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Digest SASL Mechanism September, 1999
Network Working Group Paul J. Leach, Microsoft
INTERNET-DRAFT Chris Newman, Innosoft
draft-leach-digest-sasl-04.txt
Category: Standards Track
Expires March 27, 2000 September 27, 1999
Using Digest Authentication as a SASL Mechanism
Author's draft: 15
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Distribution of this document is unlimited. Please send comments to the
authors or the SASL mailing list, ietf-sasl@imc.org.
Copyright Notice: Copyright (C) The Internet Society (1998). All Rights
Reserved. See section 8 for the full copyright notice.
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 [RFC2195]
and as a convenient way to support a single authentication mechanism for
web, mail, LDAP, and other protocols.
Leach, Newman Standards Track [Page 1]
Digest SASL Mechanism September, 1999
Table of Contents
1 INTRODUCTION........................................................3
1.1 CONVENTIONS AND NOTATION..........................................3
1.2 REQUIREMENTS......................................................4
2 AUTHENTICATION......................................................4
2.1 INITIAL AUTHENTICATION............................................4
2.1.1 Step One......................................................4
2.1.2 Step Two......................................................7
2.1.3 Step Three...................................................12
2.2 SUBSEQUENT AUTHENTICATION........................................12
2.2.1 Step one.....................................................13
2.2.2 Step Two.....................................................13
2.3 INTEGRITY PROTECTION.............................................13
2.4 CONFIDENTIALITY PROTECTION.......................................14
3 SECURITY CONSIDERATIONS............................................15
3.1 AUTHENTICATION OF CLIENTS USING DIGEST AUTHENTICATION............15
3.2 COMPARISON OF DIGEST WITH PLAINTEXT PASSWORDS....................16
3.3 REPLAY ATTACKS...................................................16
3.4 ONLINE DICTIONARY ATTACKS........................................16
3.5 OFFLINE DICTIONARY ATTACKS.......................................16
3.6 MAN IN THE MIDDLE................................................16
3.7 CHOSEN PLAINTEXT ATTACKS.........................................17
3.8 SPOOFING BY COUNTERFEIT SERVERS..................................17
3.9 STORING PASSWORDS................................................17
3.10 MULTIPLE REALMS................................................18
Leach, Newman Standards Track [Page 2]
Digest SASL Mechanism September, 1999
3.11 SUMMARY........................................................18
4 EXAMPLE............................................................18
5 REFERENCES.........................................................19
6 AUTHORS' ADDRESSES.................................................20
7 ABNF...............................................................21
7.1 AUGMENTED BNF....................................................21
7.2 BASIC RULES......................................................22
8 SAMPLE CODE........................................................24
9 FULL COPYRIGHT STATEMENT...........................................25
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
This specification uses the same ABNF notation and lexical conventions
as HTTP/1.1 specification; see appendix A.
Let { a, b, ... } be the concatenation of the octet strings a, b, ...
Let H(s) be the 16 octet MD5 hash [RFC 1321] of the octet string s.
Leach, Newman Standards Track [Page 3]
Digest SASL Mechanism September, 1999
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.
The value of a quoted string constant as an octet string does not
include any terminating null character.
1.2 Requirements
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].
An implementation is not compliant if it fails to satisfy one or more of
the MUST level requirements for the protocols it implements. An
implementation that satisfies all the MUST level and all the SHOULD
level requirements for its protocols is said to be "unconditionally
compliant"; one that satisfies all the MUST level requirements but not
all the SHOULD level requirements for its protocols is said to be
"conditionally compliant."
2 Authentication
The following sections describe how to use Digest as a SASL
authentication mechanism.
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.
2.1.1Step One
The server starts by sending a challenge. The data encoded in the
challenge contains a string formatted according to the rules for a
"digest-challenge" defined as follows:
digest-challenge =
1#( realm | nonce | qop-options | stale | maxbuf | charset
algorithm | cipher-opts | auth-param )
Leach, Newman Standards Track [Page 4]
Digest SASL Mechanism September, 1999
realm = "realm" "=" <"> realm-value <">
realm-value = qdstr-val
nonce = "nonce" "=" <"> nonce-value <">
nonce-value = qdstr-val
qop-options = "qop" "=" <"> qop-list <">
qop-list = 1#qop-value
qop-value = "auth" | "auth-int" | "auth-conf" |
token
stale = "stale" "=" "true"
maxbuf = "maxbuf" "=" maxbuf-value
maxbuf-value = 1*DIGIT
charset = "charset" "=" "utf-8"
algorithm = "algorithm" "=" "md5-sess"
cipher-opts = "cipher" "=" <"> 1#cipher-value <">
cipher-value = "3des" | "des" | "rc4-40" | "rc4" |
"rc4-56" | token
auth-param = token "=" ( token | quoted-string )
The meanings of the values of the directives used above are as follows:
realm
Mechanistically, a string which can enable users to know which
username and password to use, in case they might 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 at least 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". 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 to username
and password.
nonce
A server-specified data string which MUST be different each time a
digest-challenge is sent as part of initial authentication. It is
recommended that this string be base64 or hexadecimal data. Note that
since the string is passed as a quoted string, the double-quote
character is not allowed. The contents of the nonce are
implementation dependent. The security of the implementation depends
on a good choice. It is RECOMMENDED that it contain at least 64 bits
of entropy. The nonce is opaque to the client. 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.
Leach, Newman Standards Track [Page 5]
Digest SASL Mechanism September, 1999
qop-options
A quoted string of one or more 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 should abort the authentication exchange.
stale
The "stale" directive is not used in initial authentication. See the
next section for its use in subsequent authentications.
maxbuf
A number indicating the size of the largest buffer the server 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 client should abort
the authentication exchange.
charset
This directive, if present, specifies that the server supports UTF-8
encoding for the username and password. If not present, the username
and password must be encoded in ISO 8859-1 (of which US-ASCII 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
should abort the authentication exchange.
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 "3des" and "des" modes are mandatory-to-
implement. The client MUST ignore unrecognized options; if the client
recognizes no option, it should abort the authentication exchange.
des
the Data Encryption Standard (DES) cipher [FIPS] in cipher block
chaining (CBC) mode with a 56 bit key.
Leach, Newman Standards Track [Page 6]
Digest SASL Mechanism September, 1999
3des
the "triple DES" cipher in CBC mode with EDE with the same key for
each E stage (aka "two keys mode") for a total key length of 112
bits.
rc4, rc4-40, rc4-56
the RC4 cipher with a 128 bit, 40 bit, and 56 bit key,
respectively.
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 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.
2.1.2Step Two
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 |
maxbuf | charset | cipher | authzid |
auth-param )
username = "username" "=" <"> username-value <">
username-value = qdstr-val
cnonce = "cnonce" "=" <"> cnonce-value <">
cnonce-value = qdstr-val
nonce-count = "nc" "=" nc-value
nc-value = 8LHEX
qop = "qop" "=" qop-value
digest-uri = "digest-uri" "=" digest-uri-value
digest-uri-value = serv-type "/" host [ "/" serv-name ]
serv-type = 1*ALPHA
host = 1*( ALPHA | DIGIT | "-" | "." )
serv-name = host
response = "response" "=" <"> response-value <">
response-value = 32LHEX
LHEX = "0" | "1" | "2" | "3" |
Leach, Newman Standards Track [Page 7]
Digest SASL Mechanism September, 1999
"4" | "5" | "6" | "7" |
"8" | "9" | "a" | "b" |
"c" | "d" | "e" | "f"
cipher = "cipher" "=" cipher-value
authzid = "authzid" "=" authzid-value
authzid-value = qdstr-val
username
The user's name in the specified realm, encoded as UTF-8. This
directive is required and MUST be present exactly once; otherwise,
authentication fails.
realm
The realm containing the user's account. This directive is required
if the server provided any realms in the "digest-challenge", in which
case it may appear exactly once and its value SHOULD be one of those
realms. If the directive is missing, "realm-value" will set to the
empty string when computing A1 (see below for details).
nonce
The server-specified data string received in the preceding digest-
challenge. This directive is required and MUST be present exactly
once; otherwise, authentication fails.
cnonce
A client-specified data string which MUST be different each time a
digest-response is sent as part of initial authentication. The
cnonce-value is an opaque quoted string value provided by the client
and used by both client and server to avoid chosen plaintext attacks,
and to provide mutual authentication. The security of the
implementation depends on a good choice. It is RECOMMENDED that it
contain at least 64 bits of entropy. This directive is required and
MUST be present exactly once; otherwise, authentication fails.
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.
qop
Indicates what "quality of protection" the client accepted. If
Leach, Newman Standards Track [Page 8]
Digest SASL Mechanism September, 1999
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 "www" 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]. Regardless of
case, they are lower cased when used in hash computations.
host
The DNS host name or IP 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, 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. It is also so
that clients will be trained to provide values that will work with
implementations that use a shared back-end authentication service
that can provide server authentication.
Leach, Newman Standards Track [Page 9]
Digest SASL Mechanism September, 1999
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.
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.
maxbuf
A number indicating the size of the largest buffer the client is able
to receive. If this directive is missing, the default value is 65536.
This directive may appear at most once; if multiple instances are
present, the server should abort the authentication exchange.
charset
This directive, if present, specifies that the client has used UTF-8
encoding for the username and password. If not present, the username
and password must be encoded in ISO 8859-1 (of which US-ASCII is a
subset). The client should send this directive only if the server has
indicated it supports UTF-8. The directive is needed for backwards
compatibility with HTTP Digest, which only supports ISO 8859-1.
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.
authzid
The "authorization ID" as per RFC 2222, encoded in UTF-8. This
directive is optional. 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 will be
incorrect, and authentication will fail.
Leach, Newman Standards Track [Page 10]
Digest SASL Mechanism September, 1999
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 show
how the value is computed.
response-value =
HEX( KD ( HEX(H(A1)),
{ nonce-value, ":" nc-value, ":",
cnonce-value, ":", qop-value, ":", HEX(H(A2))
}))
If authzid is specified, then A1 is
A1 = { H( { username-value, ":", realm-value, ":", passwd } ),
":", nonce-value, ":", cnonce-value, ":", authzid-value }
If authzid is not specified, then A1 is
A1 = { H( { username-value, ":", realm-value, ":", passwd } ),
":", nonce-value, ":", cnonce-value }
where
passwd = *OCTET
The "username-value", "realm-value" and "passwd" are encoded according
to the value of the "charset" directive. If "charset=UTF-8" is present,
and all the characters of either "username-value" or "passwd" are in the
ISO 8859-1 character set, then it must be converted to ISO 8859-1 before
being hashed. 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.
If the "qop" directive's value is "auth", then A2 is:
A2 = { "AUTHENTICATE:", digest-uri-value }
If the "qop" value is "auth-int" or "auth-conf" then A2 is:
A2 = { "AUTHENTICATE:", digest-uri-value,
":00000000000000000000000000000000" }
Leach, Newman Standards Track [Page 11]
Digest SASL Mechanism September, 1999
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).
2.1.3Step Three
The server receives and validates the "digest-response". The server
checks that the nonce-count is "00000001". If it supports subsequent
authentication (see section 2.2), it saves the value of the nonce and
the nonce-count. It sends a message formatted as follows:
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, ":00000000000000000000000000000000"
}
Compared to its use in HTTP, the following Digest directives in the
"digest-response" are unused:
nextnonce
qop
cnonce
nonce-count
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
Leach, Newman Standards Track [Page 12]
Digest SASL Mechanism September, 1999
permits an initial client response, then it MAY perform "subsequent
authentication", as defined in this section.
2.2.1Step 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 above, but with a nonce-count
one greater than used in the last "digest-response".
2.2.2Step 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 the nonce-count is one greater than that used in the
previous authentication using that nonce, and saves the new value of
nonce-count.
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.
Leach, Newman Standards Track [Page 13]
Digest SASL Mechanism September, 1999
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 is initialized to zero, and
incremented by one for each message sent.
Upon receipt, MAC(Ki, SeqNum, msg) is computed and compared with the
received value; the message is discarded if they differ.
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],
"Digest H(A1) to client-to-server sealing key magic constant"})
The key for confidentiality protecting messages from server to client
is:
Kcs = MD5({H(A1)[0..n],
"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 "rc-*" ciphers is
all 16 bytes of Kcc or Kcs; the key for "des" is the first 7 bytes; the
Leach, Newman Standards Track [Page 14]
Digest SASL Mechanism September, 1999
key for "3des" is the first 14 bytes. The IV for "des" and "3des" is the
last 8 bytes of Kcc or Kcs.
If message confidentiality is negotiated, each message is encrypted with
the chosen cipher and a MAC block is appended to the message.
The MAC block is a variable length padding prefix followed by 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. 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
total length of the encrypted part of the message is a multiple of the
blocksize. The padding and first 10 bytes of the MAC block are encrypted
along with the message.
SEAL(Ki, Kc, SeqNum, msg) =
{CIPHER(Kc, {msg, pad, HMAC(Ki, {SeqNum, msg})[0..9])}), 0x0001,
SeqNum}
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 is initialized to zero, and incremented by
one for each message sent.
Upon receipt, the message is decrypted, HMAC(Ki, {SeqNum, msg}) is
computed and compared with the received value; the message is discarded
if they differ.
3 Security Considerations
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 LDAP [10], POP and
IMAP (see RFC 2195 [9]). 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
deployabilty 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.
Leach, Newman Standards Track [Page 15]
Digest SASL Mechanism September, 1999
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.
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
Leach, Newman Standards Track [Page 16]
Digest SASL Mechanism September, 1999
should never choose a scheme that does not meet its minimum
requirements.
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
cryptanalysis much easier [8].
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 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.
3.9 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, ":", passwd }), which is adequate to compute H(A1) as described
above without directly exposing the user's password.
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
Leach, Newman Standards Track [Page 17]
Digest SASL Mechanism September, 1999
realm string should include the name of the host doing the
authentication.
3.10 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 2060]. The base64 encoding of the challenges
and responses is part of the IMAP4 AUTHENTICATE command, not part of the
Digest specification itself. (Note: linebreaks added for editorial
clarity are not part of the mechanism):
Leach, Newman Standards Track [Page 18]
Digest SASL Mechanism September, 1999
* OK elwood.innosoft.com IMAP4 Server PMDF5.3-1 at Mon, 28 Sep 1998
09:16:30 -0700 (PDT)
c CAPABILITY
* CAPABILITY IMAP4 IMAP4REV1 NAMESPACE STARTTLS AUTH=CRAM-MD5
AUTH=DIGEST-MD5 AUTH=LOGIN AUTH=PLAIN
c OK CAPABILITY completed
a AUTHENTICATE DIGEST-MD5
+ cmVhbG09ImVsd29vZC5pbm5vc29mdC5jb20iLG5vbmNlPSJENlBpNXVvT2xp
RzI4WFZidVRYQ0l3Iixxb3A9ImF1dGgi
dXNlcm5hbWU9ImNocmlzIixyZWFsbT0iZWx3b29kLmlubm9zb2Z0LmNvbSIsbm
9uY2U9IkQ2UGk1dW9PbGlHMjhYVmJ1VFhDSXciLG5jPTAwMDAwMDAxLGNub25j
ZT0iZS9nWG5wRW94ODNzVzNERXU3b1FoZyIscmVzcG9uc2U9IjRmNjA2NTBhYW
FmNDQxNzkyOWViNjg3Zjc2NmNlOTMyIixxb3A9ImF1dGgi
a OK AUTHENTICATE completed
---
Decoding the base64, gets:
realm="elwood.innosoft.com",nonce="D6Pi5uoOliG28XVbuTXCIw",qop="auth
"
and
username="chris",realm="elwood.innosoft.com",nonce="D6Pi5uoOliG28XVb
uTXCIw",
nc=00000001,cnonce="e/gXnpEox83sW3DEu7oQhg",
response="4f60650aaaf4417929eb687f766ce932",qop=auth
The password was "secret".
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.
5 References
[Digest] Franks, J., et. al., "HTTP Authentication: Basic and Digest
Access Authentication", <draft-ietf-http-authentication-03>, Work in
Progress of the HTTP Working Group, August, 1998
[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.
Leach, Newman Standards Track [Page 19]
Digest SASL Mechanism September, 1999
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 822] D. H. Crocker, "Standard for The Format of ARPA Internet Text
Messages," STD 11, RFC 822, UDEL, August 1982.
[RFC 1321] R. Rivest, "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992
[RFC 2047] Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part
Three: Message Header Extensions for Non-ASCII Text", RFC 2047,
University of Tennessee, November 1996.
[RFC 2052] A. Gulbrandsen, P. Vixie, A DNS RR for specifying the
location of services (DNS SRV). October 1996.
[RFC 2060] Crispin, "Internet Message Access Protocol - Version 4rev1",
RFC 2060, University of Washington, December 1996.
[RFC 2104] H. Krawczyk, M. Bellare, R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC 2104, 02/05/1997
[RFC2195] Klensin, J., et. al., "IMAP/POP AUTHorize Extension for Simple
Challenge/Response", RFC 2195, September, 1997.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119, Harvard University, March 1997.
[USASCII] US-ASCII. Coded Character Set - 7-Bit American Standard Code
for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.
6 Authors' Addresses
Paul Leach
Microsoft
1 Microsoft Way
Redmond, WA 98052
paulle@microsoft.com
Chris Newman
Innosoft International, Inc.
1050 Lakes Drive
West Covina, CA 91790 USA
chris.newman@innosoft.com
Leach, Newman Standards Track [Page 20]
Digest SASL Mechanism September, 1999
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 (RFC 2617); 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. It is also not quite legal ABNF; again,
the errors were copied from the HTTP specifications.
7.1 Augmented BNF
All of the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur Form (BNF) similar to that used by
RFC 822 [RFC 822]. Implementers will need to be familiar with the
notation in order to understand this specification.
The augmented BNF includes the following constructs:
name = definition
The name of a rule is simply the name itself (without any enclosing
"<" and ">") and is separated from its definition by the equal "="
character. White space is only significant in that indentation of
continuation lines is used to indicate a rule definition that spans
more than one line. Certain basic rules are in uppercase, such as SP,
LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within
definitions whenever their presence will facilitate discerning the
use of rule names.
"literal"
Quotation marks surround literal text. Unless stated otherwise, the
text is case-insensitive.
rule1 | rule2
Elements separated by a bar ("|") are alternatives, e.g., "yes | no"
will accept yes or no.
(rule1 rule2)
Elements enclosed in parentheses are treated as a single element.
Thus, "(elem (foo | bar) elem)" allows the token sequences
"elem foo elem" and "elem bar elem".
*rule
The character "*" preceding an element indicates repetition. The full
form is "<n>*<m>element" indicating at least <n> and at most <m>
occurrences of element. Default values are 0 and infinity so that
Leach, Newman Standards Track [Page 21]
Digest SASL Mechanism September, 1999
"*(element)" allows any number, including zero; "1*element" requires
at least one; and "1*2element" allows one or two.
[rule]
Square brackets enclose optional elements; "[foo bar]" is equivalent
to "*1(foo bar)".
N rule
Specific repetition: "<n>(element)" is equivalent to
"<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
alphabetic characters.
#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 ))
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.
; comment
A semi-colon, set off some distance to the right of rule text, starts
a comment that continues to the end of line. This is a simple way of
including useful notes in parallel with the specifications.
implied *LWS
Except where noted otherwise, linear white space ("LWS") can be
included between any adjacent "token", "quoted-string", or
"separators" constructs, as these are defined in the basic rules
below; such LWS is ignored.
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].
OCTET = <any 8-bit sequence of data>
Leach, Newman Standards Track [Page 22]
Digest SASL Mechanism September, 1999
CHAR = <any US-ASCII character (octets 0 - 127)>
UPALPHA = <any US-ASCII uppercase letter "A".."Z">
LOALPHA = <any US-ASCII lowercase letter "a".."z">
ALPHA = UPALPHA | LOALPHA
DIGIT = <any US-ASCII digit "0".."9">
CTL = <any US-ASCII control character
(octets 0 - 31) and DEL (127)>
CR = <US-ASCII CR, carriage return (13)>
LF = <US-ASCII LF, linefeed (10)>
SP = <US-ASCII SP, space (32)>
HT = <US-ASCII HT, horizontal-tab (9)>
<"> = <US-ASCII double-quote mark (34)>
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*( SP | HT )
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 MAY contain characters from character sets other than ISO-8859-1
[ISO 8859]only when encoded according to the rules of RFC 2047 [RFC
2047].
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.
Hexadecimal numeric characters are used in several protocol elements.
HEX = "A" | "B" | "C" | "D" | "E" | "F"
| "a" | "b" | "c" | "d" | "e" | "f" | DIGIT
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*<any CHAR except CTLs or separators>
separators = "(" | ")" | "<" | ">" | "@"
| "," | ";" | ":" | "\" | <">
| "/" | "[" | "]" | "?" | "="
| "{" | "}" | SP | HT
A string of text is parsed as a single word if it is quoted using
double-quote marks.
Leach, Newman Standards Track [Page 23]
Digest SASL Mechanism September, 1999
quoted-string = ( <"> qdstr-val <"> )
qdstr-val = *( qdtext | quoted-pair )
qdtext = <any TEXT except <">>
The backslash character ("\") MAY be used as a single-character quoting
mechanism only within qdstr-val and comment constructs.
quoted-pair = "\" CHAR
The value of this construct is CHAR. Note that an effect of this rule is
that backslash must be quoted.
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.
Leach, Newman Standards Track [Page 24]
Digest SASL Mechanism September, 1999
/* 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);
}
9 Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works. However, this document itself
may not be modified in any way, such as by removing the copyright notice
or references to the Internet Society or other Internet organizations,
Leach, Newman Standards Track [Page 25]
Digest SASL Mechanism September, 1999
except as needed for the purpose of developing Internet standards in
which case the procedures for copyrights defined in the Internet
Standards process must be followed, or as required to translate it into
languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS 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.
Leach, Newman Standards Track [Page 26]
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