| # Author: Google |
| # See the LICENSE file for legal information regarding use of this file. |
| |
| # GCM derived from Go's implementation in crypto/cipher. |
| # |
| # https://golang.org/src/crypto/cipher/gcm.go |
| |
| # GCM works over elements of the field GF(2^128), each of which is a 128-bit |
| # polynomial. Throughout this implementation, polynomials are represented as |
| # Python integers with the low-order terms at the most significant bits. So a |
| # 128-bit polynomial is an integer from 0 to 2^128-1 with the most significant |
| # bit representing the x^0 term and the least significant bit representing the |
| # x^127 term. This bit reversal also applies to polynomials used as indices in a |
| # look-up table. |
| |
| from .cryptomath import bytesToNumber, numberToByteArray |
| |
| class AESGCM(object): |
| """ |
| AES-GCM implementation. Note: this implementation does not attempt |
| to be side-channel resistant. It's also rather slow. |
| """ |
| |
| def __init__(self, key, implementation, rawAesEncrypt): |
| self.isBlockCipher = False |
| self.isAEAD = True |
| self.nonceLength = 12 |
| self.tagLength = 16 |
| self.implementation = implementation |
| if len(key) == 16: |
| self.name = "aes128gcm" |
| elif len(key) == 32: |
| self.name = "aes256gcm" |
| else: |
| raise AssertionError() |
| |
| self._rawAesEncrypt = rawAesEncrypt |
| |
| # The GCM key is AES(0). |
| h = bytesToNumber(self._rawAesEncrypt(bytearray(16))) |
| |
| # Pre-compute all 4-bit multiples of h. Note that bits are reversed |
| # because our polynomial representation places low-order terms at the |
| # most significant bit. Thus x^0 * h = h is at index 0b1000 = 8 and |
| # x^1 * h is at index 0b0100 = 4. |
| self._productTable = [0] * 16 |
| self._productTable[_reverseBits(1)] = h |
| for i in range(2, 16, 2): |
| self._productTable[_reverseBits(i)] = \ |
| _gcmShift(self._productTable[_reverseBits(i/2)]) |
| self._productTable[_reverseBits(i+1)] = \ |
| _gcmAdd(self._productTable[_reverseBits(i)], h) |
| |
| def _rawAesCtrEncrypt(self, counter, inp): |
| """ |
| Encrypts (or decrypts) plaintext with AES-CTR. counter is modified. |
| """ |
| out = bytearray(len(inp)) |
| for i in range(0, len(out), 16): |
| mask = self._rawAesEncrypt(counter) |
| for j in range(i, min(len(out), i + 16)): |
| out[j] = inp[j] ^ mask[j-i] |
| _inc32(counter) |
| return out |
| |
| def _auth(self, ciphertext, ad, tagMask): |
| y = 0 |
| y = self._update(y, ad) |
| y = self._update(y, ciphertext) |
| y ^= (len(ad) << (3 + 64)) | (len(ciphertext) << 3) |
| y = self._mul(y) |
| y ^= bytesToNumber(tagMask) |
| return numberToByteArray(y, 16) |
| |
| def _update(self, y, data): |
| for i in range(0, len(data) // 16): |
| y ^= bytesToNumber(data[16*i:16*i+16]) |
| y = self._mul(y) |
| extra = len(data) % 16 |
| if extra != 0: |
| block = bytearray(16) |
| block[:extra] = data[-extra:] |
| y ^= bytesToNumber(block) |
| y = self._mul(y) |
| return y |
| |
| def _mul(self, y): |
| """ Returns y*H, where H is the GCM key. """ |
| ret = 0 |
| # Multiply H by y 4 bits at a time, starting with the highest power |
| # terms. |
| for i in range(0, 128, 4): |
| # Multiply by x^4. The reduction for the top four terms is |
| # precomputed. |
| retHigh = ret & 0xf |
| ret >>= 4 |
| ret ^= (_gcmReductionTable[retHigh] << (128-16)) |
| |
| # Add in y' * H where y' are the next four terms of y, shifted down |
| # to the x^0..x^4. This is one of the pre-computed multiples of |
| # H. The multiplication by x^4 shifts them back into place. |
| ret ^= self._productTable[y & 0xf] |
| y >>= 4 |
| assert y == 0 |
| return ret |
| |
| def seal(self, nonce, plaintext, data): |
| """ |
| Encrypts and authenticates plaintext using nonce and data. Returns the |
| ciphertext, consisting of the encrypted plaintext and tag concatenated. |
| """ |
| |
| if len(nonce) != 12: |
| raise ValueError("Bad nonce length") |
| |
| # The initial counter value is the nonce, followed by a 32-bit counter |
| # that starts at 1. It's used to compute the tag mask. |
| counter = bytearray(16) |
| counter[:12] = nonce |
| counter[-1] = 1 |
| tagMask = self._rawAesEncrypt(counter) |
| |
| # The counter starts at 2 for the actual encryption. |
| counter[-1] = 2 |
| ciphertext = self._rawAesCtrEncrypt(counter, plaintext) |
| |
| tag = self._auth(ciphertext, data, tagMask) |
| |
| return ciphertext + tag |
| |
| def open(self, nonce, ciphertext, data): |
| """ |
| Decrypts and authenticates ciphertext using nonce and data. If the |
| tag is valid, the plaintext is returned. If the tag is invalid, |
| returns None. |
| """ |
| |
| if len(nonce) != 12: |
| raise ValueError("Bad nonce length") |
| if len(ciphertext) < 16: |
| return None |
| |
| tag = ciphertext[-16:] |
| ciphertext = ciphertext[:-16] |
| |
| # The initial counter value is the nonce, followed by a 32-bit counter |
| # that starts at 1. It's used to compute the tag mask. |
| counter = bytearray(16) |
| counter[:12] = nonce |
| counter[-1] = 1 |
| tagMask = self._rawAesEncrypt(counter) |
| |
| if tag != self._auth(ciphertext, data, tagMask): |
| return None |
| |
| # The counter starts at 2 for the actual decryption. |
| counter[-1] = 2 |
| return self._rawAesCtrEncrypt(counter, ciphertext) |
| |
| def _reverseBits(i): |
| assert i < 16 |
| i = ((i << 2) & 0xc) | ((i >> 2) & 0x3) |
| i = ((i << 1) & 0xa) | ((i >> 1) & 0x5) |
| return i |
| |
| def _gcmAdd(x, y): |
| return x ^ y |
| |
| def _gcmShift(x): |
| # Multiplying by x is a right shift, due to bit order. |
| highTermSet = x & 1 |
| x >>= 1 |
| if highTermSet: |
| # The x^127 term was shifted up to x^128, so subtract a 1+x+x^2+x^7 |
| # term. This is 0b11100001 or 0xe1 when represented as an 8-bit |
| # polynomial. |
| x ^= 0xe1 << (128-8) |
| return x |
| |
| def _inc32(counter): |
| for i in range(len(counter)-1, len(counter)-5, -1): |
| counter[i] = (counter[i] + 1) % 256 |
| if counter[i] != 0: |
| break |
| return counter |
| |
| # _gcmReductionTable[i] is i * (1+x+x^2+x^7) for all 4-bit polynomials i. The |
| # result is stored as a 16-bit polynomial. This is used in the reduction step to |
| # multiply elements of GF(2^128) by x^4. |
| _gcmReductionTable = [ |
| 0x0000, 0x1c20, 0x3840, 0x2460, 0x7080, 0x6ca0, 0x48c0, 0x54e0, |
| 0xe100, 0xfd20, 0xd940, 0xc560, 0x9180, 0x8da0, 0xa9c0, 0xb5e0, |
| ] |