otp.nx

otp.nx -- HOTP (RFC 4226) + TOTP (RFC 6238) one-time passwords. Powers the 6-digit codes in Google Authenticator / Authy / Yubico Authenticator / Microsoft Authenticator / 1Password. HOTP = HMAC(key, counter) -> 6-digit code (event-counter based) TOTP = HOTP(key, floor(time / period)) -- time-based variant Standard RFC profiles use HMAC-SHA-1 for compatibility. We ship HMAC-SHA-256 variants here because: - SHA-1 is deprecated for new systems (rule: don't encourage) - RFC 6238 §5 explicitly permits SHA-256 / SHA-512 - All major authenticator apps support SHA-256 profiles Callers interoperating with legacy SHA-1 providers need a separate hotp_sha1.nx we're not shipping. Algorithm (RFC 4226): 1. HMAC = HMAC-SHA-256(key, counter as u64 big-endian) 2. offset = HMAC[31] & 0x0F (dynamic truncation) 3. truncated = read 4 bytes BE at HMAC[offset..offset+4] 4. truncated &= 0x7FFFFFFF (clear top bit) 5. code = truncated mod 10^digits Invariants: OTP1 digits in [6, 10]; most deployments use 6. We don't enforce; caller picks. OTP2 Counter is u64 big-endian; identical on every platform. OTP3 Code is zero-padded to `digits` length on render.