TOTP & HOTP

TOTP (časově vázané jednorázové heslo, RFC 6238) a HOTP (jednorázové heslo na bázi HMAC, RFC 4226) jsou dvě RFC, která implementuje každý QR kód pro registraci 2FA. Kódované schéma URI otpauth:// je de facto standard Google Authenticatoru, univerzální ve všech hlavních aplikacích pro ověřování.

Co to je

QR kód pro registraci 2FA je URL ve schématu otpauth://, které nese sdílený tajný klíč a identifikační metadata. Formát:

otpauth://TYPE/LABEL?secret=SECRET&issuer=ISSUER&algorithm=ALG&digits=N&period=SEC

• TYPE, totp (time-based) or hotp (counter-based).
• LABEL, Issuer:Account, URL-encoded. Example: GitHub:alice@example.com.
• secret, the shared key, Base32-encoded (RFC 4648 §6 alphabet: A-Z, 2-7).
• issuer, the service name, shown in the authenticator app. Redundant with the label but improves UX on apps that don't parse the label.
• algorithm, SHA1 (default), SHA256, or SHA512. Every authenticator supports SHA1; fewer support SHA256/SHA512.
• digits, 6 (default) or 8. Most consumer apps expect 6.
• period, TOTP only. 30 (default) or 60 seconds. HOTP has a counter parameter instead.

Kanonické testovací vektory

Živý validátor tajných klíčů Base32

Aplikace pro ověřování odmítají tajné klíče obsahující znaky mimo abecedu Base32 (A-Z, 2-7). Níže je stejný validátor, který Abundera spouští přímo v generátoru TOTP, běží ve vašem prohlížeči.

Časté chyby

• Base32 is not Base64. Base64 uses A-Z, a-z, 0-9, +, /, any of those lowercase letters or digits 0, 1, 8, 9 in your secret means you were handed a Base64 string and it'll be rejected by every authenticator.
• Secret length. RFC 4226 §4 recommends at least 128 bits (26 Base32 chars) for HOTP and 160 bits (32 chars) for TOTP. Secrets under 80 bits (16 chars) are technically legal but flagged weak.
• Algorithm support varies. Google Authenticator ignores the algorithm parameter and always uses SHA1. For SHA256/SHA512 to actually take effect, your users must be on 1Password, Authy, Bitwarden, or Microsoft Authenticator.
• Digits support varies. Most authenticators ignore digits=8 and silently truncate to 6. If 8-digit OTPs matter (banking, some government apps), test your target authenticator before rolling out.
• URL-encode the label. Special characters in issuer or account (:, @, space) must be percent-encoded in the label, or older authenticators drop the label entirely. Abundera handles this automatically.
• Never reuse secrets. Every account gets a fresh cryptographically-random secret. Reusing a secret across services means a compromise at one breaks all of them.
• Storage matters more than the QR. The QR is a one-time bootstrap. The authenticator app stores the secret after the first scan. If that secret is later exported in plain text (some authenticators do this), one disk leak = every 2FA compromised.

Kompatibilita aplikací pro ověřování

Viz také

• /totp-2fa-qr-code/, the TOTP generator with Base32 validator inline.
• /hotp-qr-code/, HOTP counter-based variant.
• /standards/, back to the standards index.

Reference specifikací ověřeny 2026-04-18. RFC 6238 (2011), RFC 4226 (2005), Google Key URI Format.