection; the value
"auth-conf" indicates authentication with integrity protection and
encryption. This directive is optional; if not present it
defaults to "auth". The client MUST ignore unrecognized options;
if the client recognizes no option, it MUST abort the
authentication exchange.
<<What if this directive is present multiple times? Error, or take
the union of all values?>>
stale
The "stale" directive is not used in initial authentication. See
the next section for its use in subsequent authentications. This
directive may appear at most once; if multiple instances are
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present, the client MUST abort the authentication exchange.
server_maxbuf ("maximal ciphertext buffer size")
A number indicating the size of the largest buffer (in bytes) the
server is able to receive when using "auth-int" or "auth-conf".
The value MUST be bigger than 16 and smaller or equal to 16777215
(i.e. 2**24-1). If this directive is missing, the default value is
65536. This directive may appear at most once; if multiple
instances are present, or the value is out of range the client
MUST abort the authentication exchange.
Let "maximal cleartext buffer size" (or "maximal sender size") be
the maximal size of a cleartext buffer that, after being
transformed by integrity (section 2.3) or confidentiality (section
2.4) protection function, will produce a SASL block of the maxbuf
size. As it should be clear from the name, the sender MUST never
pass a block of data bigger than the "maximal sender size" through
the selected protection function. This will guarantee that the
receiver will never get a block bigger than the maxbuf.
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charset
This directive, if present, specifies that the server supports
UTF-8 [UTF-8] encoding for the username, realm and password. If
present, the username, realm and password are encoded as UTF-8
[UTF-8]. If not present, the username, realm and password used by
the client in Step 2 MUST be encoded in ISO 8859-1 [ISO-8859] (of
which US-ASCII [USASCII] is a subset). The directive is needed for
backwards compatibility with HTTP Digest, which only supports ISO
8859-1. This directive may appear at most once; if multiple
instances are present, the client MUST abort the authentication
exchange.
Note, that this directive doesn't affect authorization id
("authzid").
algorithm
This directive is required for backwards compatibility with HTTP
Digest, which supports other algorithms. This directive is
required and MUST appear exactly once; if not present, or if
multiple instances are present, the client SHOULD abort the
authentication exchange.
cipher-opts
A list of ciphers that the server supports. This directive must be
present exactly once if "auth-conf" is offered in the
"qop-options" directive, in which case the "rc4" cipher is
mandatory-to-implement. The client MUST ignore unrecognized
ciphers; if the client recognizes no cipher, it MUST behave as if
"auth-conf" qop option wasn't provided by the server. If the
client recognizes no cipher and the server only advertised "auth-
conf" in the qop option, the client MUST abort the authentication
exchange. See section 2.4 for more detailed description of the
ciphers.
rc4, rc4-40, rc4-56
the RC4 cipher with a 128 bit, 40 bit, and 56 bit key,
respectively.
aes-ctr
the Advanced Encryption Standard (AES) cipher [AES] in counter
(CTR) mode with a 128 bit key. This mode requires an IV that
has the same size as the block size.
auth-param
This construct allows for future extensions; it may appear more
than once. The client MUST ignore any unrecognized directives.
For use as a SASL mechanism, note that the following changes are made
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to "digest-challenge" from HTTP: the following Digest options (called
"directives" in HTTP terminology) are unused (i.e., MUST NOT be sent,
and MUST be ignored if received):
opaque
domain
The size of a "digest-challenge" MUST be less than 2048 bytes.
2.1.2 Step Two
The client validates "digest-challenge" as described in the previous
section. In particular, when channel bindings are in use, client MUST
reject "digest-challenge" that contain server nonce whose channel
bindings do not match those of the actual underlying channel as
observed by the client.
The client makes note of the "digest-challenge" and then responds
with a string formatted and computed according to the rules for a
"digest-response" defined as follows:
digest-response = 1#( username | realm | nonce | cnonce |
nonce-count | qop | digest-uri | response |
client_maxbuf | charset | cipher | authzid |
auth-param )
username = "username" "=" username-value
username-value = quoted-string
cnonce = "cnonce" "=" cnonce-value
cnonce-value = nonce-value
nonce-count = "nc" "=" nc-value
nc-value = 8LHEX
client_maxbuf = "maxbuf" "=" maxbuf-value
qop = "qop" "=" qop-value
digest-uri = "digest-uri" "=" <"> digest-uri-value <">
digest-uri-value = serv-type "/" host [ "/" serv-name ]
serv-type = 1*ALPHA
serv-name = host
response = "response" "=" response-value
response-value = 32LHEX
LHEX = "0" | "1" | "2" | "3" |
"4" | "5" | "6" | "7" |
"8" | "9" | "a" | "b" |
"c" | "d" | "e" | "f"
cipher = "cipher" "=" cipher-value
authzid = "authzid" "=" authzid-value
authzid-value = quoted-string
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The 'host' non-terminal is defined in [RFC 3986] as
host = IP-literal / IPv4address / reg-name
username
The user's name in the specified realm, encoded according to the
value of the "charset" directive. This directive is REQUIRED and
MUST be present exactly once; otherwise, authentication fails.
If the charset directive is also specified (which means that the
username is encoded as UTF-8) the client MUST first check if all
the characters of the username are in the ISO 8859-1 character
set. If they are, no further changes are performed. Otherwise, the
client SHOULD prepare <<SHOULD prepare or MUST prepare, but SHOULD
use SASLPrep. The same issue elsewhere>> the username using the
"SASLPrep" profile [SASLPrep] of the "stringprep" algorithm [RFC
3454]. If the preparation of the username fails or results in an
empty string, the client SHOULD abort the authentication exchange
(*). If the preparation succeeds, the prepared value will be sent
to the server and used in hash computations described in section
2.1.2.1.
(*) An interactive client can request a repeated entry of username
value.
realm
The realm containing the user's account, encoded according to the
value of the "charset" directive. This directive MUST appear at
most once and SHOULD contain one of the realms provided by the
server in the "digest-challenge". If the directive is missing,
"realm-value" will set to the empty string when computing A1 (see
below for details).
If the realm value was provided by the client and if the charset
directive is also specified in "digest-response" (which means that
the realm is encoded as UTF-8), the client SHOULD first prepare it
using the "SASLPrep" profile [SASLPrep] of the "stringprep"
algorithm [RFC 3454]. If the preparation of the realm fails, the
client SHOULD abort the authentication exchange(*). If the
preparation succeeds, the prepared version is sent to the server
and used in hash computations described in section 2.1.2.1.
(*) An interactive client can request a repeated entry of the
realm value.
nonce
The server-specified data string received in the preceding digest-
challenge. This directive is required and MUST be present exactly
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once; otherwise, authentication fails.
cnonce
A client-specified string erstwhile intended to add entropy to the
challenge. The cnonce field may be used to exchange channel
binding data.
This directive is required and MUST be present exactly once;
otherwise, authentication fails.
Older implementations typically generate some random or pseudo-
random data and base64 [RFC 3548] or hexadecimally encode it.
When channel binding is not used the cnonce string MUST be
different each time a digest-challenge is sent as part of initial
authentication. It is RECOMMENDED that the random data contain at
least 64 bits of entropy.
When channel binding is performed, the cnonce must be generated
from the channel type, the bindings to the channel being bound to
and an actual nonce consisting of 64-bits or more of entropy and
base64-encoded, and formatted as follows:
"CB-" <channel type> ":" <channel bindings> ":" <nonce octets>
The only channel binding currently defined is to TLS channels.
The channel type for TLS is "TLS" and the channel bindings for TLS
channels consist of the TLS client and server finished messages
concatenated in that order and base64-encoded.
An actual nonce is included in order to allow for channel bindings
to possible future channels with channel bindings data which is
not necessarily unique for each instance. It is used by both
client and server to avoid chosen plaintext attacks, and to
provide mutual authentication.
nonce-count
The nc-value is the hexadecimal count of the number of requests
(including the current request) that the client has sent with the
nonce value in this request. For example, in the first request
sent in response to a given nonce value, the client sends
"nc=00000001". The purpose of this directive is to allow the
server to detect request replays by maintaining its own copy of
this count - if the same nc-value is seen twice, then the request
is a replay. See the description below of the construction of the
response value. This directive is required and MUST be present
exactly once; otherwise, or if the value is 0, authentication
fails.
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qop
Indicates what "quality of protection" the client accepted. If
present, it may appear exactly once and its value MUST be one of
the alternatives in qop-options. If not present, it defaults to
"auth". These values affect the computation of the response. Note
that this is a single token, not a quoted list of alternatives.
serv-type
Indicates the type of service, such as "http" for web service,
"ftp" for FTP service, "smtp" for mail delivery service, etc. The
service name as defined in the SASL profile for the protocol see
section 4 of [RFC 2222], registered in the IANA registry of
"service" elements for the GSSAPI host-based service name form
[RFC 2078].
host
The DNS host name or IP (IPv4 or IPv6) address for the service
requested. The DNS host name must be the fully-qualified
canonical name of the host. The DNS host name is the preferred
form; see notes on server processing of the digest-uri.
serv-name
Indicates the name of the service if it is replicated. The service
is considered to be replicated if the client's service-location
process involves resolution using standard DNS lookup operations,
and if these operations involve DNS records (such as SRV [RFC
2052], or MX) which resolve one DNS name into a set of other DNS
names. In this case, the initial name used by the client is the
"serv-name", and the final name is the "host" component. For
example, the incoming mail service for "example.com" may be
replicated through the use of MX records stored in the DNS, one of
which points at an SMTP server called "mail3.example.com"; it's
"serv-name" would be "example.com", it's "host" would be
"mail3.example.com". If the service is not replicated, or the
serv-name is identical to the host, then the serv-name component
MUST be omitted.
digest-uri
Indicates the principal name of the service with which the client
wishes to connect, formed from the serv-type, host, and serv-name.
For example, the FTP service on "ftp.example.com" would have a
"digest-uri" value of "ftp/ftp.example.com"; the SMTP server from
the example above would have a "digest-uri" value of
"SMTP/mail3.example.com/example.com".
Servers SHOULD check that the supplied value is correct. This will
detect accidental connection to the incorrect server, as well as some
redirection attacks. It is also so that clients will be trained to
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provide values that will work with implementations that use a shared
back-end authentication service that can provide server
authentication.
The serv-type component should match the service being offered. The
host component should match one of the host names of the host on
which the service is running, or it's IP address. Servers SHOULD NOT
normally support the IP address form, because server authentication
by IP address is not very useful; they should only do so if the DNS
is unavailable or unreliable. The serv-name component should match
one of the service's configured service names.
This directive is required and MUST be present exactly once; if
multiple instances are present, the server MUST abort the
authentication exchange.
Note: In the HTTP use of Digest authentication, the digest-uri is the
URI (usually a URL) of the resource requested -- hence the name of
the directive.
response
A string of 32 hex digits computed as defined below, which proves
that the user knows a password. This directive is required and
MUST be present exactly once; otherwise, authentication fails.
client_maxbuf
A number indicating the size of the largest ciphertext buffer the
client is able to receive when using "auth-int" or "auth-conf". If
this directive is missing, the default value is 65536. This
directive may appear at most once; if multiple instances are
present, the server MUST abort the authentication exchange. If the
value is less or equal to 16, or bigger than 16777215 (i.e.
2**24-1), the server MUST abort the authentication exchange.
Upon processing/sending of the client_maxbuf value both the server
and the client calculate their "maximal ciphertext buffer size" as
the minimum of the server_maxbuf (Step One) and the client_maxbuf
(Step Two). The "maximal sender size" can be calculated by
subtracting 16 from the calculated "maximal ciphertext buffer
size".
When sending a block of data the client/server MUST NOT pass more
than the "maximal sender size" bytes of data to the selected
protection function (2.3 or 2.4).
charset
This directive, if present, specifies that the client has used
UTF-8 [UTF-8] encoding for the username, realm and password. If
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present, the username, realm and password are encoded as UTF-8
[UTF-8]. If not present, the username, realm and password MUST be
encoded in ISO 8859-1 [ISO-8859] (of which US-ASCII [USASCII] is a
subset). The client should send this directive only if the server
has indicated that it supports UTF-8 [UTF-8]. The directive is
needed for backwards compatibility with HTTP Digest, which only
supports ISO 8859-1.
Note, that this directive doesn't affect the authorization
identity ("authzid").
LHEX
32 hex digits, where the alphabetic characters MUST be lower case,
because MD5 is not case insensitive.
cipher
The cipher chosen by the client. This directive MUST appear
exactly once if "auth-conf" is negotiated; if required and not
present, authentication fails. If the cipher chosen by the client
is not one of the ciphers advertised by the server, authentication
fails.
authzid
The "authorization ID" (authzid) directive may appear at most
once; if multiple instances are present, the server MUST abort the
authentication exchange. If present, and the authenticating user
has sufficient privilege, and the server supports it, then after
authentication the server will use this identity for making all
accesses and access checks. If the client specifies it, and the
server does not support it, then the response-value calculated on
the server will not match the one calculated on the client and
authentication will fail.
The authzid MUST NOT be an empty string.
The authorization identifier MUST NOT be converted to ISO 8859-1
even if the authentication identifier ("username") is converted
for compatibility as directed by "charset" directive.
The server SHOULD verify the correctness of an authzid as
specified by the corresponding SASL protocol profile.
The size of a digest-response MUST be less than 4096 bytes.
2.1.2.1 Response-value
The definition of "response-value" above indicates the encoding for
its value -- 32 lower case hex characters. The following definitions
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show how the value is computed.
Although qop-value and components of digest-uri-value may be
case-insensitive, the case which the client supplies in step two is
preserved for the purpose of computing and verifying the
response-value.
response-value =
HEX( KD ( HEX(H(A1)),
{ unq(nonce-value), ":" nc-value, ":",
unq(cnonce-value), ":", qop-value, ":",
HEX(H(A2)) }))
If authzid is specified, then A1 is
A1 = { SS, ":", unq(nonce-value), ":",
unq(cnonce-value), ":", unq(authzid-value) }
If authzid is not specified, then A1 is
A1 = { SS, ":", unq(nonce-value), ":", unq(cnonce-value) }
where
password = *OCTET
SS = H( { unq(username-value), ":",
unq(realm-value), ":", password } )
The "username-value", "realm-value" and "password" are encoded
according to the value of the "charset" directive. If "charset=UTF-8"
is present, and all the characters of "username-value" are in the ISO
8859-1 character set, then it MUST be converted to ISO 8859-1 before
being hashed. The same transformation has to be done for "realm-
value" and "password". This is so that authentication databases that
store the hashed username, realm and password (which is common) can
be shared compatibly with HTTP, which specifies ISO 8859-1. A sample
implementation of this conversion is in section 8.
Note that on the client side the "username-value", "realm-value"(*)
and "password" MUST be prepared before being encoded as described in
the previous paragraph. The [SASLPrep] profile of the [StringPrep]
algorithm is the RECOMMENDED preparation algorithm. The SASLprep
preparation algorithm is recommended to improve the likelihood that
comparisons behave in an expected manner. The SASLprep preparation
algorithm is not mandatory. This is done to allow, when appropriate,
the server to employ other preparation algorithms (including none).
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For instance, use of different preparation algorithm may be necessary
for the server to interoperate with an external system.
(*) - Note that if the "realm-value" was provided by the client (i.e.
not chosen from the list of realms returned by the server), it MUST
NOT be prepared.
On the server side the preparation of the "username-value", "realm-
value" and "password" MUST NOT be performed.
If the "qop" directive's value is "auth", then A2 is:
A2 = { "AUTHENTICATE:", digest-uri-value }
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If the "qop" value is "auth-int" or "auth-conf" then A2 is:
A2 = { "AUTHENTICATE:", digest-uri-value,
":00000000000000000000000000000000" }
Note that "AUTHENTICATE:" must be in upper case, and the second
string constant is a string with a colon followed by 32 zeros.
These apparently strange values of A2 are for compatibility with
HTTP; they were arrived at by setting "Method" to "AUTHENTICATE" and
the hash of the entity body to zero in the HTTP digest calculation of
A2.
Also, in the HTTP usage of Digest, several directives in the
"digest-challenge" sent by the server have to be returned by the
client in the "digest-response". These are:
opaque
algorithm
These directives are not needed when Digest is used as a SASL
mechanism (i.e., MUST NOT be sent, and MUST be ignored if received).
2.1.3 Step Three
The server receives and validates the "digest-response". In
particular, when channel bindings are in use, server MUST reject
"digest-response" that contain client nonce whose channel bindings do
not match those of the actual underlying channel as observed by the
server.
The server also checks that the nonce-count is "00000001". If it
supports subsequent authentication (see section 2.2), it saves the
value of the "nonce-octets" part of the nonce and the nonce-count. It
sends a message formatted as follows:
auth-info = 1#( response-auth | auth-param )
response-auth = "rspauth" "=" response-value
where response-value is calculated as above, using the values sent in
step two, except that if qop is "auth", then A2 is
A2 = { ":", digest-uri-value }
And if qop is "auth-int" or "auth-conf" then A2 is
A2 = { ":", digest-uri-value,
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":00000000000000000000000000000000" }
Note that only one occurance of the "response-auth" is allowed. If
more than one is found, the client MUST treat this as an
authentication error.
Compared to its use in HTTP, the following Digest directives in the
"auth-info" are unused:
nextnonce
qop
cnonce
nonce-count
The size of an auth-info MUST be less than 2048 bytes.
2.2 Subsequent Authentication
If the client has previously authenticated to the server, and
remembers the values of username, realm, nonce, nonce-count, cnonce,
and qop that it used in that authentication, and the SASL profile for
a protocol permits an initial client response, then it MAY perform
"subsequent authentication" (also known as "fast reauthentication"),
as defined in this section. Note, that a subsequent authentication
can be done on a different connection, or on the same connection, if
the protocol profile also permits multiple authentications.
2.2.1 Step one
The client uses the values from the previous authentication and sends
an initial response with a string formatted and computed according to
the rules for a "digest-response", as defined in section 2.1.2, after
applying the following changes:
1) the nonce-count value is one greater than used in the last
"digest-response"
2) if nonce/cnonce values contained any channel bindings information,
it
MUST be replaced with the channel bindings relevant for the new
connection.
In other words, only the "nonce-octets" part of nonce/cnonce
"nonce-data"
MUST be preserved on reauthentication.
<<Remove "cnonce" from the above, if it doesn't have to stay the
same. Section 2.2 says it does.>>
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2.2.2 Step Two
The server receives the "digest-response". If the server does not
support subsequent authentication, then it sends a
"digest-challenge", and authentication proceeds as in initial
authentication. If the server has no saved nonce and nonce-count from
a previous authentication, then it sends a "digest-challenge", and
authentication proceeds as in initial authentication. Otherwise, the
server validates the "digest-response"; checks that values of the
username, the realm, the qop and nonce-octets part of the nonce and
the cnonce are the same as in the original authentication attempt;
checks that the nonce-count is one greater than that used in the
previous authentication using that nonce, and saves the new value of
nonce-count.
<<What about cnonce? Can it be a new value>>
If the response is invalid, then the server sends a
"digest-challenge", and authentication proceeds as in initial
authentication (and should be configurable to log an authentication
failure in some sort of security audit log, since the failure may be
a symptom of an attack). The nonce-count MUST NOT be incremented in
this case: to do so would allow a denial of service attack by sending
an out-of-order nonce-count.
If the response is valid, the server MAY choose to deem that
authentication has succeeded. However, if it has been too long since
the previous authentication, or for any other reason, the server MAY
send a new "digest-challenge" with a new value for nonce. The
challenge MAY contain a "stale" directive with value "true", which
says that the client may respond to the challenge using the password
it used in the previous response; otherwise, the client must solicit
the password anew from the user. This permits the server to make sure
that the user has presented their password recently. (The directive
name refers to the previous nonce being stale, not to the last use of
the password.) Except for the handling of "stale", after sending the
"digest-challenge" authentication proceeds as in the case of initial
authentication.
2.3 Integrity Protection
If the server offered "qop=auth-int" and the client responded
"qop=auth-int", then subsequent messages, up to but not including the
next subsequent authentication, between the client and the server
MUST be integrity protected. Using as a base session key the value of
H(A1), as defined above the client and server calculate a pair of
message integrity keys as follows.
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The key for integrity protecting messages from client to server is:
Kic = MD5({H(A1),
"Digest session key to client-to-server signing key magic constant"})
The key for integrity protecting messages from server to client is:
Kis = MD5({H(A1),
"Digest session key to server-to-client signing key magic constant"})
where MD5 is as specified in [RFC 1321]. If message integrity is
negotiated, a MAC block for each message is appended to the message.
The MAC block is 16 bytes: the first 10 bytes of the HMAC-MD5 [RFC
2104] of the message, a 2-byte message type number in network byte
order with value 1, and the 4-byte sequence number in network byte
order. The message type is to allow for future extensions such as
rekeying.
MAC(Ki, SeqNum, msg) = (HMAC(Ki, {SeqNum, msg})[0..9], 0x0001,
SeqNum)
where Ki is Kic for messages sent by the client and Kis for those
sent by the server. The sequence number (SeqNum) is an unsigned
number initialized to zero after initial or subsequent
authentication, and incremented by one for each message
sent/successfully verified. (Note, that there are two independent
counters for sending and receiving.) The sequence number wraps around
to 0 after 2**32-1.
Upon receipt, MAC(Ki, SeqNum, msg) is computed and compared with the
received value; the message is discarded if they differ and as the
result the connection being used MUST be dropped. The receiver's
sequence counter is incremented if they match.
2.4 Confidentiality Protection
If the server sent a "cipher-opts" directive and the client responded
with a "cipher" directive, then subsequent messages between the
client and the server MUST be confidentiality protected. Using as a
base session key the value of H(A1) as defined above the client and
server calculate a pair of message integrity keys as follows.
The key for confidentiality protecting messages from client to server
is:
Kcc = MD5({H(A1)[0..n-1],
"Digest H(A1) to client-to-server sealing key magic constant"})
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The key for confidentiality protecting messages from server to client
is:
Kcs = MD5({H(A1)[0..n-1],
"Digest H(A1) to server-to-client sealing key magic constant"})
where MD5 is as specified in [RFC 1321]. For cipher "rc4-40" n is 5;
for "rc4-56" n is 7; for the rest n is 16. The key for the "rc4-*"
and "aes-ctr" ciphers is all 16 bytes of Kcc or Kcs.
"aes-ctr" cipher works as described in section 2.4.1.
rc4 cipher state MUST NOT be reset before sending/receiving a next
buffer of protected data.
If the blocksize of the chosen cipher is not 1 byte, the padding
prefix is one or more octets each containing the number of padding
bytes, such that the total length of the encrypted part of the
message is a multiple of the blocksize.
The MAC block is 16 bytes formatted as follows: the first 10 bytes of
the HMAC-MD5 [RFC 2104] of the message, a 2-byte message type number
in network byte order with value 1, and the 4-byte sequence number in
network byte order.
The padding and first 10 bytes of the MAC block are encrypted with
the chosen cipher along with the message.
SEAL(Ki, Kc, SeqNum, msg) = CIPHER(Kc, {msg, pad, MAC})
MAC(Ki, SeqNum, msg) = {HMAC(Ki, {SeqNum, msg})[0..9],
packet_type_data, SeqNum}
packet_type_data = 0x0001
where CIPHER is the chosen cipher, Ki and Kc are Kic and Kcc for
messages sent by the client and Kis and Kcs for those sent by the
server. The sequence number (SeqNum) is an unsigned number
initialized to zero after initial or subsequent authentication, and
incremented by one for each message sent/successfully verified.
(Note, that there are two independent counters for sending and
receiving.) The sequence number wraps around to 0 after 2**32-1.
Upon receipt, the message is decrypted, HMAC(Ki, {SeqNum, msg}) is
computed and compared with the received value; the padding and the
packet type are verified. The message is discarded if the received
and the calculated HMACs differ and/or the padding is invalid. See
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also section <<3.8>> for important information about MAC and padding
verification. The receiver's sequence counter is then compared with
the received SeqNum value; the message is discarded if they differ
and, as the result, the connection being used MUST be dropped. The
receiver's sequence counter is incremented if they match.
2.4.1 AES cipher in "stateful-decryption counter" mode ("aes-ctr")
In stateful-decryption counter mode, both the sender and the receiver
maintain an internal 128-bit counter CTRBLK.
The initial value of the CTRLBLK is calculated as follows:
The counter for the first SASL packet going from the client
to the server consists of 16 bytes calculated as follows:
CTRBLK = MD5({H(A1), "aes-128 counter client-to-server", nc-
value})
The counter for the first SASL packet going from the server
to the client consists of 16 bytes calculated as follows:
CTRBLK = MD5({H(A1), "aes-128 counter server-to-client", nc-
value})
<<Add qop-list (as sent by the server) and list of ciphers?
Add other options?>>
<<An alternative is to add a new option containing 128bit of random
data, which is sent with successful authentication and is used to
construct the initial counter.>>
For each buffer of cleartext data to be encrypted the sender performs
the following procedure:
1) padding and MAC block are constructed (see section 2.4) and
appended to the end of the plaintext. After this step the data
to be encrypted will look like:
{msg, pad, MAC}
As the total length of the data will be multiple of AES block size
(i.e. 128 bit), this can also be represented as
{P[1], P[2], P[3], ..., P[m]}
where P[i] is a chunk of data of the length 128 bit.
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2) Data is encrypted as follows:
FOR i := 1 to m DO
E[i] := P[i] XOR CIPHER ( Kc, CTRBLK )
CTRBLK := CTRBLK + 1
END
This will generate ciphertext {E[1], ..., E[m]} to be sent as a
single
SASL packet.
The initial CTRBLK value is constructed as described at the
beginning of
this section. The last CTRBLK value produced after encrypting P[m]
is
used to encrypt the first 128bit chunk of the next sent SASL
packet
(if any), end so on.
If CTRBLK = (2**128)-1, then "CTRBLK + 1" has the traditional
semantics of "set CTRBLK to 0."
The receiver performs the following steps:
1) Data is decrypted as follows:
FOR i := 1 to m DO
P[i] := E[i] XOR CIPHER ( Kc, CTRBLK )
CTRBLK := CTRBLK + 1
END
This will generate plaintext {P[1], ..., P[m]}, which is
{msg, pad, MAC}.
The initial CTRBLK value is constructed as described at the
beginning of
this section. The last CTRBLK value produced after decrypting P[m]
is used to decrypt the first 128bit chunk of the next received
SASL packet
(if any), end so on.
If CTRBLK = (2**128)-1, then "CTRBLK + 1" has the traditional
semantics of "set CTRBLK to 0."
2) pad and MAC block are verified as described in section 2.4.
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3 Security Considerations
General SASL security considerations apply to this mechanism.
"stringprep" and Unicode security considerations also apply.
Detailed discussion of other DIGEST-MD5 specific security issues is
below.
3.1 Authentication of Clients using Digest Authentication
Digest Authentication does not provide a strong authentication
mechanism, when compared to public key based mechanisms, for example.
However, since it prevents chosen plaintext attacks, it is stronger
than (e.g.) CRAM-MD5, which has been proposed for use with ACAP [RFC
2244], POP and IMAP [RFC 2195]. It is intended to replace the much
weaker and even more dangerous use of plaintext passwords; however,
since it is still a password based mechanism it avoids some of the
potential deployability issues with public-key, OTP or similar
mechanisms.
Digest Authentication offers no confidentiality protection beyond
protecting the actual password. All of the rest of the challenge and
response are available to an eavesdropper, including the user's name
and authentication realm.
3.2 Comparison of Digest with Plaintext Passwords
The greatest threat to the type of transactions for which these
protocols are used is network snooping. This kind of transaction
might involve, for example, online access to a mail service whose use
is restricted to paying subscribers. With plaintext password
authentication an eavesdropper can obtain the password of the user.
This not only permits him to access anything in the database, but,
often worse, will permit access to anything else the user protects
with the same password.
3.3 Replay Attacks
Replay attacks are defeated if the client or the server chooses a
fresh nonce for each authentication, as this specification requires.
As a security precaution, the server, when verifying a response from
the client, must use the original server nonce ("nonce") it sent, not
the one returned by the client in the response, as it might have been
modified by an attacker.
To prevent some redirection attacks it is recommended that the server
verifies that the "serv-type" part of the "digest-uri" matches the
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service name and that the hostname/IP address belongs to the server.
3.4 Online dictionary attacks
If the attacker can eavesdrop, then it can test any overheard
nonce/response pairs against a (potentially very large) list of
common words. Such a list is usually much smaller than the total
number of possible passwords. The cost of computing the response for
each password on the list is paid once for each challenge.
The server can mitigate this attack by not allowing users to select
passwords that are in a dictionary.
3.5 Offline dictionary attacks
If the attacker can choose the challenge, then it can precompute the
possible responses to that challenge for a list of common words. Such
a list is usually much smaller than the total number of possible
passwords. The cost of computing the response for each password on
the list is paid just once.
Offline dictionary attacks are defeated if the client chooses a fresh
nonce for each authentication, as this specification requires.
3.6 Man in the Middle
Digest authentication is vulnerable to "man in the middle" (MITM)
attacks. Clearly, a MITM would present all the problems of
eavesdropping. But it also offers some additional opportunities to
the attacker.
A possible man-in-the-middle attack would be to substitute a weaker
qop scheme for the one(s) sent by the server; the server will not be
able to detect this attack. For this reason, the client should always
use the strongest scheme that it understands from the choices
offered, and should never choose a scheme that does not meet its
minimum requirements.
A man-in-the-middle attack may also make the client and the server
that agreed to use confidentiality protection to use different (and
possibly weaker) cipher's. This is because the chosen cipher is not
used in the shared secret calculation.
3.7 Chosen plaintext attacks
A chosen plaintext attack is where a MITM or a malicious server can
arbitrarily choose the challenge that the client will use to compute
the response. The ability to choose the challenge is known to make
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cryptanalysis much easier [MD5].
However, Digest does not permit the attack to choose the challenge as
long as the client chooses a fresh nonce for each authentication, as
this specification requires.
3.8 <<CBC Mode attacks>>
<<The following attack can be launched when the connection uses
Confidentiality protection with ciphers in CBC mode. If bad padding
is treated differently from bad MACs when decrypting a DIGEST-MD5
buffer of protected data, the attacker may be able to launch
Vaudenay's [VAUDENAY] attack on padding.>>
An error logfile will suffice to launch the attack if it reveals the
type of error -- even if file permissions prevent the attacker from
actually reading the file (the file length increase cause by the
attack is likely to reveal which of the two errors occured).
<<Is the following still relevant for the counter mode?:>> A
different approach to distinguish these two error cases and launch
the attack is to examine the timing of error responses: if the MAC
verification is skipped when bad padding has been found, the error
will appear quicker in the case of incorrect block cipher padding
than in the case of an incorrect MAC.
A countermeasure is to compute a MAC of the plaintext anyway, even if
the usual padding removal step fails because of incorrect padding, to
obtain (nearly) uniform timing.
3.9 Spoofing by Counterfeit Servers
If a user can be led to believe that she is connecting to a host
containing information protected by a password she knows, when in
fact she is connecting to a hostile server, then the hostile server
can obtain challenge/response pairs where it was able to partly
choose the challenge. There is no known way that this can be
exploited.
3.10 Storing passwords
Digest authentication requires that the authenticating agent (usually
the server) store some data derived from the user's name and password
in a "password file" associated with a given realm. Normally this
might contain pairs consisting of username and H({ username-value,
":", realm-value, ":", password }), which is adequate to compute
H(A1) as described above without directly exposing the user's
password.
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The security implications of this are that if this password file is
compromised, then an attacker gains immediate access to documents on
the server using this realm. Unlike, say a standard UNIX password
file, this information need not be decrypted in order to access
documents in the server realm associated with this file. On the other
hand, decryption, or more likely a brute force attack, would be
necessary to obtain the user's password. This is the reason that the
realm is part of the digested data stored in the password file. It
means that if one Digest authentication password file is compromised,
it does not automatically compromise others with the same username
and password (though it does expose them to brute force attack).
There are two important security consequences of this. First the
password file must be protected as if it contained plaintext
passwords, because for the purpose of accessing documents in its
realm, it effectively does.
A second consequence of this is that the realm string should be
unique among all realms that any single user is likely to use. In
particular a realm string should include the name of the host doing
the authentication.
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3.11 Multiple realms
Use of multiple realms may mean both that compromise of a the
security database for a single realm does not compromise all
security, and that there are more things to protect in order to keep
the whole system secure.
3.11 Summary
By modern cryptographic standards Digest Authentication is weak,
compared to (say) public key based mechanisms. But for a large range
of purposes it is valuable as a replacement for plaintext passwords.
Its strength may vary depending on the implementation.
4 Example
This example shows the use of the Digest SASL mechanism with the
IMAP4 AUTHENTICATE command [RFC 3501].
In this example, "C:" and "S:" represent a line sent by the client or
server respectively including a CRLF at the end. Linebreaks and
indentation within a "C:" or "S:" are editorial and not part of the
protocol. The password in this example was "secret". Note that the
base64 encoding of the challenges and responses is part of the IMAP4
AUTHENTICATE command, not part of the Digest specification itself.
S: * OK elwood.innosoft.com PMDF IMAP4rev1 V6.0-9
C: c CAPABILITY
S: * CAPABILITY IMAP4 IMAP4rev1 ACL LITERAL+ NAMESPACE QUOTA
UIDPLUS AUTH=CRAM-MD5 AUTH=DIGEST-MD5 AUTH=PLAIN
S: c OK Completed
C: a AUTHENTICATE DIGEST-MD5
S: + cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJPQTZNRzl0
RVFHbTJoaCIscW9wPSJhdXRoIixhbGdvcml0aG09bWQ1LXNlc3MsY2hh
cnNldD11dGYtOA==
C: Y2hhcnNldD11dGYtOCx1c2VybmFtZT0iY2hyaXMiLHJlYWxtPSJlbHdvb2
QuaW5ub3NvZnQuY29tIixub25jZT0iT0E2TUc5dEVRR20yaGgiLG5jPTAw
MDAwMDAxLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLGRpZ2VzdC11cmk9Im
ltYXAvZWx3b29kLmlubm9zb2Z0LmNvbSIscmVzcG9uc2U9ZDM4OGRhZDkw
ZDRiYmQ3NjBhMTUyMzIxZjIxNDNhZjcscW9wPWF1dGg=
S: + cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZA==
C:
S: a OK User logged in
---
The base64-decoded version of the SASL exchange is:
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S: realm="elwood.innosoft.com",nonce="OA6MG9tEQGm2hh",qop="auth",
algorithm=md5-sess,charset=utf-8
C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
nonce="OA6MG9tEQGm2hh",nc=00000001,cnonce="OA6MHXh6VqTrRk",
digest-uri="imap/elwood.innosoft.com",
response=d388dad90d4bbd760a152321f2143af7,qop=auth
S: rspauth=ea40f60335c427b5527b84dbabcdfffd
The password in this example was "secret".
This example shows the use of the Digest SASL mechanism with the
ACAP, using the same notational conventions and password as in the
previous example. Note that ACAP does not base64 encode and uses
fewer round trips that IMAP4.
S: * ACAP (IMPLEMENTATION "Test ACAP server") (SASL "CRAM-MD5"
"DIGEST-MD5" "PLAIN")
C: a AUTHENTICATE "DIGEST-MD5"
S: + {94}
S: realm="elwood.innosoft.com",nonce="OA9BSXrbuRhWay",qop="auth",
algorithm=md5-sess,charset=utf-8
C: {206}
C: charset=utf-8,username="chris",realm="elwood.innosoft.com",
nonce="OA9BSXrbuRhWay",nc=00000001,cnonce="OA9BSuZWMSpW8m",
digest-uri="acap/elwood.innosoft.com",
response=6084c6db3fede7352c551284490fd0fc,qop=auth
S: a OK (SASL {40}
S: rspauth=2f0b3d7c3c2e486600ef710726aa2eae) "AUTHENTICATE
Completed"
---
The server uses the values of all the directives, plus knowledge of
the users password (or the hash of the user's name, server's realm
and the user's password) to verify the computations above. If they
check, then the user has authenticated.
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5 References
5.1 Normative references
[Digest] Franks, J., et al., "HTTP Authentication: Basic and Digest
Access Authentication", RFC 2617, June 1999.
[ISO-8859] ISO-8859. International Standard--Information Processing--
8-bit Single-Byte Coded Graphic Character Sets --
Part 1: Latin alphabet No. 1, ISO-8859-1:1987.
Part 2: Latin alphabet No. 2, ISO-8859-2, 1987.
Part 3: Latin alphabet No. 3, ISO-8859-3, 1988.
Part 4: Latin alphabet No. 4, ISO-8859-4, 1988.
Part 5: Latin/Cyrillic alphabet, ISO-8859-5, 1988.
Part 6: Latin/Arabic alphabet, ISO-8859-6, 1987.
Part 7: Latin/Greek alphabet, ISO-8859-7, 1987.
Part 8: Latin/Hebrew alphabet, ISO-8859-8, 1988.
Part 9: Latin alphabet No. 5, ISO-8859-9, 1990.
[RFC 1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC 2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[RFC 2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2222] Melnikov, A. (editor), "Simple Authentication and Security
Layer (SASL)", draft-ietf-sasl-rfc2222bis-xx.txt, a work
in progress.
[RFC 3454] Hoffman, P., Blanchet, M., "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
3.2.0", defined by: The Unicode Standard, Version 3.0
(Reading, MA, Addison-Wesley, 2000. ISBN 0-201-61633-5),
as amended by the Unicode Standard Annex #28: Unicode 3.2
(http://www.unicode.org/reports/tr28/tr28-3.html).
[UTF-8] Yergeau, "UTF-8, a transformation format of ISO 10646",
RFC 2279, Janyary 1998.
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[USASCII] US-ASCII. Coded Character Set - 7-Bit American Standard
Code for Information Interchange. Standard ANSI X3.4-1986,
ANSI, 1986.
[SASLPrep] Zeilenga, K., "SASLprep: Stringprep profile for user names
and passwords", RFC 4013, February 2005.
[RFC 3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", RFC 3986,
January 2005.
[AES] Daemen, J., Rijmen, V., "The Rijndael Block Cipher",
http://csrc.nist.gov/encryption/aes/rijndael/Rijndael.pdf,
3rd September 1999.
[RFC 2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
5.2 Informative references
[RFC 2195] Klensin, J., Catoe, R. and P. Krumviede, "IMAP/POP
AUTHorize Extension for Simple Challenge/Response", RFC
2195, September 1997.
[MD5] Kaliski, B.,Robshaw, M., "Message Authentication with
MD5", CryptoBytes, Sping 1995, RSA Inc,
(http://www.rsa.com/rsalabs/pubs/cryptobytes/spring95/md5.htm)
[ABNF] Crocker, D. (Ed.) and P. Overell , "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
[RFC 2078] Linn, J., "Generic Security Service Application Program
Interface, Version 2", RFC 2078, January 1997.
[RFC 3501] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, March 2003.
[RFC 2244] Newman, C., Myers, J., "ACAP -- Application Configuration
Access Protocol", RFC 2244, November 1997.
[RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
<<[CBCATT] Canvel, B., "Password Interception in a SSL/TLS
Channel", published 2003-02-20:
http://lasecwww.epfl.ch/memo_ssl.shtml>>
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[VAUDENAY] Serge Vaudenay, "Security Flaws Induced by CBC Padding -
Applications to SSL, IPSEC, WTLS ...". L.R. Knudsen (Ed.):
EUROCRYPT 2002, LNCS 2332, pp. 534-545, 2002.
[RFC 3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[IANA-SASL] IANA, "SIMPLE AUTHENTICATION AND SECURITY LAYER (SASL)
MECHANISMS", <http://www.iana.org/assignments/sasl-
mechanisms>.
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6 IANA Considerations
It is requested that the SASL Mechanism registry [IANA-SASL] entry
for the DIGEST-MD5 mechanism be updated to reflect that this document
now provides its technical specification.
To: iana@iana.org
Subject: Updated Registration of SASL mechanism DIGEST-MD5
Family of SASL mechanisms: NO
SASL mechanism name: DIGEST-MD5
Security considerations: See RFC XXXX.
Published specification (optional, recommended): RFC XXXX
Person & email address to contact for further information:
Alexey Melnikov <alexey.melnikov@isode.com>
IETF SASL WG <ietf-sasl@imc.org>
Intended usage: COMMON
Author/Change controller: IESG <iesg@ietf.org>
Note: Updates existing entry for DIGEST-MD5
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7 ABNF
<<What follows is the definition of the notation as is used in the
HTTP/1.1 specification [RFC 2616] and the HTTP authentication
specification [Digest]; it is reproduced here for ease of reference.
Since it is intended that a single Digest implementation can support
both HTTP and SASL-based protocols, the same notation is used in both
to facilitate comparison and prevention of unwanted differences.
Since it is cut-and-paste from the HTTP specifications, not all
productions may be used in this specification.>>
7.1 Augmented BNF
All of the mechanisms specified in this document are described in
both prose and an Augmented Backus-Naur Form (BNF) which is a
superset of the ABNF defined in [ABNF]. The Augmented BNF used by
this document defines the following extra syntactic rule:
#rule
A construct "#" is defined, similar to "*", for defining lists of
elements. The full form is "<n>#<m>element" indicating at least
<n> and at most <m> elements, each separated by one or more commas
(",") and OPTIONAL linear white space (LWS). This makes the usual
form of lists very easy; a rule such as
( *LWS element *( *LWS "," *LWS element ) *LWS )
can be shown as
1#element
Wherever this construct is used, null elements are allowed, but do
not contribute to the count of elements present. That is,
"(element), , (element) " is permitted, but counts as only two
elements. Therefore, where at least one element is required, at
least one non-null element MUST be present. Default values are 0
and infinity so that "#element" allows any number, including zero;
"1#element" requires at least one; and "1#2element" allows one or
two.
Other differences from [ABNF]:
<<"literal"
Quotation marks surround literal text. Unless stated otherwise,
the text is case-insensitive.
- Seem to be the same as in 4234. There every literal string is
case-insensitive.>>
implied *LWS
The grammar described by this specification is word-based. Except
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where noted otherwise, linear white space (LWS) can be included
between any two adjacent words (token or quoted-string), and
between adjacent words and separators, without changing the
interpretation of a field. At least one delimiter (LWS and/or
separators) MUST exist between any two tokens (for the definition
of "token" below), since they would otherwise be interpreted as a
single token.
7.2 Basic Rules
The following rules are used throughout this specification to
describe basic parsing constructs. The US-ASCII coded character set
is defined by ANSI X3.4-1986 [USASCII]. Non-terminals not defined in
this document can be found in [ABNF].
TEXTCHAR = <any OCTET except CTLs, but including HTAB>
All linear white space, including folding, has the same semantics as
SP. A recipient MAY replace any linear white space with a single SP
before interpreting the field value or forwarding the message
downstream.
LWS = [CRLF] 1*WSP
<<Replace with:
LWSP = *(WSP / CRLF WSP)
? Our LWS is more restrictive, as only single CRLF is allowed. But
it doesn't seem that this was the intent.>>
The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words
of TEXT contains characters either from ISO-8859-1 [ISO-8859]
character set or UTF-8 [UTF-8].
TEXT = <any *OCTET except CTLs,
but including LWS>
A CRLF is allowed in the definition of TEXT only as part of a header
field continuation. It is expected that the folding LWS will be
replaced with a single SP before interpretation of the TEXT value.
Many HTTP/1.1 header field values consist of words separated by LWS
or special characters. These special characters MUST be in a quoted
string to be used within a parameter value.
token = 1*TOKENCHAR
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BACKSLASH = %x5C
; character
separators = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | BACKSLASH | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HTAB
TOKENCHAR = <any CHAR except CTLs or separators>
A string of text is parsed as a single word if it is quoted using
double-quote marks.
quoted-string = DQUOTE qdstr-val DQUOTE
qdstr-val = *( qdtext | quoted-pair )
qdtext = <any TEXTCHAR except DQUOTE and BACKSLASH>
Note that LWS is NOT implicit between the double-quote marks (DQUOTE)
surrounding a qdstr-val and the qdstr-val; any LWS will be considered
part of the qdstr-val. This is also the case for quotation marks
surrounding any other construct.
The backslash character (BACKSLASH) MAY be used as a single-character
quoting mechanism only within qdstr-val and comment constructs.
quoted-pair = BACKSLASH CHAR
The value of this construct is CHAR. Note that an effect of this rule
is that backslash itself MUST be quoted.
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8 Sample Code
The sample implementation in [Digest] also applies to DIGEST-MD5.
The following code implements the conversion from UTF-8 to 8859-1 if
necessary.
/* if the string is entirely in the 8859-1 subset of UTF-8, then
* translate to 8859-1 prior to MD5
*/
void MD5_UTF8_8859_1(MD5_CTX *ctx, const unsigned char *base,
int len)
{
const unsigned char *scan, *end;
unsigned char cbuf;
end = base + len;
for (scan = base; scan < end; ++scan) {
if (*scan > 0xC3) break; /* abort if outside 8859-1 */
if (*scan >= 0xC0 && *scan <= 0xC3) {
if (++scan == end || *scan < 0x80 || *scan > 0xBF)
break;
}
}
/* if we found a character outside 8859-1, don't alter string
*/
if (scan < end) {
MD5Update(ctx, base, len);
return;
}
/* convert to 8859-1 prior to applying hash
*/
do {
for (scan = base; scan < end && *scan < 0xC0; ++scan)
;
if (scan != base) MD5Update(ctx, base, scan - base);
if (scan + 1 >= end) break;
cbuf = ((scan[0] & 0x3) << 6) | (scan[1] & 0x3f);
MD5Update(ctx, &cbuf, 1);
base = scan + 2;
} while (base < end);
}
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9 Authors' Addresses
Paul Leach
Microsoft
1 Microsoft Way
Redmond, WA 98052, USA
EMail: paulle@microsoft.com
Chris Newman
Sun Microsystems
1050 Lakes Drive
West Covina, CA 91790, USA
EMail: Chris.Newman@Sun.COM
Alexey Melnikov
Isode Ltd.
5 Castle Business Village,
36 Station Road,
Hampton,
Middlesex,
TW12 2BX,
United Kingdom
Email: Alexey.Melnikov@isode.com
10 Acknowledgements
The following people had substantial contributions to the development
and/or refinement of this document:
Lawrence Greenfield
John Gardiner Myers
Simon Josefsson
RL Bob Morgan
Jeff Hodges
Claus Assmann
Tony Hansen
Ken Murchison
Sam Hartman
Kurt D. Zeilenga
Hallvard B. Furuseth
Abhijit Menon-Sen
Nicolas Williams
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Tom Yu
as well as other members of the SASL mailing list.
<<The text used is section 3.8 was taken from <<[TLS-CBC]>> by Bodo
Moeller.>>
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11 Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
12 Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
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Appendix A: Changes from 2831
1). Fixed various typos in formulas.
2). Dropped DES as mandatory to implement cipher (rc4 is mandatory to
implement). Removed "des" and "3des" ciphers because of known
interoperability problems and vulnerability to CBC mode attack.
3). Tighten ABNF. Fixed some bugs.
4). Clarified nc-value verification and which side is aborting
exchange.
5). Added text saying that for interoperability
username/password/realm SHOULD be prepared using the "SASLPrep"
profile [SASLPrep] of the "stringprep" algorithm [RFC 3454].
6). Clarified that unquoted version of the username, etc. used in A1
calculation.
7). Various cleanup to References section. Split all references to
Normative and Informative.
8). Added minimal and maximal limits on maxbuf. Clarified how to
calculate "maximal sender size".
9). Change ABNF for host to allow for IPv6 addresses. ABNF now
references RFC 3986.
10). Added man-in-the-middle considerations for ciphers.
11). Clarified how sequence counters are updated.
12). Addition warnings about preventing reply/redirection attacks.
13). Specified that "charset" directive affects "realm" and doesn't
affect "authzid".
14). Removed text that described that "authzid" is in Unicode in
Normalization Form KC, encoded as UTF-8.
15). Clarified that rc4 state is not reset between two consecutive
sent/received buffers of protected data.
16). Allow for extensibility in step 3. Use "auth-info" as in RFC
2617.
17). Prohibit an empty authzid, as this caused interoperability
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problems.
18). Added AES cipher defined in "AES Ciphersuite for DIGEST-MD5 SASL
mechanism" document (expired draft-ietf-sasl-digest-aes-00.txt).
Use aes cipher in CTR mode.
18). Cleaned up Confidentiality protection section. Added step by
step exlanation how CBC mode is used.
19). Clarified client behavior, if it recognizes no ciphers.
20). Clarified that the server is not required to advertise all
realms it supports.
21). Clarified how UIs should present realms.
22). Changed some informative text to normative MUST/SHOULDs.
23). Changed nonce/cnonce to allow for channel bindings.
24). Replace RFC 822 ABNF with [ABNF].
(in progress)
And other minor text clarifications.
Appendix B: Open Issues/ToDo List
1). Normative vs. Informative references must be carefully rechecked.
2). Replace ABNF with the reference to RFC 4234?
3). Resolve ISO-8859-1 and SASLPrep interaction issue as reported by
Simon Josefsson.
4). Can we drop ISO-8859-1 stuff in favor of UTF-8? This is what
people have suggested for HTTP Digest.
5). "SHOULD SASLprep" / "MUST fail authentication exchange" language
should be checked for consistency. Also, should SASLPrep be
applied BEFORE or AFTER checking for ISO-8859-1 subset? (I think
it should be applied before)
<<From RFC 2617: auth-param
This directive allows for future extensions. Any unrecognized
directive MUST be ignored.>>
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