Network Working Group                                           J. Myers
Request for Comments: 1731                               Carnegie Mellon
Category: Standards Track                                  December 1994


                   IMAP4 Authentication Mechanisms

Status of this Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.


1. Introduction

  The Internet Message Access Protocol, Version 4 [IMAP4] contains the
  AUTHENTICATE command, for identifying and authenticating a user to an
  IMAP4 server and for optionally negotiating a protection mechanism
  for subsequent protocol interactions.  This document describes
  several authentication mechanisms for use by the IMAP4 AUTHENTICATE
  command.


2. Kerberos version 4 authentication mechanism

  The authentication type associated with Kerberos version 4 is
  "KERBEROS_V4".

  The data encoded in the first ready response contains a random 32-bit
  number in network byte order.  The client should respond with a
  Kerberos ticket and an authenticator for the principal
  "imap.hostname@realm", where "hostname" is the first component of the
  host name of the server with all letters in lower case and where
  "realm" is the Kerberos realm of the server.  The encrypted checksum
  field included within the Kerberos authenticator should contain the
  server provided 32-bit number in network byte order.

  Upon decrypting and verifying the ticket and authenticator, the
  server should verify that the contained checksum field equals the
  original server provided random 32-bit number.  Should the
  verification be successful, the server must add one to the checksum
  and construct 8 octets of data, with the first four octets containing
  the incremented checksum in network byte order, the fifth octet
  containing a bit-mask specifying the protection mechanisms supported
  by the server, and the sixth through eighth octets containing, in



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  network byte order, the maximum cipher-text buffer size the server is
  able to receive.  The server must encrypt the 8 octets of data in the
  session key and issue that encrypted data in a second ready response.
  The client should consider the server authenticated if the first four
  octets the un-encrypted data is equal to one plus the checksum it
  previously sent.

  The client must construct data with the first four octets containing
  the original server-issued checksum in network byte order, the fifth
  octet containing the bit-mask specifying the selected protection
  mechanism, the sixth through eighth octets containing in network byte
  order the maximum cipher-text buffer size the client is able to
  receive, and the following octets containing a user name string.  The
  client must then append from one to eight octets so that the length
  of the data is a multiple of eight octets. The client must then PCBC
  encrypt the data with the session key and respond to the second ready
  response with the encrypted data.  The server decrypts the data and
  verifies the contained checksum.  The username field identifies the
  user for whom subsequent IMAP operations are to be performed; the
  server must verify that the principal identified in the Kerberos
  ticket is authorized to connect as that user.  After these
  verifications, the authentication process is complete.

  The protection mechanisms and their corresponding bit-masks are as
  follows:

     1 No protection mechanism
     2 Integrity (krb_mk_safe) protection
     4 Privacy (krb_mk_priv) protection


  EXAMPLE: The following are two Kerberos version 4 login scenarios
  (note that the line breaks in the sample authenticators are for
  editorial clarity and are not in real authenticators)

     S: * OK IMAP4 Server
     C: A001 AUTHENTICATE KERBEROS_V4
     S: + AmFYig==
     C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
        +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
        WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
     S: + or//EoAADZI=
     C: DiAF5A4gA+oOIALuBkAAmw==
     S: A001 OK Kerberos V4 authentication successful







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     S: * OK IMAP4 Server
     C: A001 AUTHENTICATE KERBEROS_V4
     S: + gcfgCA==
     C: BAcAQU5EUkVXLkNNVS5FRFUAOCAsho84kLN3/IJmrMG+25a4DT
        +nZImJjnTNHJUtxAA+o0KPKfHEcAFs9a3CL5Oebe/ydHJUwYFd
        WwuQ1MWiy6IesKvjL5rL9WjXUb9MwT9bpObYLGOKi1Qh
     S: A001 NO Kerberos V4 authentication failed


3. GSSAPI authentication mechanism

  The authentication type associated with all mechanisms employing the
  GSSAPI [RFC1508] is "GSSAPI".

  The first ready response issued by the server contains no data.  The
  client should call GSS_Init_sec_context, passing in 0 for
  input_context_handle (initially) and a targ_name equal to output_name
  from GSS_Import_Name called with input_name_type of NULL and
  input_name_string of "SERVICE:imap@hostname" where "hostname" is the
  fully qualified host name of the server with all letters in lower
  case.  The client must then respond with the resulting output_token.
  If GSS_Init_sec_context returns GSS_CONTINUE_NEEDED, then the client
  should expect the server to issue a token in a subsequent ready
  response.  The client must pass the token to another call to
  GSS_Init_sec_context.

  If GSS_Init_sec_context returns GSS_COMPLETE, then the client should
  respond with any resulting output_token.  If there is no
  output_token, the client should respond with no data.  The client
  should then expect the server to issue a token in a subsequent ready
  response.  The client should pass this token to GSS_Unseal and
  interpret the first octet of resulting cleartext as a bit-mask
  specifying the protection mechanisms supported by the server and the
  second through fourth octets as the maximum size output_message to
  send to the server.  The client should construct data, with the first
  octet containing the bit-mask specifying the selected protection
  mechanism, the second through fourth octets containing in network
  byte order the maximum size output_message the client is able to
  receive, and the remaining octets containing a user name string.  The
  client must pass the data to GSS_Seal with conf_flag set to FALSE,
  and respond with the generated output_message.  The client can then
  consider the server authenticated.

  The server must issue a ready response with no data and pass the
  resulting client supplied token to GSS_Accept_sec_context as
  input_token, setting acceptor_cred_handle to NULL (for "use default
  credentials"), and 0 for input_context_handle (initially).  If
  GSS_Accept_sec_context returns GSS_CONTINUE_NEEDED, the server should



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  return the generated output_token to the client in a ready response
  and pass the resulting client supplied token to another call to
  GSS_Accept_sec_context.

  If GSS_Accept_sec_context returns GSS_COMPLETE, then if an
  output_token is returned, the server should return it to the client
  in a ready response and expect a reply from the client with no data.
  Whether or not an output_token was returned, the server then should
  then construct 4 octets of data, with the first octet containing a
  bit-mask specifying the protection mechanisms supported by the server
  and the second through fourth octets containing in network byte order
  the maximum size output_token the server is able to receive.  The
  server must then pass the plaintext to GSS_Seal with conf_flag set to
  FALSE and issue the generated output_message to the client in a ready
  response.  The server must then pass the resulting client supplied
  token to GSS_Unseal and interpret the first octet of resulting
  cleartext as the bit-mask for the selected protection mechanism, the
  second through fourth octets as the maximum size output_message to
  send to the client, and the remaining octets as the user name.  Upon
  verifying the src_name is authorized to authenticate as the user
  name, The server should then consider the client authenticated.

  The protection mechanisms and their corresponding bit-masks are as
  follows:

     1 No protection mechanism
     2 Integrity protection.
       Sender calls GSS_Seal with conf_flag set to FALSE
     4 Privacy protection.
       Sender calls GSS_Seal with conf_flag set to TRUE


4. S/Key authentication mechanism

  The authentication type associated with S/Key [SKEY] is "SKEY".

  The first ready response issued by the server contains no data.  The
  client responds with the user name string.

  The data encoded in the second ready response contains the decimal
  sequence number followed by a single space and the seed string for
  the indicated user.  The client responds with the one-time-password,
  as either a 64-bit value in network byte order or encoded in the "six
  English words" format.

  Upon successful verification of the one-time-password, the server
  should consider the client authenticated.




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  S/Key authentication does not provide for any protection mechanisms.


  EXAMPLE: The following are two S/Key login scenarios.

     S: * OK IMAP4 Server
     C: A001 AUTHENTICATE SKEY
     S: +
     C: bW9yZ2Fu
     S: + OTUgUWE1ODMwOA==
     C: Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
     S: A001 OK S/Key authentication successful


     S: * OK IMAP4 Server
     C: A001 AUTHENTICATE SKEY
     S: +
     C: c21pdGg=
     S: + OTUgUWE1ODMwOA==
     C: BsAY3g4gBNo=
     S: A001 NO S/Key authentication failed


5. References

  [IMAP4] Crispin, M., "Internet Message Access Protocol - Version 4",
  RFC 1730, University of Washington, December 1994.

  [RFC1508] Linn, J., "Generic Security Service Application Program
  Interface", RFC 1508, Geer Zolot Associates, September 1993.

  [SKEY] Haller, Neil M. "The S/Key One-Time Password System",
  Bellcore, Morristown, New Jersey, October 1993,
  thumper.bellcore.com:pub/nmh/docs/ISOC.symp.ps

















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RFC 1731            IMAP4 Authentication Mechanisms        December 1994


6. Security Considerations

  Security issues are discussed throughout this memo.


7. Author's Address

  John G. Myers
  Carnegie-Mellon University
  5000 Forbes Ave.
  Pittsburgh PA, 15213-3890

  EMail: [email protected]






































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