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rfc:rfc1731

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

Myers [Page 1] RFC 1731 IMAP4 Authentication Mechanisms December 1994

 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

Myers [Page 2] RFC 1731 IMAP4 Authentication Mechanisms December 1994

    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

Myers [Page 3] RFC 1731 IMAP4 Authentication Mechanisms December 1994

 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.

Myers [Page 4] RFC 1731 IMAP4 Authentication Mechanisms December 1994

 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

Myers [Page 5] 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: jgm+@cmu.edu

Myers [Page 6]

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