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Network Working Group                                           J. MyersRequest for Comments: 1731                               Carnegie MellonCategory: Standards Track                                  December 1994IMAP4 Authentication MechanismsStatus 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, inMyers                                                           [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 successfulMyers                                                           [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 failed3. 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 shouldMyers                                                           [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 TRUE4. 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 failed5. 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.psMyers                                                           [Page 5]

RFC 1731            IMAP4 Authentication Mechanisms        December 19946. 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.eduMyers                                                           [Page 6]