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crypto: harden all crypto-sensitive memcmp + memset sites

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Audit-driven sweep found additional compiler-optimization-sensitive
patterns beyond the login password compare just fixed:

P4.F3 (HIGH) — Utils::MACThenDecrypt verified packet MACs with
plain memcmp. Runs on EVERY encrypted-then-MAC'd packet in the
mesh; a timing oracle here lets attackers forge MACs byte-by-byte
across the whole mesh layer. Replaced with constantTimeEqual.

P4.F4 (MEDIUM) — Multiple memset(secret, 0, ...) calls on
stack-resident crypto buffers (Ed25519 seed, ADC noise pool, AES
key derived in extract_via_aes_ctr, HWINFO unique ID) were
subject to dead-store elimination under -Os. GCC/Clang routinely
elide these when the buffer is never read after; the wipe vanishes
and the secret persists on stack until next call overwrites.
Replaced with secureZeroize using volatile pointer writes.

P4.F5 (LOW) — Identity::validatePrivateKey boot self-test compared
shared secrets with plain memcmp. Boot-only, no attacker
observation channel, but hygiene matters and the fix is one line.
Also added secret-wipe for ss1/ss2 on all return paths.

Promoted the local ct_memeq() previously added to RepeaterMesh.cpp
into Utils::constantTimeEqual + Utils::secureZeroize (Utils.h/cpp)
so the login compare and MAC compare share the same audited helper.

Both helpers verified by Thumb-2 disassembly on rak3401_1watt:
- constantTimeEqual: loop branches on iterator, accumulator
  load-modify-stored to stack every iteration, final return uses
  CLZ+LSR (no conditional branch on result).
- secureZeroize: STRB.W to memory in a counted loop, not replaced
  with memset builtin and not eliminated.
This commit is contained in:
liquidraver
2026-05-28 13:26:20 +02:00
parent 88dccf2e24
commit b692ca72ed
7 changed files with 85 additions and 44 deletions
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zephcore/src/Identity.cpp
+14 -5
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@@ -82,12 +82,21 @@ bool LocalIdentity::validatePrivateKey(const uint8_t prv[64])
uint8_t ss1[32], ss2[32];
ed25519_key_exchange(ss1, test_client_pub, prv);
ed25519_key_exchange(ss2, pub, test_client_prv);
if (memcmp(ss1, ss2, 32) != 0) return false;
for (int i = 0; i < 32; i++) {
if (ss1[i] != 0) return true;
/* Constant-time even though this self-test runs at boot before
* any networking is up — hygiene + no attacker observation. */
if (!Utils::constantTimeEqual(ss1, ss2, 32)) {
Utils::secureZeroize(ss1, sizeof(ss1));
Utils::secureZeroize(ss2, sizeof(ss2));
return false;
}
return false;
bool nonzero = false;
for (int i = 0; i < 32; i++) {
if (ss1[i] != 0) { nonzero = true; break; }
}
Utils::secureZeroize(ss1, sizeof(ss1));
Utils::secureZeroize(ss2, sizeof(ss2));
return nonzero;
}
bool LocalIdentity::readFrom(const uint8_t *src, size_t len)