otp_crypto/hkdf library

OTP Crypto – HKDF-SHA256 (extract + expand)

Implements HKDF (RFC 5869) over SHA-256 in pure Dart using our HMAC construction (HmacSha256). Used to derive two 32-byte keys:

• enc_key (AES-256 key)
• mac_key (HMAC-SHA256 key)

SCHEME:

• PRK = HMAC(salt, IKM) // extract
• OKM = T(1) || T(2) || … up to L bytes // expand where T(i) = HMAC(PRK, T(i-1) || info || i)

NOTES:

• HashLen = 32 (SHA-256)
• Max output length L ≤ 255 * HashLen (per RFC 5869).
• If salt is null/empty, RFC recommends a zero-array of HashLen.
• We derive 64 bytes once and split: enc_key = first 32, mac_key = next 32.

HINTS:

• Use deriveKeys(...) for the common enc/mac pair derivation.
• Keep inputs as bytes; avoid string conversions for secrets.