diff --git a/cmd_chat/client/_srp_pure.py b/cmd_chat/client/_srp_pure.py new file mode 100644 index 0000000..bc728a8 --- /dev/null +++ b/cmd_chat/client/_srp_pure.py @@ -0,0 +1,336 @@ +"""Pure-Python SRP-6a client — a drop-in fallback for the ``srp`` PyPI package. + +The ``srp`` PyPI package is a C extension with no reliable aarch64 wheel, so it +often fails to build under Termux/Android. This module reproduces the *exact* +client-side derivation of that package's pure-Python reference (``srp._pysrp``) +with ``rfc5054_enable()`` active, using only the standard library. + +Only the surface ``cmd_chat/client/client.py`` consumes is provided: + + rfc5054_enable() # module-level toggle + SHA256 # hash-alg constant + usr = User(b"chat", password, hash_alg=SHA256) + _, A = usr.start_authentication() # -> (username, A_bytes) + M = usr.process_challenge(salt, B) # -> M_bytes or None + usr.verify_session(H_AMK) # sets authenticated flag + usr.authenticated() # -> bool + +The server uses the real ``srp`` library (see cmd_chat/server/srp_auth.py), so +every constant, padding rule, and hash input here mirrors ``srp._pysrp`` byte +for byte — otherwise the M1 / H_AMK proofs would not match and auth would fail. +Correctness is gated by tests/test_srp_pure_interop.py. +""" + +import hashlib +import os + +# ── RFC 5054 compatibility toggle ──────────────────────────────────────────── +# Mirrors srp._pysrp: when enabled, integer hash inputs are zero-padded to the +# width of N and the generator is zero-padded to N's length inside H(N, g). +_rfc5054_compat = False +_no_username_in_x = False + + +def rfc5054_enable(enable: bool = True) -> None: + global _rfc5054_compat + _rfc5054_compat = enable + + +def no_username_in_x(enable: bool = True) -> None: + global _no_username_in_x + _no_username_in_x = enable + + +# ── Hash-algorithm constants (values match srp._pysrp exactly) ─────────────── +SHA1 = 0 +SHA224 = 1 +SHA256 = 2 +SHA384 = 3 +SHA512 = 4 + +# ── N/g group selectors (values match srp._pysrp exactly) ──────────────────── +NG_1024 = 0 +NG_2048 = 1 +NG_4096 = 2 +NG_8192 = 3 +NG_CUSTOM = 4 + +_hash_map = { + SHA1: hashlib.sha1, + SHA224: hashlib.sha224, + SHA256: hashlib.sha256, + SHA384: hashlib.sha384, + SHA512: hashlib.sha512, +} + +# The RFC 5054 groups, copied verbatim from srp._pysrp so the numeric N/g are +# bit-identical to the server's. Only NG_2048 is used by the hack-house client +# (User's default ng_type), but the full table is kept for parity. +_ng_const = ( + # 1024-bit + ( + "EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256576D674DF7496" + "EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D6089DAD15DC7D7B46154D6B6CE8E" + "F4AD69B15D4982559B297BCF1885C529F566660E57EC68EDBC3C05726CC02FD4CBF4976EAA" + "9AFD5138FE8376435B9FC61D2FC0EB06E3", + "2", + ), + # 2048-bit + ( + "AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050A37329CBB4" + "A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50E8083969EDB767B0CF60" + "95179A163AB3661A05FBD5FAAAE82918A9962F0B93B855F97993EC975EEAA80D740ADBF4FF" + "747359D041D5C33EA71D281E446B14773BCA97B43A23FB801676BD207A436C6481F1D2B907" + "8717461A5B9D32E688F87748544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB37861" + "60279004E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DB" + "FBB694B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73", + "2", + ), + # 4096-bit + ( + "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08" + "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B" + "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9" + "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6" + "49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8" + "FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D" + "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C" + "180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718" + "3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D" + "04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D" + "B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226" + "1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C" + "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC" + "E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26" + "99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB" + "04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2" + "233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127" + "D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199" + "FFFFFFFFFFFFFFFF", + "5", + ), + # 8192-bit + ( + "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08" + "8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B" + "302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9" + "A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6" + "49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8" + "FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D" + "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C" + "180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718" + "3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D" + "04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D" + "B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226" + "1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C" + "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC" + "E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26" + "99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB" + "04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2" + "233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127" + "D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934028492" + "36C3FAB4D27C7026C1D4DCB2602646DEC9751E763DBA37BDF8FF9406" + "AD9E530EE5DB382F413001AEB06A53ED9027D831179727B0865A8918" + "DA3EDBEBCF9B14ED44CE6CBACED4BB1BDB7F1447E6CC254B33205151" + "2BD7AF426FB8F401378CD2BF5983CA01C64B92ECF032EA15D1721D03" + "F482D7CE6E74FEF6D55E702F46980C82B5A84031900B1C9E59E7C97F" + "BEC7E8F323A97A7E36CC88BE0F1D45B7FF585AC54BD407B22B4154AA" + "CC8F6D7EBF48E1D814CC5ED20F8037E0A79715EEF29BE32806A1D58B" + "B7C5DA76F550AA3D8A1FBFF0EB19CCB1A313D55CDA56C9EC2EF29632" + "387FE8D76E3C0468043E8F663F4860EE12BF2D5B0B7474D6E694F91E" + "6DBE115974A3926F12FEE5E438777CB6A932DF8CD8BEC4D073B931BA" + "3BC832B68D9DD300741FA7BF8AFC47ED2576F6936BA424663AAB639C" + "5AE4F5683423B4742BF1C978238F16CBE39D652DE3FDB8BEFC848AD9" + "22222E04A4037C0713EB57A81A23F0C73473FC646CEA306B4BCBC886" + "2F8385DDFA9D4B7FA2C087E879683303ED5BDD3A062B3CF5B3A278A6" + "6D2A13F83F44F82DDF310EE074AB6A364597E899A0255DC164F31CC5" + "0846851DF9AB48195DED7EA1B1D510BD7EE74D73FAF36BC31ECFA268" + "359046F4EB879F924009438B481C6CD7889A002ED5EE382BC9190DA6" + "FC026E479558E4475677E9AA9E3050E2765694DFC81F56E880B96E71" + "60C980DD98EDD3DFFFFFFFFFFFFFFFFF", + "0x13", + ), +) + + +def _get_ng(ng_type: int, n_hex, g_hex): + if ng_type < NG_CUSTOM: + n_hex, g_hex = _ng_const[ng_type] + return int(n_hex, 16), int(g_hex, 16) + + +# ── Integer <-> big-endian byte conversions (match srp._pysrp exactly) ─────── +def _bytes_to_long(s: bytes) -> int: + return int.from_bytes(s, "big") + + +def _long_to_bytes(n: int) -> bytes: + # srp._pysrp yields the minimal big-endian encoding with no leading zero + # byte, and b"" for zero. int.to_bytes with the exact bit-length does the + # same for n > 0; special-case 0 to match the empty-string behaviour. + if n == 0: + return b"" + return n.to_bytes((n.bit_length() + 7) // 8, "big") + + +def _get_random(nbytes: int) -> int: + return _bytes_to_long(os.urandom(nbytes)) + + +def _get_random_of_length(nbytes: int) -> int: + offset = (nbytes * 8) - 1 + return _get_random(nbytes) | (1 << offset) + + +def _H(hash_class, *args, width=None) -> bytes: + """Concatenating hash. Integer args become minimal big-endian bytes; when + ``width`` is set and RFC 5054 mode is on, each arg is left zero-padded to + ``width`` bytes before hashing (mirrors srp._pysrp.H).""" + h = hash_class() + for s in args: + if s is None: + continue + data = _long_to_bytes(s) if isinstance(s, int) else s + if width is not None and _rfc5054_compat: + h.update(bytes(width - len(data))) + h.update(data) + return h.digest() + + +def _HNxorg(hash_class, N: int, g: int) -> bytes: + bin_N = _long_to_bytes(N) + bin_g = _long_to_bytes(g) + padding = len(bin_N) - len(bin_g) if _rfc5054_compat else 0 + hN = hash_class(bin_N).digest() + hg = hash_class(b"".join([b"\0" * padding, bin_g])).digest() + return bytes(hN[i] ^ hg[i] for i in range(len(hN))) + + +def _gen_x(hash_class, salt: bytes, username, password) -> int: + username = username.encode() if hasattr(username, "encode") else username + password = password.encode() if hasattr(password, "encode") else password + if _no_username_in_x: + username = b"" + return _bytes_to_long( + _H(hash_class, salt, _H(hash_class, username + b":" + password)) + ) + + +def _calculate_M(hash_class, N, g, I, s, A, B, K) -> bytes: + I = I.encode() if hasattr(I, "encode") else I + h = hash_class() + h.update(_HNxorg(hash_class, N, g)) + h.update(hash_class(I).digest()) + if isinstance(s, int): + s = _long_to_bytes(s) + h.update(s) + h.update(_long_to_bytes(A)) + h.update(_long_to_bytes(B)) + h.update(K) + return h.digest() + + +def _calculate_H_AMK(hash_class, A, M, K) -> bytes: + h = hash_class() + h.update(_long_to_bytes(A)) + h.update(M) + h.update(K) + return h.digest() + + +class User: + """Client side of the SRP-6a exchange, matching srp._pysrp.User's surface.""" + + def __init__( + self, + username, + password, + hash_alg=SHA1, + ng_type=NG_2048, + n_hex=None, + g_hex=None, + bytes_a=None, + bytes_A=None, + k_hex=None, + ): + if ng_type == NG_CUSTOM and (n_hex is None or g_hex is None): + raise ValueError("Both n_hex and g_hex are required when ng_type = NG_CUSTOM") + if bytes_a and len(bytes_a) != 256: + raise ValueError("256 bytes required for bytes_a") + + N, g = _get_ng(ng_type, n_hex, g_hex) + hash_class = _hash_map[hash_alg] + if k_hex is None: + k = _bytes_to_long(_H(hash_class, N, g, width=len(_long_to_bytes(N)))) + else: + k = int(k_hex, 16) + + self.I = username + self.p = password + if bytes_a: + self.a = _bytes_to_long(bytes_a) + else: + self.a = _get_random_of_length(256) + if bytes_A: + self.A = _bytes_to_long(bytes_A) + else: + self.A = pow(g, self.a, N) + self.v = None + self.M = None + self.K = None + self.H_AMK = None + self._authenticated = False + + self.hash_class = hash_class + self.N = N + self.g = g + self.k = k + + def authenticated(self) -> bool: + return self._authenticated + + def get_username(self): + return self.I + + def get_ephemeral_secret(self) -> bytes: + return _long_to_bytes(self.a) + + def get_session_key(self): + return self.K if self._authenticated else None + + def start_authentication(self): + return (self.I, _long_to_bytes(self.A)) + + def process_challenge(self, bytes_s, bytes_B): + """Returns M (bytes) or None if an SRP-6a safety check is violated.""" + self.s = bytes_s + self.B = _bytes_to_long(bytes_B) + + N = self.N + g = self.g + k = self.k + hash_class = self.hash_class + + # SRP-6a safety check + if (self.B % N) == 0: + return None + + self.u = _bytes_to_long(_H(hash_class, self.A, self.B, width=len(_long_to_bytes(N)))) + + # SRP-6a safety check + if self.u == 0: + return None + + self.x = _gen_x(hash_class, self.s, self.I, self.p) + self.v = pow(g, self.x, N) + self.S = pow((self.B - k * self.v), (self.a + self.u * self.x), N) + + self.K = hash_class(_long_to_bytes(self.S)).digest() + self.M = _calculate_M(hash_class, N, g, self.I, self.s, self.A, self.B, self.K) + self.H_AMK = _calculate_H_AMK(hash_class, self.A, self.M, self.K) + + return self.M + + def verify_session(self, host_HAMK) -> None: + if self.H_AMK == host_HAMK: + self._authenticated = True