use HChaCha20 KDF for 128-bit key expansion in ChaCha20Poly1305#59
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Replace the naive K128 | K128 key doubling with HChaCha20-based key derivation to avoid any structural relation between the two 128-bit halves of the expanded 256-bit key. The two halves are now derived independently using distinct nonces (1 and 2), ensuring the expanded key is indistinguishable from a uniformly random 256-bit key under the PRF security of HChaCha20. Signed-off-by: Andreas Auernhammer <git@aead.dev>
klauspost
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Apr 29, 2026
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Yeah, my initial through was to "stretch" with sha256, but this is much cleaner and doesn't bring in anything new.
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Replace the naive K128 | K128 key doubling with HChaCha20-based key derivation to avoid any structural relation between the two 128-bit halves of the expanded 256-bit key. The two halves are now derived independently using distinct nonces (1 and 2), ensuring the expanded key is indistinguishable from a uniformly random 256-bit key under the PRF security of HChaCha20.