keep-core: zeroize NIP-49 ncryptsec assembly buffer on drop

[kwsantiago]

Ncryptsec::encrypt assembles the version/log_n/salt/nonce/security-byte/ciphertext into a plain Vec<u8> for bech32 encoding, then drops it without clearing. It never holds the plaintext key (that stays in the Zeroizing symmetric key and the secret_key slice), but the salt and nonce are worth clearing from memory rather than leaving them for a later heap read.

Change

• Wrap the assembly buffer in Zeroizing so it is cleared on drop. Vec::with_capacity(91) is the exact ncryptsec byte length, so no reallocation leaves an un-zeroed intermediate copy behind.

No behavior change: the bech32 output is byte-identical.

Verification

• cargo test -p keep-core --lib keys: passed (ncryptsec encrypt/decrypt round-trips). cargo clippy -p keep-core --all-targets: clean.

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📥 Commits

Reviewing files that changed from the base of the PR and between 89ee825 and fdba31a.

📒 Files selected for processing (1)

• keep-core/src/keys.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/nip49-zeroize-concat-buffer

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