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package gssapi
import (
From RFC 4121, section
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
2 Flags Attributes field, as described in section
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
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
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 (
HdrLen = 16 // Length of the Wrap Token's header
FillerByte byte = 0xFF
// WrapToken represents a GSS API Wrap token, as defined in RFC 4121.
// It contains the header fields, the payload and the checksum, and provides
// the logic for converting to/from bytes plus computing and verifying checksums
type WrapToken struct {
// const GSS Token ID: 0x0504
Flags byte // contains three flags: acceptor, sealed, acceptor subkey
// const Filler: 0xFF
EC uint16 // checksum length. big-endian
RRC uint16 // right rotation count. big-endian
SndSeqNum uint64 // sender's sequence number. big-endian
Payload []byte // your data! :)
CheckSum []byte // authenticated checksum of { payload | header }
// Return the 2 bytes identifying a GSS API Wrap token
func getGssWrapTokenId() *[2]byte {
return &[2]byte{0x05, 0x04}
// Marshal the WrapToken into a byte slice.
// The payload should have been set and the checksum computed, otherwise an error is returned.
func (wt *WrapToken) Marshal() ([]byte, error) {
if wt.CheckSum == nil {
return nil, errors.New("checksum has not been set")
if wt.Payload == nil {
return nil, errors.New("payload has not been set")
pldOffset := HdrLen // Offset of the payload in the token
chkSOffset := HdrLen + len(wt.Payload) // Offset of the checksum in the token
bytes := make([]byte, chkSOffset+int(wt.EC))
copy(bytes[0:], getGssWrapTokenId()[:])
bytes[2] = wt.Flags
bytes[3] = FillerByte
binary.BigEndian.PutUint16(bytes[4:6], wt.EC)
binary.BigEndian.PutUint16(bytes[6:8], wt.RRC)
binary.BigEndian.PutUint64(bytes[8:16], wt.SndSeqNum)
copy(bytes[pldOffset:], wt.Payload)
copy(bytes[chkSOffset:], wt.CheckSum)
return bytes, nil
// SetCheckSum uses the passed encryption key and key usage to compute the checksum over the payload and
// the header, and sets the CheckSum field of this WrapToken.
// If the payload has not been set or the checksum has already been set, an error is returned.
func (wt *WrapToken) SetCheckSum(key types.EncryptionKey, keyUsage uint32) error {
if wt.Payload == nil {
return errors.New("payload has not been set")
if wt.CheckSum != nil {
return errors.New("checksum has already been computed")
chkSum, cErr := wt.computeCheckSum(key, keyUsage)
if cErr != nil {
return cErr
wt.CheckSum = chkSum
return nil
// ComputeCheckSum computes and returns the checksum of this token, computed using the passed key and key usage.
// Conforms to RFC 4121 in that the checksum will be computed over { body | header },
// with the EC and RRC flags zeroed out.
// In the context of Kerberos Wrap tokens, mostly keyusage GSSAPI_ACCEPTOR_SEAL (=22)
// and GSSAPI_INITIATOR_SEAL (=24) will be used.
// Note: This will NOT update the struct's Checksum field.
func (wt *WrapToken) computeCheckSum(key types.EncryptionKey, keyUsage uint32) ([]byte, error) {
if wt.Payload == nil {
return nil, errors.New("cannot compute checksum with uninitialized payload")
// Build a slice containing { payload | header }
checksumMe := make([]byte, HdrLen+len(wt.Payload))
copy(checksumMe[0:], wt.Payload)
copy(checksumMe[len(wt.Payload):], getChecksumHeader(wt.Flags, wt.SndSeqNum))
encType, err := crypto.GetEtype(key.KeyType)
if err != nil {
return nil, err
return encType.GetChecksumHash(key.KeyValue, checksumMe, keyUsage)
// Build a header suitable for a checksum computation
func getChecksumHeader(flags byte, senderSeqNum uint64) []byte {
header := make([]byte, 16)
copy(header[0:], []byte{0x05, 0x04, flags, 0xFF, 0x00, 0x00, 0x00, 0x00})
binary.BigEndian.PutUint64(header[8:], senderSeqNum)
return header
// Verify computes the token's checksum with the provided key and usage,
// and compares it to the checksum present in the token.
// In case of any failure, (false, Err) is returned, with Err an explanatory error.
func (wt *WrapToken) Verify(key types.EncryptionKey, keyUsage uint32) (bool, error) {
computed, cErr := wt.computeCheckSum(key, keyUsage)
if cErr != nil {
return false, cErr
if !hmac.Equal(computed, wt.CheckSum) {
return false, fmt.Errorf(
"checksum mismatch. Computed: %s, Contained in token: %s",
hex.EncodeToString(computed), hex.EncodeToString(wt.CheckSum))
return true, nil
// Unmarshal bytes into the corresponding WrapToken.
// If expectFromAcceptor is true, we expect the token to have been emitted by the gss acceptor,
// and will check the according flag, returning an error if the token does not match the expectation.
func (wt *WrapToken) Unmarshal(b []byte, expectFromAcceptor bool) error {
// Check if we can read a whole header
if len(b) < 16 {
return errors.New("bytes shorter than header length")
// Is the Token ID correct?
if !bytes.Equal(getGssWrapTokenId()[:], b[0:2]) {
return fmt.Errorf("wrong Token ID. Expected %s, was %s",
// Check the acceptor flag
flags := b[2]
isFromAcceptor := flags&0x01 == 1
if isFromAcceptor && !expectFromAcceptor {
return errors.New("unexpected acceptor flag is set: not expecting a token from the acceptor")
if !isFromAcceptor && expectFromAcceptor {
return errors.New("expected acceptor flag is not set: expecting a token from the acceptor, not the initiator")
// Check the filler byte
if b[3] != FillerByte {
return fmt.Errorf("unexpected filler byte: expecting 0xFF, was %s ", hex.EncodeToString(b[3:4]))
checksumL := binary.BigEndian.Uint16(b[4:6])
// Sanity check on the checksum length
if int(checksumL) > len(b)-HdrLen {
return fmt.Errorf("inconsistent checksum length: %d bytes to parse, checksum length is %d", len(b), checksumL)
wt.Flags = flags
wt.EC = checksumL
wt.RRC = binary.BigEndian.Uint16(b[6:8])
wt.SndSeqNum = binary.BigEndian.Uint64(b[8:16])
wt.Payload = b[16 : len(b)-int(checksumL)]
wt.CheckSum = b[len(b)-int(checksumL):]
return nil
// NewInitiatorWrapToken builds a new initiator token (acceptor flag will be set to 0) and computes the authenticated checksum.
// Other flags are set to 0, and the RRC and sequence number are initialized to 0.
// Note that in certain circumstances you may need to provide a sequence number that has been defined earlier.
// This is currently not supported.
func NewInitiatorWrapToken(payload []byte, key types.EncryptionKey) (*WrapToken, error) {
encType, err := crypto.GetEtype(key.KeyType)
if err != nil {
return nil, err
token := WrapToken{
Flags: 0x00, // all zeroed out (this is a token sent by the initiator)
// Checksum size: length of output of the HMAC function, in bytes.
EC: uint16(encType.GetHMACBitLength() / 8),
RRC: 0,
SndSeqNum: 0,
Payload: payload,
if err := token.SetCheckSum(key, keyusage.GSSAPI_INITIATOR_SEAL); err != nil {
return nil, err
return &token, nil