Pragya Arya | 324337e | 2020-02-20 14:35:08 +0530 | [diff] [blame^] | 1 | package rfc3961 |
| 2 | |
| 3 | import ( |
| 4 | "bytes" |
| 5 | |
| 6 | "gopkg.in/jcmturner/gokrb5.v7/crypto/etype" |
| 7 | ) |
| 8 | |
| 9 | const ( |
| 10 | prfconstant = "prf" |
| 11 | ) |
| 12 | |
| 13 | // DeriveRandom implements the RFC 3961 defined function: DR(Key, Constant) = k-truncate(E(Key, Constant, initial-cipher-state)). |
| 14 | // |
| 15 | // key: base key or protocol key. Likely to be a key from a keytab file. |
| 16 | // |
| 17 | // usage: a constant. |
| 18 | // |
| 19 | // n: block size in bits (not bytes) - note if you use something like aes.BlockSize this is in bytes. |
| 20 | // |
| 21 | // k: key length / key seed length in bits. Eg. for AES256 this value is 256. |
| 22 | // |
| 23 | // e: the encryption etype function to use. |
| 24 | func DeriveRandom(key, usage []byte, e etype.EType) ([]byte, error) { |
| 25 | n := e.GetCypherBlockBitLength() |
| 26 | k := e.GetKeySeedBitLength() |
| 27 | //Ensure the usage constant is at least the size of the cypher block size. Pass it through the nfold algorithm that will "stretch" it if needs be. |
| 28 | nFoldUsage := Nfold(usage, n) |
| 29 | //k-truncate implemented by creating a byte array the size of k (k is in bits hence /8) |
| 30 | out := make([]byte, k/8) |
| 31 | |
| 32 | /*If the output of E is shorter than k bits, it is fed back into the encryption as many times as necessary. |
| 33 | The construct is as follows (where | indicates concatenation): |
| 34 | |
| 35 | K1 = E(Key, n-fold(Constant), initial-cipher-state) |
| 36 | K2 = E(Key, K1, initial-cipher-state) |
| 37 | K3 = E(Key, K2, initial-cipher-state) |
| 38 | K4 = ... |
| 39 | |
| 40 | DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...)*/ |
| 41 | _, K, err := e.EncryptData(key, nFoldUsage) |
| 42 | if err != nil { |
| 43 | return out, err |
| 44 | } |
| 45 | for i := copy(out, K); i < len(out); { |
| 46 | _, K, _ = e.EncryptData(key, K) |
| 47 | i = i + copy(out[i:], K) |
| 48 | } |
| 49 | return out, nil |
| 50 | } |
| 51 | |
| 52 | // DeriveKey derives a key from the protocol key based on the usage and the etype's specific methods. |
| 53 | func DeriveKey(protocolKey, usage []byte, e etype.EType) ([]byte, error) { |
| 54 | r, err := e.DeriveRandom(protocolKey, usage) |
| 55 | if err != nil { |
| 56 | return nil, err |
| 57 | } |
| 58 | return e.RandomToKey(r), nil |
| 59 | } |
| 60 | |
| 61 | // RandomToKey returns a key from the bytes provided according to the definition in RFC 3961. |
| 62 | func RandomToKey(b []byte) []byte { |
| 63 | return b |
| 64 | } |
| 65 | |
| 66 | // DES3RandomToKey returns a key from the bytes provided according to the definition in RFC 3961 for DES3 etypes. |
| 67 | func DES3RandomToKey(b []byte) []byte { |
| 68 | r := fixWeakKey(stretch56Bits(b[:7])) |
| 69 | r2 := fixWeakKey(stretch56Bits(b[7:14])) |
| 70 | r = append(r, r2...) |
| 71 | r3 := fixWeakKey(stretch56Bits(b[14:21])) |
| 72 | r = append(r, r3...) |
| 73 | return r |
| 74 | } |
| 75 | |
| 76 | // DES3StringToKey returns a key derived from the string provided according to the definition in RFC 3961 for DES3 etypes. |
| 77 | func DES3StringToKey(secret, salt string, e etype.EType) ([]byte, error) { |
| 78 | s := secret + salt |
| 79 | tkey := e.RandomToKey(Nfold([]byte(s), e.GetKeySeedBitLength())) |
| 80 | return e.DeriveKey(tkey, []byte("kerberos")) |
| 81 | } |
| 82 | |
| 83 | // PseudoRandom function as defined in RFC 3961 |
| 84 | func PseudoRandom(key, b []byte, e etype.EType) ([]byte, error) { |
| 85 | h := e.GetHashFunc()() |
| 86 | h.Write(b) |
| 87 | tmp := h.Sum(nil)[:e.GetMessageBlockByteSize()] |
| 88 | k, err := e.DeriveKey(key, []byte(prfconstant)) |
| 89 | if err != nil { |
| 90 | return []byte{}, err |
| 91 | } |
| 92 | _, prf, err := e.EncryptData(k, tmp) |
| 93 | if err != nil { |
| 94 | return []byte{}, err |
| 95 | } |
| 96 | return prf, nil |
| 97 | } |
| 98 | |
| 99 | func stretch56Bits(b []byte) []byte { |
| 100 | d := make([]byte, len(b), len(b)) |
| 101 | copy(d, b) |
| 102 | var lb byte |
| 103 | for i, v := range d { |
| 104 | bv, nb := calcEvenParity(v) |
| 105 | d[i] = nb |
| 106 | if bv != 0 { |
| 107 | lb = lb | (1 << uint(i+1)) |
| 108 | } else { |
| 109 | lb = lb &^ (1 << uint(i+1)) |
| 110 | } |
| 111 | } |
| 112 | _, lb = calcEvenParity(lb) |
| 113 | d = append(d, lb) |
| 114 | return d |
| 115 | } |
| 116 | |
| 117 | func calcEvenParity(b byte) (uint8, uint8) { |
| 118 | lowestbit := b & 0x01 |
| 119 | // c counter of 1s in the first 7 bits of the byte |
| 120 | var c int |
| 121 | // Iterate over the highest 7 bits (hence p starts at 1 not zero) and count the 1s. |
| 122 | for p := 1; p < 8; p++ { |
| 123 | v := b & (1 << uint(p)) |
| 124 | if v != 0 { |
| 125 | c++ |
| 126 | } |
| 127 | } |
| 128 | if c%2 == 0 { |
| 129 | //Even number of 1s so set parity to 1 |
| 130 | b = b | 1 |
| 131 | } else { |
| 132 | //Odd number of 1s so set parity to 0 |
| 133 | b = b &^ 1 |
| 134 | } |
| 135 | return lowestbit, b |
| 136 | } |
| 137 | |
| 138 | func fixWeakKey(b []byte) []byte { |
| 139 | if weak(b) { |
| 140 | b[7] ^= 0xF0 |
| 141 | } |
| 142 | return b |
| 143 | } |
| 144 | |
| 145 | func weak(b []byte) bool { |
| 146 | // weak keys from https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-67r1.pdf |
| 147 | weakKeys := [4][]byte{ |
| 148 | {0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01}, |
| 149 | {0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE}, |
| 150 | {0xE0, 0xE0, 0xE0, 0xE0, 0xF1, 0xF1, 0xF1, 0xF1}, |
| 151 | {0x1F, 0x1F, 0x1F, 0x1F, 0x0E, 0x0E, 0x0E, 0x0E}, |
| 152 | } |
| 153 | semiWeakKeys := [12][]byte{ |
| 154 | {0x01, 0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E}, |
| 155 | {0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E, 0x01}, |
| 156 | {0x01, 0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1}, |
| 157 | {0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1, 0x01}, |
| 158 | {0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE}, |
| 159 | {0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01}, |
| 160 | {0x1F, 0xE0, 0x1F, 0xE0, 0x0E, 0xF1, 0x0E, 0xF1}, |
| 161 | {0xE0, 0x1F, 0xE0, 0x1F, 0xF1, 0x0E, 0xF1, 0x0E}, |
| 162 | {0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E, 0xFE}, |
| 163 | {0xFE, 0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E}, |
| 164 | {0xE0, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1, 0xFE}, |
| 165 | {0xFE, 0xE0, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1}, |
| 166 | } |
| 167 | for _, k := range weakKeys { |
| 168 | if bytes.Equal(b, k) { |
| 169 | return true |
| 170 | } |
| 171 | } |
| 172 | for _, k := range semiWeakKeys { |
| 173 | if bytes.Equal(b, k) { |
| 174 | return true |
| 175 | } |
| 176 | } |
| 177 | return false |
| 178 | } |