Function HashOTP

internal/crypto/otp.go:15–19

HashOTP returns the hex-encoded HMAC-SHA256 of the OTP value under the supplied server key. OTPs are short-lived (minutes) so we do not need a reversible primitive — only the verifier needs to be able to recompute the digest from a candidate value. This means an offline DB dump no longer reveals usa

(otp, key string)

// the digest from a candidate value. This means an offline DB dump no
// longer reveals usable OTPs.
func HashOTP(otp, key string) string {
	h := hmac.New(sha256.New, []byte(key))
	h.Write([]byte(otp))
	return hex.EncodeToString(h.Sum(nil))
}

// VerifyOTPHash compares a candidate plaintext OTP against a stored HMAC
// digest in constant time.