gssapi

const (
	HdrLen          = 16 // Length of the Wrap Token's header
	FillerByte byte = 0xFF
)

From RFC 4121, section 4.2.6.2:

Use of the GSS_Wrap() call yields a token (referred as the Wrap token
in this document), which consists of a descriptive header, followed
by a body portion that contains either the input user data in
plaintext concatenated with the checksum, or the input user data
encrypted.  The GSS_Wrap() token SHALL have the following format:

      Octet no   Name        Description
      --------------------------------------------------------------
       0..1     TOK_ID    Identification field.  Tokens emitted by
                          GSS_Wrap() contain the hex value 05 04
                          expressed in big-endian order in this
                          field.
       2        Flags     Attributes field, as described in section
                          4.2.2.
       3        Filler    Contains the hex value FF.
       4..5     EC        Contains the "extra count" field, in big-
                          endian order as described in section 4.2.3.
       6..7     RRC       Contains the "right rotation count" in big-
                          endian order, as described in section
                          4.2.5.
       8..15    SndSeqNum   Sequence number field in clear text,
                          expressed in big-endian order.
       16..last Data      Encrypted data for Wrap tokens with
                          confidentiality, or plaintext data followed
                          by the checksum for Wrap tokens without
                          confidentiality, as described in section
                          4.2.4.

Quick notes:

• "EC" or "Extra Count" refers to the length of the checksum.
• "Flags" (complete details in section 4.2.2) is a set of bits:
• if bit 0 is set, it means the token was sent by the acceptor (generally the kerberized service).
• bit 1 indicates that the token's payload is encrypted
• bit 2 indicates if the message is protected using a subkey defined by the acceptor.
• When computing checksums, EC and RRC MUST be set to 0.
• Wrap Tokens are not ASN.1 encoded.

const (
	TOK_ID_KRB_AP_REQ = "0100"
	TOK_ID_KRB_AP_REP = "0200"
	TOK_ID_KRB_ERROR  = "0300"

	GSS_C_DELEG_FLAG    = 1
	GSS_C_MUTUAL_FLAG   = 2
	GSS_C_REPLAY_FLAG   = 4
	GSS_C_SEQUENCE_FLAG = 8
	GSS_C_CONF_FLAG     = 16
	GSS_C_INTEG_FLAG    = 32
)

GSSAPI MechToken IDs and flags.