Scram as a gs2 mechanism ABNF

[Sam Hartman <hartmans-ietf@xxxxxxx>]

Here are Nico and my notes on scram as a GSS-API mechanism.

My hope is that Chris and others will find this acceptable.

If not, there are changes to GS2 that could almost certainly make
Chris happy.

    gs2-length = 4*octet
    ctx-length = gs2-length
    wrap-length = gs2-length

    gs2-header = ctx-length wrap-length

    der-definite-length = %00-%7F / %81 octet / %82 2*octet / %83 3*octet / %84 4*octet

    gss-init-ctx-header = %60 der-definite-length "<constant encoded OID octet string here>"

    scram-c1 = gs2-header gss-init-ctx-header username "," hash-list "," nonce
	       ;; wrap-length here is 4*%00

    scram-s1 = gs2-header nonce "," hash-list "," salt "," iteration-count
	       ;; wrap-length here is 4*%00

    mac = "m=" base64 ";"
          ;; This is a GSS wrap with integrity token for SCRAM; normally
	  ;; this is all it will be, but a true SCRAM GSS mechanism may
	  ;; have sequence numbers too
    channel-binding = *(octet)

    scram-c2-ctx = nonce "," proof
    scram-c2-wrap = mac "," 4*%00 channel-binding 4*octet %00 [channel-binding] [authzid]
    scram-c2 = gs2-header scram-c2-ctx scram-c2-wrap

    scram-s2-ctx = verifier
    scram-c2-wrap = mac "," ( %08 / %00 ) 3*%00
    scram-s2 = gs2-header scram-s2-ctx scram-s2-wrap