Avoid using PyCryptodome for AES-SIV encryption of zero-length plaintext

lib_cryptomator_utils/aes.py

@@ -14,8 +14,11 @@
 #   You should have received a copy of the GNU Affero General Public License
 #   along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-from Crypto.Cipher import AES
 from cryptography.hazmat.primitives.ciphers.aead import AESSIV
+from cryptography.hazmat.primitives.ciphers.algorithms import AES
+from cryptography.hazmat.primitives.cmac import CMAC
+
+import struct
 
 def aes_siv_encrypt(primary_master_key, hmac_master_key, plaintext, associated_data):
 	"""
@@ -23,33 +26,72 @@ def aes_siv_encrypt(primary_master_key, hmac_master_key, plaintext, associated_d
 	"""
 	
 	if len(plaintext) == 0:
-		# Must use PyCryptodome
 		# https://github.com/pyca/cryptography/issues/10958 - cryptography AESSIV does not accept empty plaintext (e.g. root directory has empty directory ID)
-		if associated_data:
-			if len(associated_data) > 1:
-				# Incompatible with PyCryptodome
-				raise ValueError('Cannot encrypt zero-length plaintext with AES-SIV with >1 associated data')
-			
-			if not associated_data[0]:
-				# Incompatible with PyCryptodome
-				raise ValueError('Cannot encrypt zero-length plaintext with AES-SIV with zero-length associated data')
-			
-			# If there is only one associated data, this is equivalent to the nonce, so we can use PyCryptodome
-			ciphertext, tag = AES.new(hmac_master_key + primary_master_key, AES.MODE_SIV, nonce=associated_data[0]).encrypt_and_digest(plaintext)
-			return tag + ciphertext
 		
-		# Zero-length plaintext with no AAD - encrypt with PyCryptodome
-		ciphertext, tag = AES.new(hmac_master_key + primary_master_key, AES.MODE_SIV).encrypt_and_digest(plaintext)
-		return tag + ciphertext
+		# Manually calculate the synthetic IV
+		siv = aes_siv_s2v(hmac_master_key, plaintext, associated_data)
+		
+		# Empty plaintext equals empty ciphertext, so result is just the SIV
+		return siv
 	
 	# In all other cases, use cryptography AESSIV
-	tag_and_ciphertext = AESSIV(hmac_master_key + primary_master_key).encrypt(plaintext, associated_data)
-	return tag_and_ciphertext
+	siv_and_ciphertext = AESSIV(hmac_master_key + primary_master_key).encrypt(plaintext, associated_data)
+	return siv_and_ciphertext
 
-def aes_siv_decrypt(primary_master_key, hmac_master_key, tag_and_ciphertext, associated_data):
+def aes_siv_decrypt(primary_master_key, hmac_master_key, siv_and_ciphertext, associated_data):
 	"""
 	Decrypt the given AES-SIV ciphertext
 	"""
 	
-	# Use cryptography AESSIV
-	return AESSIV(hmac_master_key + primary_master_key).decrypt(tag_and_ciphertext, associated_data)
+	return AESSIV(hmac_master_key + primary_master_key).decrypt(siv_and_ciphertext, associated_data)
+
+def aes_cmac(hmac_master_key, data):
+	mac = CMAC(AES(hmac_master_key))
+	mac.update(data)
+	return mac.finalize()
+
+def aes_siv_dbl(data):
+	# Based on miscreant.py by Phil Rogaway, MIT License
+	
+	overflow = 0
+	words = struct.unpack(b'!LLLL', data)
+	output_words = []
+	
+	for word in reversed(words):
+		new_word = (word << 1) & 0xFFFFFFFF
+		new_word |= overflow
+		overflow = int((word & 0x80000000) >= 0x80000000)
+		output_words.append(new_word)
+	
+	result = bytearray(struct.pack(b'!LLLL', *reversed(output_words)))
+	if overflow:
+		result[-1] ^= 0x87  # Foot-gun! Not constant time
+	
+	return result
+
+def aes_siv_s2v(hmac_master_key, plaintext, associated_data):
+	# Based on miscreant.py by Phil Rogaway, MIT License
+	
+	# Note: The standalone S2V returns CMAC(1) if the number of passed vectors is zero, however in SIV construction this case is never triggered, since we always pass plaintext as the last vector, so we omit this case.
+	
+	d = bytes(128//8)  # 128-bit blocks
+	d = aes_cmac(hmac_master_key, d)
+	
+	if associated_data:
+		for ad in associated_data:
+			d = aes_siv_dbl(d)
+			d = bytes(x ^ y for x, y in zip(d, aes_cmac(hmac_master_key, ad)))  # d ^= aes_cmac(hmac_master_key, ad)
+	
+	if len(plaintext) >= 128//8:  # 128 bits
+		mac = CMAC(AES(hmac_master_key))
+		difference = len(plaintext) - 128/8
+		mac.update(plaintext[:difference])
+		d = bytes(x ^ y for x, y in zip(d, plaintext[difference:]))  # d ^= plaintext[difference:]
+		mac.update(d)
+		return mac.finalize()
+	
+	d = aes_siv_dbl(d)
+	for i in range(len(plaintext)):
+		d[i] ^= plaintext[i]
+	d[len(plaintext)] ^= 0x80
+	return aes_cmac(hmac_master_key, d)