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beardthelion
9a46e8cbb7
x25519_secret_from_seed derived the X25519 private scalar from the Ed25519 seed and returned it as a bare [u8; 32]. Because [u8; 32] is Copy with no Drop (and sha2's digest output likewise has no zeroize), the scalar, the SHA-512 digest, and the returned temporary were released without being scrubbed, leaving secret-derived material in freed memory. This is the same Copy-no-Drop class fixed for the raw seed in #41, on the derived secret. Return Zeroizing<[u8; 32]>, build the scalar directly into a zeroizing buffer so no bare secret local persists, and explicitly wipe the SHA-512 digest before it drops. Callers deref the result into crypto_box::SecretKey, which already scrubs its own copy. No behavior change: the derived scalar and all decryption output are byte-identical, covered by the existing ed25519_to_x25519_keypair_agrees and seal_open_round_trip_for_recipients tests. Stacked on #41 (#64).