                         Digest SASL Mechanism          September, 1999 




Network Working Group               Paul J. Leach, Microsoft 
INTERNET-DRAFT                        Chris Newman, Innosoft 
draft-leach-digest-sasl-05.txt 
Category: Standards Track 
Expires April 21, 2000                      October 21, 1999 
                                     


            Using Digest Authentication as a SASL Mechanism 

                           Author's draft: 16 

 


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. 




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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........................................13 

  2.2.1 Step one.....................................................13 

  2.2.2 Step Two.....................................................13 

 2.3 INTEGRITY PROTECTION.............................................14 

 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................................................17 

 3.7 CHOSEN PLAINTEXT ATTACKS.........................................17 

 3.8 SPOOFING BY COUNTERFEIT SERVERS..................................17 

 3.9 STORING PASSWORDS................................................17 

 3.10 MULTIPLE REALMS................................................18 




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 3.11 SUMMARY........................................................18 

4 EXAMPLE............................................................18 

5 REFERENCES.........................................................20 

6 AUTHORS' ADDRESSES.................................................21 

7 ABNF...............................................................21 
 7.1 AUGMENTED BNF....................................................22 

 7.2 BASIC RULES......................................................23 

8 SAMPLE CODE........................................................25 

9 FULL COPYRIGHT STATEMENT...........................................26 
 


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. 



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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 ) 
 



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     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 unless escaped (see section 7.2). 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.  



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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. This 
  directive may appear at most once; if multiple instances are present, 
  the client should abort the authentication exchange. 

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. 






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  des 
     the Data Encryption Standard (DES) cipher [FIPS] in cipher block 
     chaining (CBC) mode with a 56 bit key. 

  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 | "-" | "." ) 



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    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    = qdstr-val 
 
 

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. 

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 



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  description below of the construction of the response value. This 
  directive may appear at most once; if multiple instances are present, 
  the client should abort the authentication exchange. 

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 "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". 




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   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. 

  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 may appear at most once; if multiple instances are 
  present, the client should 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. 

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. 



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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. 

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: 




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   A2       = { "AUTHENTICATE:", digest-uri-value } 
 
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). 

2.1.3Step 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: 









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    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 
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 




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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. 

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], 



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"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 
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 



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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. 

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. 






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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. 

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 



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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.  

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]. 

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. 













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    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: + cnNwYXV0aD00YjJiYjM3ZjA0OTEwNTA1Nzc3YzJmNjM4YzkyMjcyNQ== 
    C: 
    S: a OK User logged in 
    --- 
     
    The base64-decoded version of the SASL exchange is: 
     
    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=4b2bb37f04910505777c2f638c922725 
     
    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. 


















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    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=d84489141f9d86605c6a77b95cb5365a) "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. 


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. 
  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 






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[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 


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 




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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 
  "*(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 



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  "<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  
   The grammar described by this specification is word-based. Except 
  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]. 

       OCTET          = <any 8-bit sequence of data> 
       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"> 



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       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)> 
       CRLF           = CR LF 
 
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. 

      quoted-string  = ( <"> qdstr-val <"> ) 
      qdstr-val      = *( qdtext | quoted-pair ) 
      qdtext         = <any TEXT except <">> 
 



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Note that LWS is NOT implicit between the double-quote marks (<">) 
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 ("\") 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. 

































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    /* 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 implementation 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, 



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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.  

 





































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