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Warning

This is a work in progress, largely intended to solicit the opinion of interested parties.

Introduction

GSSAPI is a standard for secure communications between two parties. Notably, it has become the de-facto standard portable API for Kerberos, so much so that “GSSAPI” often implies “Kerberos” despite the existence of things like SPKM and LIPKEY.

GSSAPI is, at its core, an online client/server (“initiator” and “acceptor” in its terminology) architecture. The basic flow of communication is to initiate a “context” (and have that context be accepted) before messages may be passed within that context, using, optionally, confidentiality and integrity mechanisms provided by the underlying cryptographic mechanism. That is, Alice may establish a context for secure communication with Bob, and another for Carol, and so on. (Other things are possible, including delegation of identity or rights and replay detection on the stream of messages, but these are not germane to this document.)

However, GSSAPI has no intrinsic mechanism for asynchronous communication with another party; there is no mechanism, Alice having established a context with Bob (and, optionally, Carol), for Alice (or Bob) to seal or box a message such that it can be passed to Carol, who can then forward it to Bob in a verifiable, confidential way. Bob should be able to verify that

  • Alice sealed this message,
  • Bob was indeed the intended recipient,
  • (optionally) that Carol was the intended relaying party,
  • (optionally) that the message was sealed recently, and
  • (optionally) that the message has not been replayed by Carol (or another agent).

Neither Carol nor any other agent should be able to learn anything about the message body (except an approximation of its length) or the identities of Alice, Bob, or Carol from the message itself.

Another way to think about the intended use of sealing is that Alice may wish to allow Carol to prove to Bob that Alice and Carol are in communication and to reliably prove Alice’s opinions of that communication to Bob. For intuition, imagine Alice as something akin to a Kerberos TGS, having active contexts with both Bob and Carol; Alice’s sealing of a message for Carol to give to Bob is akin to granting a service ticket. The key is that the sealed message carries the identities of its originator (Alice) and relaying party (Carol) and may only be unsealed by its recipient (Bob).

API Calls

  • GSS_Wrap2 (as performed by Alice):

    • Inputs:
      • recipient_context_handle CONTEXT HANDLE (with Bob),
      • relaying_context_handle CONTEXT HANDLE (with Carol, optional),
      • freshness_stamp INTEGER,
      • input_message OCTET STRING
    • Outputs:
      • major_status INTEGER,
      • minor_status INTEGER,
      • output_message OCTET STRING – caller to release with GSS_Release_buffer()

    The freshness_stamp parameter is one of GSS_WRAP_FRESH_TIMESERIAL, GSS_WRAP_FRESH_TIMEONLY, or GSS_WRAP_FRESH_NONE.

    The existing GSS_Wrap call is equivalent to GSS_Wrap2 with relaying_context_handle = NULL and freshness_stamp = GSS_WRAP_FRESH_NONE, and a call to GSS_Wrap2 satisfying these conditions may not fail unless the equivalent call to GSS_Wrap would.

  • GSS_Unwrap2 (as performed by Bob):

    • Inputs:
      • context_handle CONTEXT HANDLE (with Carol),
      • req_freshness INTEGER,
      • input_message OCTET STRING (from Carol, originally produced by Alice)
    • Outputs:
      • major_status INTEGER,
      • minor_status INTEGER,
      • output_message OCTET STRING – caller to release with GSS_Release_buffer()
      • cred_name INTERNAL NAME – caller to release with GSS_Release_name()
      • is_relayer INTEGER

    is_relayer indicates whether the remote credential of context_handle is the same credential as the remote credential of the relaying_context_handle given to GSS_Wrap2. Possible values are

    • GSS_WRAP_ISRELAY_POSITIVE, indicating explicit confirmation;
    • GSS_WRAP_ISRELAY_NEGATIVE, indicating explicit denial; or
    • GSS_WRAP_ISRELAY_UNSPECIFIED, indicating that no relaying_context_handle was given to GSS_Wrap2.

    If GSS_Unwrap2 is successful, output_message must match the input_message used in the call to GSS_Wrap2 to produce the input_message fed to GSS_Unwrap2 and cred_name must describe Alice. (In particular, cred_name should be equal to the name that would be obtained as the remote half of Bob’s half of Alice’s recipient_context_handle.)

    Note that GSS_Unwrap2 is a superset of GSS_Unwrap. If the input_message to GSS_Unwrap2 was produced with GSS_Wrap or with an equivalent GSS_Wrap2 call, cred_name must be NULL and is_relayer must be GSS_WRAP_ISRELAY_UNSPECIFIED.