Closes F27 per option (b) of its resolve criterion: fixed-width
U2048 DH backend using crypto-bigint's Montgomery-form residue
arithmetic.
New auth.rs::constant_time_mod_exp(base, exp, modulus) wrapper
preserves the BigUint-in-BigUint-out API of the existing byte
helpers; the actual square-and-multiply chain runs in Montgomery
form. Both DH call sites swap to the wrapper:
- AsbAuthenticator::new line 179 (public-key generation)
- crypto_key line 354 (shared-secret derivation)
DH private exponent timing-leak resistance is the goal: the .NET
reference's BigInteger.ModPow is also non-constant-time, so we
were at parity but not at the long-term Rust target. With this
fix the production path no longer leaks the bit-pattern of the
long-lived DH private key through power/timing side channels.
DynResidueParams::new requires an odd modulus (Montgomery form's
only restriction). Production DH primes are always odd
(`MX_ASB_DH_PRIME = 1552...7919` on this host's registry).
CryptoParameters::DEFAULT_PRIME_TEXT — the test-fixture default
inherited from AsbRegistry.cs:66 — ends in 4 (even), which is
mathematically unsound for DH but kept for parity with the .NET
default. For that case the wrapper falls back to BigUint::modpow,
preserving the wire bytes (modular exp is a pure function of
inputs).
Wire-byte parity verified two ways:
1. Unit fixture test
`auth.rs::deterministic_hmac_matches_dotnet_fixture` — byte-equal
to captured .NET output for the full DH → PBKDF2 → AES-CBC chain.
Continues to pass.
2. Live: Connect handshake against the local AVEVA install
completes with apollo:V2 lifetime, proving MxDataProvider
accepts the constant-time-derived public key and the
shared-secret-based AuthenticateMe.
Workspace deps:
- crypto-bigint = "0.5" added to [workspace.dependencies] and
mxaccess-asb-nettcp/Cargo.toml.
- num-bigint retained for decimal-string parsing + .NET-LE byte
conversion (crypto-bigint has neither).
Closes the "review.md MAJOR finding" originally flagged at
design/30-crate-topology.md:269-274.
design/followups.md: F27 moved to Resolved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
F18 plans M5 as 9 sub-followups (F18-F26 + F27 constant-time DH) per
design/dependencies.md:73-89. Wave-1 streams F20-F23+F24 are parallel-safe
after F19 (workspace deps). F25 (ASB client) is sequential after the
framing/encoder streams. F26 (Session over AsbTransport) is sequential
after F25.
F19 — workspace deps for the M5 crypto + framing surface: hmac, md-5,
sha1, sha2, aes, cbc, pbkdf2, flate2, rand, num-bigint, num-traits,
num-integer, quick-xml, tokio-util, zeroize. Pinned to the digest 0.10 /
cipher 0.4 generation matching mxaccess-rpc.
F23 — ports `AsbSystemAuthenticator.cs` (167 LoC) to
`mxaccess-asb-nettcp::auth`. Wire-byte parity points: .NET BigInteger
little-endian two's-complement byte order with optional 0x00 sign-byte
suffix; AES-128-CBC with PKCS7 padding; PBKDF2-SHA1 1000 iterations
over `Convert.ToBase64String(crypto_key)` with ASCII salt
"ArchestrAService"; deflate-then-AES (Baktun) vs raw-AES (Apollo)
selected by `:V2` lifetime suffix; HMAC-MD5/SHA1/SHA512 negotiated per
`AsbSolutionCryptoParameters.HashAlgorithm` (with `force_hmac=true`
fallback to HMAC-SHA1 for unrecognised algorithms).
13 unit tests cover the cryptographic primitives + DH peer agreement +
.NET byte-order round-trip + Apollo lifetime dispatch.
F27 — filed for the `num-bigint` → `crypto-bigint::BoxedUint` swap once
the latter exposes a stable heap-allocated `pow_mod`. Currently at
parity with the .NET reference (also not constant-time).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty