TOTP & HOTP

TOTP (Time-based One-Time Password, RFC 6238) and HOTP (HMAC-based One-Time Password, RFC 4226) are the two RFCs every 2FA enrolment QR implements. The encoded otpauth:// URI scheme is Google Authenticator's de-facto standard — universal across every major authenticator app.

TOTP spec: RFC 6238 (time-based, 2011).
HOTP spec: RFC 4226 (counter-based, 2005).
URI scheme: Google Authenticator Key URI Format — the format every authenticator agrees on.

What it is

A 2FA enrolment QR is a URL in the otpauth:// scheme that carries the shared secret plus identifying metadata. Format:

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.

Canonical test vectors

Case	Inputs	Expected otpauth:// URI
TOTP, minimal (SHA1, 6 digits, 30 s)	`issuer=GitHub account=alice@example.com secret=JBSWY3DPEHPK3PXP`	`otpauth://totp/GitHub:alice@example.com?secret=JBSWY3DPEHPK3PXP&issuer=GitHub`
TOTP, SHA256, 8 digits	`issuer=Corp account=bob secret=JBSWY3DPEHPK3PXP algorithm=SHA256 digits=8`	`otpauth://totp/Corp:bob?secret=JBSWY3DPEHPK3PXP&issuer=Corp&algorithm=SHA256&digits=8`
HOTP, counter=0	`issuer=YubiKey account=carol secret=JBSWY3DPEHPK3PXP counter=0`	`otpauth://hotp/YubiKey:carol?secret=JBSWY3DPEHPK3PXP&issuer=YubiKey&counter=0`
TOTP, RFC 6238 reference secret	`issuer=Example account=test secret=GEZDGNBVGY3TQOJQGEZDGNBVGY3TQOJQ digits=8`	Standard RFC test secret (`12345678901234567890`). Produces TOTP `94287082` at T=59 s.

Live Base32 secret validator

Authenticator apps reject secrets that contain anything outside the Base32 alphabet (A-Z, 2-7). Below is the same validator Abundera runs inline inside the TOTP generator — runs in your browser.

Base32 secret

Enter a secret to validate.

Common pitfalls

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.

Authenticator compatibility

App	TOTP SHA1	TOTP SHA256/512	HOTP	8-digit	Notes
Google Authenticator	Yes	Ignored	Yes	Ignored	The de facto baseline. Always targets this first.
1Password	Yes	Yes	Yes	Yes	Full RFC support.
Authy	Yes	Yes	No	Yes	Drops HOTP in newer versions.
Bitwarden	Yes	Yes	Yes	Yes	Full RFC support.
Microsoft Authenticator	Yes	Yes	Yes	Yes	Full RFC support.
YubiKey Authenticator	Yes	Yes	Yes	Yes	HOTP is the canonical YubiKey mode.
Duo Mobile	Yes	Ignored	No	Ignored	Uses its own push flow; TOTP is fallback only.