func (alg *AesCbcHmac) computeAuthTag(aad []byte, iv []byte, cipherText []byte, hmacKey []byte) (signature []byte) {
al := arrays.UInt64ToBytes(uint64(len(aad) << 3))
hmacInput := arrays.Concat(aad, iv, cipherText, al)
hmac := calculateHmac(alg.keySizeBits, hmacInput, hmacKey)
return hmac[0 : len(hmac)/2]
}
// KeyWrap encrypts provided key (CEK) with KEK key using AES Key Wrap (rfc 3394) algorithm
func KeyWrap(cek, kek []byte) ([]byte, error) {
// 1) Initialize variables
a := defaultIV // Set A = IV, an initial value
r := arrays.Slice(cek, 8) // For i = 1 to n
// R[0][i] = P[i]
n := uint64(len(r))
// 2) Calculate intermediate values.
var j, i, t uint64
for j = 0; j < 6; j++ { // For j = 0 to 5
for i = 0; i < n; i++ { // For i=1 to n
t = n*j + i + 1
b, e := aesEnc(kek, arrays.Concat(a, r[i])) // B=AES(K, A | R[i])
if e != nil {
return nil, e
}
a = b[:len(b)/2] // A=MSB(64,B) ^ t where t = (n*j)+i
r[i] = b[len(b)/2:] // R[i] = LSB(64, B)
a = arrays.Xor(a, arrays.UInt64ToBytes(t))
}
}
// 3) Output the results
c := make([][]byte, n+1, n+1)
c[0] = a // Set C[0] = A
for i = 1; i <= n; i++ { // For i = 1 to n
c[i] = r[i-1] // C[i] = R[i]
}
return arrays.Unwrap(c), nil
}
// KeyUnwrap decrypts previously encrypted key (CEK) with KEK key using AES Key Wrap (rfc 3394) algorithm
func KeyUnwrap(encryptedCek, kek []byte) ([]byte, error) {
// 1) Initialize variables
c := arrays.Slice(encryptedCek, 8)
a := c[0] // Set A = C[0]
r := make([][]byte, len(c)-1)
for i := 1; i < len(c); i++ { // For i = 1 to n
r[i-1] = c[i] // R[i] = C[i]
}
n := uint64(len(r))
// 2) Calculate intermediate values
var t, j uint64
for j = 6; j > 0; j-- { // For j = 5 to 0
for i := n; i > 0; i-- { // For i = n to 1
t = n*(j-1) + i
a = arrays.Xor(a, arrays.UInt64ToBytes(t))
b, e := aesDec(kek, arrays.Concat(a, r[i-1])) // B = AES-1(K, (A ^ t) | R[i]) where t = n*j+i
if e != nil {
return nil, e
}
a = b[:len(b)/2] // A = MSB(64, B)
r[i-1] = b[len(b)/2:] // R[i] = LSB(64, B)
}
}
// 3) Output the results
if !hmac.Equal(defaultIV, a) { // If A is an appropriate initial value
return nil, errors.New("aes.KeyUnwrap(): integrity check failed.")
}
// For i = 1 to n
return arrays.Unwrap(r), nil // P[i] = R[i]
}