Pragya Arya | 324337e | 2020-02-20 14:35:08 +0530 | [diff] [blame] | 1 | package gssapi |
| 2 | |
| 3 | import ( |
| 4 | "bytes" |
| 5 | "crypto/hmac" |
| 6 | "encoding/binary" |
| 7 | "encoding/hex" |
| 8 | "errors" |
| 9 | "fmt" |
| 10 | |
| 11 | "gopkg.in/jcmturner/gokrb5.v7/crypto" |
| 12 | "gopkg.in/jcmturner/gokrb5.v7/iana/keyusage" |
| 13 | "gopkg.in/jcmturner/gokrb5.v7/types" |
| 14 | ) |
| 15 | |
| 16 | /* |
| 17 | From RFC 4121, section 4.2.6.1: |
| 18 | |
| 19 | Use of the GSS_GetMIC() call yields a token (referred as the MIC |
| 20 | token in this document), separate from the user data being protected, |
| 21 | which can be used to verify the integrity of that data as received. |
| 22 | The token has the following format: |
| 23 | |
| 24 | Octet no Name Description |
| 25 | -------------------------------------------------------------- |
| 26 | 0..1 TOK_ID Identification field. Tokens emitted by |
| 27 | GSS_GetMIC() contain the hex value 04 04 |
| 28 | expressed in big-endian order in this |
| 29 | field. |
| 30 | 2 Flags Attributes field, as described in section |
| 31 | 4.2.2. |
| 32 | 3..7 Filler Contains five octets of hex value FF. |
| 33 | 8..15 SND_SEQ Sequence number field in clear text, |
| 34 | expressed in big-endian order. |
| 35 | 16..last SGN_CKSUM Checksum of the "to-be-signed" data and |
| 36 | octet 0..15, as described in section 4.2.4. |
| 37 | |
| 38 | The Filler field is included in the checksum calculation for |
| 39 | simplicity. |
| 40 | |
| 41 | */ |
| 42 | |
| 43 | const ( |
| 44 | // MICTokenFlagSentByAcceptor - this flag indicates the sender is the context acceptor. When not set, it indicates the sender is the context initiator |
| 45 | MICTokenFlagSentByAcceptor = 1 << iota |
| 46 | // MICTokenFlagSealed - this flag indicates confidentiality is provided for. It SHALL NOT be set in MIC tokens |
| 47 | MICTokenFlagSealed |
| 48 | // MICTokenFlagAcceptorSubkey - a subkey asserted by the context acceptor is used to protect the message |
| 49 | MICTokenFlagAcceptorSubkey |
| 50 | ) |
| 51 | |
| 52 | const ( |
| 53 | micHdrLen = 16 // Length of the MIC Token's header |
| 54 | ) |
| 55 | |
| 56 | // MICToken represents a GSS API MIC token, as defined in RFC 4121. |
| 57 | // It contains the header fields, the payload (this is not transmitted) and |
| 58 | // the checksum, and provides the logic for converting to/from bytes plus |
| 59 | // computing and verifying checksums |
| 60 | type MICToken struct { |
| 61 | // const GSS Token ID: 0x0404 |
| 62 | Flags byte // contains three flags: acceptor, sealed, acceptor subkey |
| 63 | // const Filler: 0xFF 0xFF 0xFF 0xFF 0xFF |
| 64 | SndSeqNum uint64 // sender's sequence number. big-endian |
| 65 | Payload []byte // your data! :) |
| 66 | Checksum []byte // checksum of { payload | header } |
| 67 | } |
| 68 | |
| 69 | // Return the 2 bytes identifying a GSS API MIC token |
| 70 | func getGSSMICTokenID() *[2]byte { |
| 71 | return &[2]byte{0x04, 0x04} |
| 72 | } |
| 73 | |
| 74 | // Return the filler bytes used in header |
| 75 | func fillerBytes() *[5]byte { |
| 76 | return &[5]byte{0xFF, 0xFF, 0xFF, 0xFF, 0xFF} |
| 77 | } |
| 78 | |
| 79 | // Marshal the MICToken into a byte slice. |
| 80 | // The payload should have been set and the checksum computed, otherwise an error is returned. |
| 81 | func (mt *MICToken) Marshal() ([]byte, error) { |
| 82 | if mt.Checksum == nil { |
| 83 | return nil, errors.New("checksum has not been set") |
| 84 | } |
| 85 | |
| 86 | bytes := make([]byte, micHdrLen+len(mt.Checksum)) |
| 87 | copy(bytes[0:micHdrLen], mt.getMICChecksumHeader()[:]) |
| 88 | copy(bytes[micHdrLen:], mt.Checksum) |
| 89 | |
| 90 | return bytes, nil |
| 91 | } |
| 92 | |
| 93 | // SetChecksum uses the passed encryption key and key usage to compute the checksum over the payload and |
| 94 | // the header, and sets the Checksum field of this MICToken. |
| 95 | // If the payload has not been set or the checksum has already been set, an error is returned. |
| 96 | func (mt *MICToken) SetChecksum(key types.EncryptionKey, keyUsage uint32) error { |
| 97 | if mt.Checksum != nil { |
| 98 | return errors.New("checksum has already been computed") |
| 99 | } |
| 100 | checksum, err := mt.checksum(key, keyUsage) |
| 101 | if err != nil { |
| 102 | return err |
| 103 | } |
| 104 | mt.Checksum = checksum |
| 105 | return nil |
| 106 | } |
| 107 | |
| 108 | // Compute and return the checksum of this token, computed using the passed key and key usage. |
| 109 | // Confirms to RFC 4121 in that the checksum will be computed over { body | header }. |
| 110 | // In the context of Kerberos MIC tokens, mostly keyusage GSSAPI_ACCEPTOR_SIGN (=23) |
| 111 | // and GSSAPI_INITIATOR_SIGN (=25) will be used. |
| 112 | // Note: This will NOT update the struct's Checksum field. |
| 113 | func (mt *MICToken) checksum(key types.EncryptionKey, keyUsage uint32) ([]byte, error) { |
| 114 | if mt.Payload == nil { |
| 115 | return nil, errors.New("cannot compute checksum with uninitialized payload") |
| 116 | } |
| 117 | d := make([]byte, micHdrLen+len(mt.Payload)) |
| 118 | copy(d[0:], mt.Payload) |
| 119 | copy(d[len(mt.Payload):], mt.getMICChecksumHeader()) |
| 120 | |
| 121 | encType, err := crypto.GetEtype(key.KeyType) |
| 122 | if err != nil { |
| 123 | return nil, err |
| 124 | } |
| 125 | return encType.GetChecksumHash(key.KeyValue, d, keyUsage) |
| 126 | } |
| 127 | |
| 128 | // Build a header suitable for a checksum computation |
| 129 | func (mt *MICToken) getMICChecksumHeader() []byte { |
| 130 | header := make([]byte, micHdrLen) |
| 131 | copy(header[0:2], getGSSMICTokenID()[:]) |
| 132 | header[2] = mt.Flags |
| 133 | copy(header[3:8], fillerBytes()[:]) |
| 134 | binary.BigEndian.PutUint64(header[8:16], mt.SndSeqNum) |
| 135 | return header |
| 136 | } |
| 137 | |
| 138 | // Verify computes the token's checksum with the provided key and usage, |
| 139 | // and compares it to the checksum present in the token. |
| 140 | // In case of any failure, (false, err) is returned, with err an explanatory error. |
| 141 | func (mt *MICToken) Verify(key types.EncryptionKey, keyUsage uint32) (bool, error) { |
| 142 | computed, err := mt.checksum(key, keyUsage) |
| 143 | if err != nil { |
| 144 | return false, err |
| 145 | } |
| 146 | if !hmac.Equal(computed, mt.Checksum) { |
| 147 | return false, fmt.Errorf( |
| 148 | "checksum mismatch. Computed: %s, Contained in token: %s", |
| 149 | hex.EncodeToString(computed), hex.EncodeToString(mt.Checksum)) |
| 150 | } |
| 151 | return true, nil |
| 152 | } |
| 153 | |
| 154 | // Unmarshal bytes into the corresponding MICToken. |
| 155 | // If expectFromAcceptor is true we expect the token to have been emitted by the gss acceptor, |
| 156 | // and will check the according flag, returning an error if the token does not match the expectation. |
| 157 | func (mt *MICToken) Unmarshal(b []byte, expectFromAcceptor bool) error { |
| 158 | if len(b) < micHdrLen { |
| 159 | return errors.New("bytes shorter than header length") |
| 160 | } |
| 161 | if !bytes.Equal(getGSSMICTokenID()[:], b[0:2]) { |
| 162 | return fmt.Errorf("wrong Token ID, Expected %s, was %s", |
| 163 | hex.EncodeToString(getGSSMICTokenID()[:]), |
| 164 | hex.EncodeToString(b[0:2])) |
| 165 | } |
| 166 | flags := b[2] |
| 167 | isFromAcceptor := flags&MICTokenFlagSentByAcceptor != 0 |
| 168 | if isFromAcceptor && !expectFromAcceptor { |
| 169 | return errors.New("unexpected acceptor flag is set: not expecting a token from the acceptor") |
| 170 | } |
| 171 | if !isFromAcceptor && expectFromAcceptor { |
| 172 | return errors.New("unexpected acceptor flag is not set: expecting a token from the acceptor, not in the initiator") |
| 173 | } |
| 174 | if !bytes.Equal(b[3:8], fillerBytes()[:]) { |
| 175 | return fmt.Errorf("unexpected filler bytes: expecting %s, was %s", |
| 176 | hex.EncodeToString(fillerBytes()[:]), |
| 177 | hex.EncodeToString(b[3:8])) |
| 178 | } |
| 179 | |
| 180 | mt.Flags = flags |
| 181 | mt.SndSeqNum = binary.BigEndian.Uint64(b[8:16]) |
| 182 | mt.Checksum = b[micHdrLen:] |
| 183 | return nil |
| 184 | } |
| 185 | |
| 186 | // NewInitiatorMICToken builds a new initiator token (acceptor flag will be set to 0) and computes the authenticated checksum. |
| 187 | // Other flags are set to 0. |
| 188 | // Note that in certain circumstances you may need to provide a sequence number that has been defined earlier. |
| 189 | // This is currently not supported. |
| 190 | func NewInitiatorMICToken(payload []byte, key types.EncryptionKey) (*MICToken, error) { |
| 191 | token := MICToken{ |
| 192 | Flags: 0x00, |
| 193 | SndSeqNum: 0, |
| 194 | Payload: payload, |
| 195 | } |
| 196 | |
| 197 | if err := token.SetChecksum(key, keyusage.GSSAPI_INITIATOR_SIGN); err != nil { |
| 198 | return nil, err |
| 199 | } |
| 200 | |
| 201 | return &token, nil |
| 202 | } |