feat(client): pure-Python SRP-6a fallback for Termux/aarch64
The `srp` PyPI package is a C extension with no reliable aarch64 wheel and often fails to build under Termux. Vendor _srp_pure.py, a stdlib-only port of srp._pysrp with rfc5054_enable() active, exposing the exact client surface client.py uses (rfc5054_enable, SHA256, User with start_authentication / process_challenge / verify_session / authenticated). Constants (N/g/k), x/u derivation, M1 and H_AMK match srp byte-for-byte — verified by a live handshake of this pure User against the real srp Verifier (the same backend the server runs). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
leetcrypt
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"""Pure-Python SRP-6a client — a drop-in fallback for the ``srp`` PyPI package.
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The ``srp`` PyPI package is a C extension with no reliable aarch64 wheel, so it
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often fails to build under Termux/Android. This module reproduces the *exact*
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client-side derivation of that package's pure-Python reference (``srp._pysrp``)
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with ``rfc5054_enable()`` active, using only the standard library.
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Only the surface ``cmd_chat/client/client.py`` consumes is provided:
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rfc5054_enable() # module-level toggle
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SHA256 # hash-alg constant
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usr = User(b"chat", password, hash_alg=SHA256)
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_, A = usr.start_authentication() # -> (username, A_bytes)
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M = usr.process_challenge(salt, B) # -> M_bytes or None
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usr.verify_session(H_AMK) # sets authenticated flag
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usr.authenticated() # -> bool
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The server uses the real ``srp`` library (see cmd_chat/server/srp_auth.py), so
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every constant, padding rule, and hash input here mirrors ``srp._pysrp`` byte
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for byte — otherwise the M1 / H_AMK proofs would not match and auth would fail.
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Correctness is gated by tests/test_srp_pure_interop.py.
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"""
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import hashlib
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import os
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# ── RFC 5054 compatibility toggle ────────────────────────────────────────────
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# Mirrors srp._pysrp: when enabled, integer hash inputs are zero-padded to the
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# width of N and the generator is zero-padded to N's length inside H(N, g).
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_rfc5054_compat = False
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_no_username_in_x = False
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def rfc5054_enable(enable: bool = True) -> None:
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global _rfc5054_compat
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_rfc5054_compat = enable
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def no_username_in_x(enable: bool = True) -> None:
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global _no_username_in_x
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_no_username_in_x = enable
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# ── Hash-algorithm constants (values match srp._pysrp exactly) ───────────────
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SHA1 = 0
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SHA224 = 1
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SHA256 = 2
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SHA384 = 3
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SHA512 = 4
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# ── N/g group selectors (values match srp._pysrp exactly) ────────────────────
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NG_1024 = 0
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NG_2048 = 1
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NG_4096 = 2
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NG_8192 = 3
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NG_CUSTOM = 4
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_hash_map = {
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SHA1: hashlib.sha1,
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SHA224: hashlib.sha224,
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SHA256: hashlib.sha256,
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SHA384: hashlib.sha384,
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SHA512: hashlib.sha512,
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}
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# The RFC 5054 groups, copied verbatim from srp._pysrp so the numeric N/g are
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# bit-identical to the server's. Only NG_2048 is used by the hack-house client
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# (User's default ng_type), but the full table is kept for parity.
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_ng_const = (
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# 1024-bit
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(
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"EEAF0AB9ADB38DD69C33F80AFA8FC5E86072618775FF3C0B9EA2314C9C256576D674DF7496"
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"EA81D3383B4813D692C6E0E0D5D8E250B98BE48E495C1D6089DAD15DC7D7B46154D6B6CE8E"
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"F4AD69B15D4982559B297BCF1885C529F566660E57EC68EDBC3C05726CC02FD4CBF4976EAA"
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"9AFD5138FE8376435B9FC61D2FC0EB06E3",
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"2",
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),
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# 2048-bit
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(
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"AC6BDB41324A9A9BF166DE5E1389582FAF72B6651987EE07FC3192943DB56050A37329CBB4"
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"A099ED8193E0757767A13DD52312AB4B03310DCD7F48A9DA04FD50E8083969EDB767B0CF60"
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"95179A163AB3661A05FBD5FAAAE82918A9962F0B93B855F97993EC975EEAA80D740ADBF4FF"
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"747359D041D5C33EA71D281E446B14773BCA97B43A23FB801676BD207A436C6481F1D2B907"
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"8717461A5B9D32E688F87748544523B524B0D57D5EA77A2775D2ECFA032CFBDBF52FB37861"
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"60279004E57AE6AF874E7303CE53299CCC041C7BC308D82A5698F3A8D0C38271AE35F8E9DB"
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"FBB694B5C803D89F7AE435DE236D525F54759B65E372FCD68EF20FA7111F9E4AFF73",
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"2",
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),
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# 4096-bit
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(
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"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
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"8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
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"302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
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"A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
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"49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8"
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"FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D"
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"670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C"
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"180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718"
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"3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D"
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"04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D"
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"B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226"
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"1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
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"BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC"
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"E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26"
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"99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB"
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"04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2"
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"233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127"
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"D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934063199"
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"FFFFFFFFFFFFFFFF",
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"5",
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),
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# 8192-bit
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(
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"FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD129024E08"
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"8A67CC74020BBEA63B139B22514A08798E3404DDEF9519B3CD3A431B"
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"302B0A6DF25F14374FE1356D6D51C245E485B576625E7EC6F44C42E9"
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"A637ED6B0BFF5CB6F406B7EDEE386BFB5A899FA5AE9F24117C4B1FE6"
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"49286651ECE45B3DC2007CB8A163BF0598DA48361C55D39A69163FA8"
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"FD24CF5F83655D23DCA3AD961C62F356208552BB9ED529077096966D"
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"670C354E4ABC9804F1746C08CA18217C32905E462E36CE3BE39E772C"
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"180E86039B2783A2EC07A28FB5C55DF06F4C52C9DE2BCBF695581718"
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"3995497CEA956AE515D2261898FA051015728E5A8AAAC42DAD33170D"
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"04507A33A85521ABDF1CBA64ECFB850458DBEF0A8AEA71575D060C7D"
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"B3970F85A6E1E4C7ABF5AE8CDB0933D71E8C94E04A25619DCEE3D226"
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"1AD2EE6BF12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
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"BBE117577A615D6C770988C0BAD946E208E24FA074E5AB3143DB5BFC"
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"E0FD108E4B82D120A92108011A723C12A787E6D788719A10BDBA5B26"
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"99C327186AF4E23C1A946834B6150BDA2583E9CA2AD44CE8DBBBC2DB"
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"04DE8EF92E8EFC141FBECAA6287C59474E6BC05D99B2964FA090C3A2"
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"233BA186515BE7ED1F612970CEE2D7AFB81BDD762170481CD0069127"
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"D5B05AA993B4EA988D8FDDC186FFB7DC90A6C08F4DF435C934028492"
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"36C3FAB4D27C7026C1D4DCB2602646DEC9751E763DBA37BDF8FF9406"
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"AD9E530EE5DB382F413001AEB06A53ED9027D831179727B0865A8918"
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"DA3EDBEBCF9B14ED44CE6CBACED4BB1BDB7F1447E6CC254B33205151"
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"2BD7AF426FB8F401378CD2BF5983CA01C64B92ECF032EA15D1721D03"
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"F482D7CE6E74FEF6D55E702F46980C82B5A84031900B1C9E59E7C97F"
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"BEC7E8F323A97A7E36CC88BE0F1D45B7FF585AC54BD407B22B4154AA"
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"CC8F6D7EBF48E1D814CC5ED20F8037E0A79715EEF29BE32806A1D58B"
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"B7C5DA76F550AA3D8A1FBFF0EB19CCB1A313D55CDA56C9EC2EF29632"
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"387FE8D76E3C0468043E8F663F4860EE12BF2D5B0B7474D6E694F91E"
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"6DBE115974A3926F12FEE5E438777CB6A932DF8CD8BEC4D073B931BA"
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"3BC832B68D9DD300741FA7BF8AFC47ED2576F6936BA424663AAB639C"
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"5AE4F5683423B4742BF1C978238F16CBE39D652DE3FDB8BEFC848AD9"
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"22222E04A4037C0713EB57A81A23F0C73473FC646CEA306B4BCBC886"
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"2F8385DDFA9D4B7FA2C087E879683303ED5BDD3A062B3CF5B3A278A6"
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"6D2A13F83F44F82DDF310EE074AB6A364597E899A0255DC164F31CC5"
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"0846851DF9AB48195DED7EA1B1D510BD7EE74D73FAF36BC31ECFA268"
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"359046F4EB879F924009438B481C6CD7889A002ED5EE382BC9190DA6"
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"FC026E479558E4475677E9AA9E3050E2765694DFC81F56E880B96E71"
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"60C980DD98EDD3DFFFFFFFFFFFFFFFFF",
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"0x13",
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),
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)
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def _get_ng(ng_type: int, n_hex, g_hex):
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if ng_type < NG_CUSTOM:
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n_hex, g_hex = _ng_const[ng_type]
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return int(n_hex, 16), int(g_hex, 16)
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# ── Integer <-> big-endian byte conversions (match srp._pysrp exactly) ───────
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def _bytes_to_long(s: bytes) -> int:
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return int.from_bytes(s, "big")
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def _long_to_bytes(n: int) -> bytes:
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# srp._pysrp yields the minimal big-endian encoding with no leading zero
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# byte, and b"" for zero. int.to_bytes with the exact bit-length does the
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# same for n > 0; special-case 0 to match the empty-string behaviour.
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if n == 0:
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return b""
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return n.to_bytes((n.bit_length() + 7) // 8, "big")
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def _get_random(nbytes: int) -> int:
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return _bytes_to_long(os.urandom(nbytes))
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def _get_random_of_length(nbytes: int) -> int:
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offset = (nbytes * 8) - 1
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return _get_random(nbytes) | (1 << offset)
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def _H(hash_class, *args, width=None) -> bytes:
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"""Concatenating hash. Integer args become minimal big-endian bytes; when
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``width`` is set and RFC 5054 mode is on, each arg is left zero-padded to
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``width`` bytes before hashing (mirrors srp._pysrp.H)."""
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h = hash_class()
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for s in args:
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if s is None:
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continue
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data = _long_to_bytes(s) if isinstance(s, int) else s
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if width is not None and _rfc5054_compat:
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h.update(bytes(width - len(data)))
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h.update(data)
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return h.digest()
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def _HNxorg(hash_class, N: int, g: int) -> bytes:
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bin_N = _long_to_bytes(N)
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bin_g = _long_to_bytes(g)
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padding = len(bin_N) - len(bin_g) if _rfc5054_compat else 0
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hN = hash_class(bin_N).digest()
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hg = hash_class(b"".join([b"\0" * padding, bin_g])).digest()
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return bytes(hN[i] ^ hg[i] for i in range(len(hN)))
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def _gen_x(hash_class, salt: bytes, username, password) -> int:
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username = username.encode() if hasattr(username, "encode") else username
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password = password.encode() if hasattr(password, "encode") else password
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if _no_username_in_x:
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username = b""
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return _bytes_to_long(
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_H(hash_class, salt, _H(hash_class, username + b":" + password))
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)
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def _calculate_M(hash_class, N, g, I, s, A, B, K) -> bytes:
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I = I.encode() if hasattr(I, "encode") else I
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h = hash_class()
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h.update(_HNxorg(hash_class, N, g))
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h.update(hash_class(I).digest())
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if isinstance(s, int):
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s = _long_to_bytes(s)
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h.update(s)
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h.update(_long_to_bytes(A))
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h.update(_long_to_bytes(B))
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h.update(K)
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return h.digest()
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def _calculate_H_AMK(hash_class, A, M, K) -> bytes:
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h = hash_class()
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h.update(_long_to_bytes(A))
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h.update(M)
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h.update(K)
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return h.digest()
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class User:
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"""Client side of the SRP-6a exchange, matching srp._pysrp.User's surface."""
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def __init__(
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self,
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username,
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password,
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hash_alg=SHA1,
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ng_type=NG_2048,
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n_hex=None,
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g_hex=None,
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bytes_a=None,
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bytes_A=None,
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k_hex=None,
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):
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if ng_type == NG_CUSTOM and (n_hex is None or g_hex is None):
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raise ValueError("Both n_hex and g_hex are required when ng_type = NG_CUSTOM")
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if bytes_a and len(bytes_a) != 256:
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raise ValueError("256 bytes required for bytes_a")
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N, g = _get_ng(ng_type, n_hex, g_hex)
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hash_class = _hash_map[hash_alg]
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if k_hex is None:
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k = _bytes_to_long(_H(hash_class, N, g, width=len(_long_to_bytes(N))))
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else:
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k = int(k_hex, 16)
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self.I = username
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self.p = password
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if bytes_a:
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self.a = _bytes_to_long(bytes_a)
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else:
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self.a = _get_random_of_length(256)
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if bytes_A:
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self.A = _bytes_to_long(bytes_A)
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else:
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self.A = pow(g, self.a, N)
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self.v = None
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self.M = None
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self.K = None
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self.H_AMK = None
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self._authenticated = False
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self.hash_class = hash_class
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self.N = N
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self.g = g
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self.k = k
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def authenticated(self) -> bool:
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return self._authenticated
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def get_username(self):
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return self.I
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def get_ephemeral_secret(self) -> bytes:
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return _long_to_bytes(self.a)
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def get_session_key(self):
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return self.K if self._authenticated else None
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def start_authentication(self):
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return (self.I, _long_to_bytes(self.A))
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def process_challenge(self, bytes_s, bytes_B):
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"""Returns M (bytes) or None if an SRP-6a safety check is violated."""
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self.s = bytes_s
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self.B = _bytes_to_long(bytes_B)
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N = self.N
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g = self.g
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k = self.k
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hash_class = self.hash_class
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# SRP-6a safety check
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if (self.B % N) == 0:
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return None
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self.u = _bytes_to_long(_H(hash_class, self.A, self.B, width=len(_long_to_bytes(N))))
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# SRP-6a safety check
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if self.u == 0:
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return None
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self.x = _gen_x(hash_class, self.s, self.I, self.p)
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self.v = pow(g, self.x, N)
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self.S = pow((self.B - k * self.v), (self.a + self.u * self.x), N)
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self.K = hash_class(_long_to_bytes(self.S)).digest()
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self.M = _calculate_M(hash_class, N, g, self.I, self.s, self.A, self.B, self.K)
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self.H_AMK = _calculate_H_AMK(hash_class, self.A, self.M, self.K)
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return self.M
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def verify_session(self, host_HAMK) -> None:
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if self.H_AMK == host_HAMK:
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self._authenticated = True
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Block a user