nx_hotp_sha1.nx

hotp_sha1.nx -- RFC 4226 HOTP with SHA-1. Counter-based OTP. Used by: - Yubico OTP hardware tokens (YubiKey in HOTP mode) - Microsoft ActiveSync + Exchange legacy provisioning - Some corporate 2FA where server tracks a per-user counter TOTP (totp_sha1.nx) is the time-based variant that builds on HOTP by setting counter = floor(unix_time / step). HOTP is the lower-level primitive. Algorithm: HOTP(K, C) = DT(HMAC-SHA-1(K, C_be64)) mod 10^digits DT(mac) = big-endian u32 at offset (mac[19] & 0x0F) with high bit of first byte masked off Invariants: H1 Counter is serialised as 8-byte big-endian u64. H2 Dynamic truncation: offset = low 4 bits of mac[19] (valid 0..15; always 4 bytes left in 20-byte MAC). H3 Output value in range [0, 10^digits).