totp-strict

RFC 6238 TOTP and RFC 4226 HOTP — full HMAC-SHA-1/256/512 support, zero external dependencies.

Why not pyotp?

pyotp is the standard choice and works fine for HMAC-SHA-1. The gap shows up with HMAC-SHA-512.

RFC 6238 errata #2866 corrects a mistake in the original spec's test vectors: the secret length must match the HMAC block size for each algorithm. SHA-512 uses a 128-byte block, so the key should be 64 bytes — not the 20-byte key used for SHA-1. pyotp does not enforce this. The result is a silent mismatch against the official test vectors.

import base64, hashlib, pyotp
from totp_strict import TOTP

# RFC 6238 Appendix B test vector: T=59, HMAC-SHA-512, 8 digits
short_key = b"12345678901234567890"                                                # 20 bytes — pre-errata
long_key  = b"1234567890123456789012345678901234567890123456789012345678901234"    # 64 bytes — errata #2866

p = pyotp.TOTP(base64.b32encode(short_key).decode(), digits=8, digest=hashlib.sha512)
print(p.at(59))   # 69342147  ← does not match RFC 6238

t = TOTP(long_key, digits=8, algorithm="sha512")
print(t.at(59))   # 90693936  ← matches RFC 6238 ✓

totp-strict passes all 18 RFC 6238 test vectors across SHA-1, SHA-256, and SHA-512.

Feature	pyotp	totp-strict
HMAC-SHA-1	✓	✓
HMAC-SHA-256	✓	✓
HMAC-SHA-512 (errata-correct)	✗	✓
All RFC 4226 test vectors	✓	✓
All RFC 6238 test vectors (SHA-1/256/512)	partial	✓
QR provisioning URI (Key URI Format)	✓	✓
Zero external dependencies	✓	✓
Python 3.11+ typed	—	✓

Installation

pip install totp-strict

Requires Python 3.11+. No external dependencies — only the standard library.

Usage

TOTP (time-based)

from totp_strict import TOTP

# Standard 6-digit TOTP (Google Authenticator compatible)
totp = TOTP(secret=b"your-secret-key", digits=6, algorithm="sha1")
print(totp.now())           # current code, e.g. "123456"
print(totp.at(1234567890))  # code at a specific Unix timestamp

# Verify with ±1-window drift tolerance (default)
totp.verify("123456")

# Strict — accept only the current window
totp.verify("123456", window=0)

# 8-digit TOTP with HMAC-SHA-256
totp = TOTP(secret=b"your-32-byte-key-here-xxxxxxxxxxx", digits=8, algorithm="sha256")
print(totp.now())

# Generate a QR code URI for Google Authenticator
uri = totp.provisioning_uri("alice@example.com", issuer="MyApp")
# → otpauth://totp/MyApp:alice@example.com?secret=...&algorithm=SHA1&digits=6&period=30

HOTP (counter-based)

from totp_strict import HOTP

hotp = HOTP(secret=b"12345678901234567890", digits=6)
print(hotp.at(0))                    # "755224"  (RFC 4226 test vector)
print(hotp.at(1))                    # "287082"
hotp.verify("755224", counter=0)     # True

API reference

TOTP(secret, digits=6, algorithm="sha1", interval=30, t0=0)

Parameter	Type	Default	Description
secret	bytes	—	Raw secret bytes (not base32-encoded)
digits	int	6	OTP length, 1–10
algorithm	"sha1" | "sha256" | "sha512"	"sha1"	HMAC hash function
interval	int	30	Time-step in seconds
t0	int	0	Unix epoch start (T0)

Method	Returns	Description
.now()	str	Current TOTP code
.at(timestamp)	str	TOTP code at a Unix timestamp
.verify(code, timestamp=None, window=1)	bool	Constant-time verification with drift tolerance
.provisioning_uri(name, issuer=None)	str	otpauth:// URI — see below

window=1 (default) accepts codes from the previous, current, and next interval (±30 s with the default step).

Provisioning URI

.provisioning_uri() produces an otpauth://totp/ URI that follows the Google Authenticator Key URI Format:

• Label format: issuer:account — the colon and @ are left unencoded, all other special characters are percent-encoded
• Secret: base32-encoded, padding stripped (no =)
• Algorithm: uppercase (SHA1, SHA256, SHA512)

Scannable by Google Authenticator, Authy, and any app that supports the Key URI Format.

HOTP(secret, digits=6, algorithm="sha1")

Method	Returns	Description
.at(counter)	str	HOTP code at a counter value
.verify(code, counter)	bool	Constant-time verification

RFC compliance

RFC 4226 — HOTP test vectors

Secret: b"12345678901234567890", HMAC-SHA-1, 6 digits (Appendix D)

Counter	Expected	Status
0	755224	✓
1	287082	✓
2	359152	✓
3	969429	✓
4	338314	✓
5	254676	✓
6	287922	✓
7	162583	✓
8	399871	✓
9	520489	✓

RFC 6238 — TOTP test vectors

8-digit TOTP, interval=30, T0=0 (Appendix B + errata #2866)

Unix time	SHA-1	SHA-256	SHA-512
59	94287082 ✓	46119246 ✓	90693936 ✓
1111111109	07081804 ✓	68084774 ✓	25091201 ✓
1111111111	14050471 ✓	67062674 ✓	99943326 ✓
1234567890	89005924 ✓	91819424 ✓	93441116 ✓
2000000000	69279037 ✓	90698825 ✓	38618901 ✓
20000000000	65353130 ✓	77737706 ✓	47863826 ✓

Run pytest tests/ -v to verify locally.

Development

git clone https://github.com/Suhas-123-cell/totp-strict
cd totp-strict
pip install -e ".[dev]"
pytest tests/ -v

Contributing

Bug reports and pull requests are welcome on GitHub.

When filing a bug report, include the secret, algorithm, and timestamp (or counter) that produces the wrong output, plus the expected value and a reference (RFC section or errata ID).

When opening a pull request, add a test case that covers the change and make sure all existing tests pass.

License

MIT — see LICENSE.