OTP Strength Calculator

• Search space: S = N^L, where N is charset size and L is OTP length.
• Entropy (bits): H = L · log₂(N).
• Median guesses: G₅₀ = S / 2 (random guessing).
• Median time: T₅₀ = G₅₀ / r, where r is guesses per second.
• Online effective rate: r_online = attempts / window_seconds (simple lockout model).

1. Choose Analyze an OTP to assess a sample code’s character classes.
2. Or choose Design parameters to compare lengths and charsets.
3. Set a realistic online lockout (tries per time window) and validity duration.
4. Review the entropy rating and the online time estimate; online limits matter most.
5. Export the result to CSV or PDF for documentation.

• Prefer short validity windows and strict rate limits for login OTPs.
• Use monitoring and anomaly detection for repeated failures.
• Avoid predictable OTP generation; uniform randomness is essential.
• For high-risk actions, combine OTPs with device binding and step-up checks.

Entropy explains how many OTPs exist

Entropy is a log-based measure of uncertainty in a code. With length L and character set size N, the search space is N^L and entropy is L·log2(N). Higher entropy means more possible OTPs, increasing guessing cost. This calculator shows both bits and the approximate search space, so teams can compare numeric-only codes against mixed-character codes quickly. Use it to document improvement.

Rate limits dominate real-world guessing risk

Online attacks rarely achieve large guessing speeds because systems throttle attempts, lock accounts, or require step-up checks. The calculator models an effective online rate using attempts per time window, then estimates median time-to-guess. Even a moderate-entropy OTP becomes highly resistant when the service allows only a few tries per five minutes, especially when monitoring and device signals trigger additional controls.

Validity windows reduce exposure time

OTP security is not just about hardness; it is also about how long a code remains valid. Short validity windows limit how many attempts can occur before replacement. In workflows like authenticator apps or email links, five minutes is common. The calculator includes validity to help reviewers justify tighter windows for high-value actions, while balancing usability for slower channels. Consider clock drift and delivery delays when setting timeouts.

Offline scenarios are different and require protection

If an attacker can validate guesses offline, rate limits disappear and the guessing rate can jump dramatically. That is why OTPs should be verified server-side and never stored in a reversible form. Where temporary secrets or tokens are hashed, strong hashing and secret handling still matter. The offline estimate in this tool is illustrative, intended for risk discussions, not for predicting exact attacker capability. Prevent replay by binding codes to sessions or transaction context.

Using results for policy and audit evidence

Security reviews benefit from consistent, explainable metrics. Record the chosen length, charset, entropy, and enforcement settings such as lockout thresholds and validity. Exported CSV supports control testing, while the PDF snapshot supports change approvals. Over time, compare results across products to standardize OTP requirements and align them with authentication assurance levels and incident learnings. Re-run the calculator after changes to rate limiting, channel, or user population.

What does entropy mean for an OTP?

Entropy estimates how many different OTPs could exist given length and character set. More bits generally means more resistance to random guessing under similar conditions.

Why do online and offline times differ?

Online guessing is constrained by lockouts, rate limits, and monitoring. Offline guessing assumes an attacker can test codes without those controls, so the assumed guess rate can be far higher.

Should I enter a real production OTP here?

No. Use a representative example or leave the field blank and evaluate parameters. Avoid sharing live codes or sensitive values in any tool or screenshot.

What charset size should I choose?

Choose the set your system actually allows. Digits-only is common for usability; alphanumeric or symbols can add entropy, but only if generation is truly random and input is practical.

How should I set validity and lockout windows?

Short validity and strict attempt limits reduce risk most effectively. Tune values to your channel latency and user base, then monitor failures to ensure usability stays acceptable.

Does a stronger OTP replace other controls?

No. OTP strength complements secure delivery, phishing resistance, device signals, and logging. For high-risk actions, combine OTPs with additional verification and anomaly detection.