Ava Chow 288018131e Merge bitcoin/bitcoin#35254: crypto: cleanse HMAC stack buffers after use and ChainCode
21a1380c13 key: cleanse ChainCode on destruction (Thomas)
b3a3f88346 crypto: cleanse HMAC stack buffers after use (Thomas)

Pull request description:

  `CHMAC_SHA256` and `CHMAC_SHA512` leave two stack buffers populated on return: `rkey[]` holds `K' ⊕ ipad` after the constructor, and `temp[]` holds the inner-hash output after `Finalize()`.

  When the HMAC is keyed with sensitive material (chain code in `BIP32Hash()` in `hash.cpp` for BIP32 child key derivation; PRK in HKDF-Expand in `hkdf_sha256_32.cpp`, used for BIP324 transport keying), `rkey` is one constant XOR from that key, and `temp` is a one-way digest covering it.

  This PR cleanses both buffers with `memory_cleanse()`, matching the convention already used in `chacha20.cpp` and `chacha20poly1305.cpp`. No observable change for callers.

  Update: Cleansing the HMAC primitive's internal buffers still leaves a caller's `ChainCode` value populated in memory after use. The second commit promotes `ChainCode` from `typedef uint256` to a `base_blob<256>` subclass with a `memory_cleanse()` destructor, so chain codes in `CExtKey`, `CExtPubKey`, and local variables are cleansed on scope exit. `MUSIG_CHAINCODE` is retyped from `constexpr uint256` to `const ChainCode` to match its BIP328 semantic role; this also removes the GCC-14 consteval lambda workaround.

ACKs for top commit:
  davidgumberg:
    crACK 21a1380c13
  optout21:
    ACK 21a1380c13
  achow101:
    ACK 21a1380c13
  winterrdog:
    ACK 21a1380c13

Tree-SHA512: 022c8372da3e2c9c269ef55b695d8415241acf64be04692f30da0e682dd1d05178f95601a3bd208573fd0630656b3dedcf6de34a2a3cf794515c0268e710af75
2026-06-10 10:47:16 -07:00
2026-02-06 13:40:59 +00:00
2026-06-04 10:09:19 +01:00
2026-06-08 15:16:29 +02:00
2026-05-29 08:59:43 +01:00
2026-04-29 21:50:13 +01:00
2025-12-29 17:50:43 +00:00
2025-06-19 11:22:14 +01:00

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