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>
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