An OTP, short for one-time password, is a single-use code, valid for only a short window, that a user enters to prove they are who they claim to be. It is the six-digit OTP code your bank texts before a transfer, or the code your authenticator app shows at login.

An OTP works as a second layer on top of a password or PIN. The app generates or requests a fresh code, sends it to something only the real user holds, and approves the action only when the code matches and has not been used. Because it changes every time and expires fast, a stolen password alone is no longer enough to break into an account or authorise a payment.

OTP at a glance

• What it is: a single-use code, valid for a short time, tied to one login or one transaction.
• What OTP stands for: one-time password (also called a one-time PIN or one-time code).
• How it reaches you: SMS, an authenticator app, email or a voice call. USSD lets Ghanaian users view an SMS code they missed.
• What it is for: confirming identity, usually as a second factor alongside a password.
• Why it is secure: it expires within minutes and works only once.

If you run an app or a payment flow, the code is only half the story. The other half is making sure it reaches every customer’s phone, including on the day a text does not arrive.

What is OTP authentication?

OTP authentication is a security method that asks a user to enter a unique, single-use code (a one-time password) to confirm their identity. The code is generated from a shared secret or a server-side random value, stays valid for one login or one transaction, and expires within minutes.

An OTP often works as a second factor on top of something the user already knows, such as a password or PIN. That pairing closes the gap between something the user knows (which can be stolen) and something the user has (a phone or an authenticator app that receives the code).

How you sequence that prompt across sign-up, login and high-value actions is a design choice in itself. See designing effective user authentication flows with OTPs.

Because the code is single-use and short-lived, it protects the account even when the password is compromised. This is one reason teams move from static credentials to one-time codes. Our comparison of OTPs vs static passwords covers the full trade-off.

What is an OTP code?

An OTP code is the actual value the user types in. It is usually a string of digits, though some systems use letters too. Each code is generated for one specific action and stops working the moment it is used or the moment its validity window closes.

You have received one many times. The six digits your bank texts before a transfer, the code an online store emails at checkout, and the number your authenticator app shows for a work login are all OTP codes. Same idea every time: a fresh, disposable credential that proves you hold the phone or device, then expires.

How many digits should an OTP code have? 4-digit vs 6-digit

Six digits is the baseline the main standards set, not a style choice. A 4-digit code is much easier to guess, so it leans far harder on limits to how many attempts a person gets.

The arithmetic is the whole argument. A 4-digit code has 10,000 possible values. A 6-digit code has 1,000,000, so an attacker guessing blindly needs a hundred times more attempts to cover the same ground.

Two standards set that six-digit floor:

• The HOTP algorithm. The IETF’s RFC 4226 says implementations must extract a 6-digit code at a minimum, and possibly a 7- or 8-digit code. That rule governs counter-based codes such as hardware tokens.
• NIST’s digital identity guidelines. NIST SP 800-63B-4, a US federal guideline, requires codes sent out-of-band, such as by SMS or voice call, to be at least six decimal digits, accepted only once, and completed within 10 minutes.

How long should an OTP code stay valid?

It depends on the type of code. For authenticator apps, the TOTP specification recommends a default time step of 30 seconds, balancing security against the time a person needs to read and type. For codes sent by SMS or voice, NIST sets 10 minutes as the outer limit.

Shorter is safer, as long as your customers can realistically receive and enter the code in time. A shorter window also shrinks the time an attacker has to guess, which is why a short code and a long expiry are a poor pairing. Our guide to OTP expiration and rate limiting walks through setting both.

If you send codes through Arkesel Phone Number Verification, Arkesel codes run 6 to 15 digits, and you set an expiry between 1 and 10 minutes to match the action you are protecting.

How does an OTP work? Step by step

Here is how OTP works, and the flow is the same across SMS, email and authenticator-app OTPs:

1. The user triggers an event: signing in, confirming a transfer, or resetting a password.
2. The server generates a code from a shared secret combined with a counter, a timestamp, or a server-side random value.
3. The code reaches the user: the server sends it to the user’s phone by SMS or to their email inbox, or an authenticator app on the user’s phone generates the matching code itself. If an SMS does not arrive, the user asks for a fresh code.
4. The user enters the code in the app within a short validity window.
5. The server validates it and, if it matches and has not been used, approves the action.

A sound OTP design enforces short expiry, single use and per-account rate limits. A weak one accepts old codes or leaves attackers an open guessing surface.

Types of OTP: HOTP, TOTP, and SMS OTP

One-time passwords are generated in one of three common ways.

HOTP (HMAC-based One-Time Password) generates codes from a shared secret plus a counter that increments with each use. It is specified in RFC 4226 and is common in hardware tokens.

TOTP (Time-based One-Time Password) generates codes from a shared secret plus the current time window, with 30 seconds as the recommended default. Specified in RFC 6238 as a time-based extension of HOTP, it is one of two code types Google supports in its open-source Google Authenticator. The other is HOTP.

Server-generated SMS or email OTP creates a random code on demand and sends it to the user’s phone by SMS or to their email inbox. No shared secret sits on the device, and nothing needs to be installed.

HOTP vs TOTP in one line: HOTP advances with an event counter, while TOTP advances with the clock, so each TOTP code expires on its own when its time step ends.

Server-generated SMS OTP is a common choice for banks, fintechs and mobile-money platforms in Africa, because it reaches any phone that can receive a text, with nothing to install.

OTP delivery channels compared

Each channel trades security against reach. The right choice is the one your users can actually receive.

Only passkeys rate high on phishing. NIST’s guideline says codes typed in by hand, including authenticator-app and texted codes, are not phishing-resistant, because typing a code does not tie it to the session being authenticated.

The pattern is clear. The more phishing- and SIM-swap-resistant a channel is, the more device and setup it demands, which shrinks how many of your users can reach it.

Voice calls and USSD are not in the table. With Arkesel, both apply to Ghanaian numbers only: a voice code when your request names voice as the channel, and a USSD code a customer can dial to view an SMS code they missed.

For a closer look at the delivery rails themselves, compare SMS, USSD and voice for OTP delivery. To weigh three widely used OTP channels head to head, see SMS OTP vs authenticator app vs email OTP.

What is OTP in banking and fintech?

Banks, mobile-money operators and e-commerce platforms use OTP to confirm that the person approving a login or a payment is the real account holder, not someone holding a stolen password. It is the checkpoint between “the credentials look right” and “the right person is here.”

The stakes are highest where money moves at scale. Every account opening, transfer and PIN reset on a mobile-money or banking platform rests on verifying identity at the moment of action. Not every flow uses an SMS OTP, since mobile-money PINs often travel over USSD, but all of them protect the same thing: the person behind the transaction.

For banking and fintech specifically, OTP guards high-value events: transaction confirmation, beneficiary changes, card enrolment and password resets. Because a code is tied to a single action and expires quickly, an intercepted or reused code fails. For the deeper playbook, see OTP for fintech and banking transaction security and our OTP API security best practices.

Is OTP secure? Risks and best practices

OTP security comes down to how you use the code, not just the code itself. An OTP raises the bar, but it does not close every gap. Three risks matter most:

• SIM swap. An attacker moves a victim’s number to a new SIM and starts receiving their SMS codes. Bank logins, mobile-money confirmations and password resets can all land in the wrong hands.
• Phishing. A fake login page tricks the user into entering both password and OTP, which the attacker relays to the real site within the code’s short window.
• SMS pumping. Fraudsters trigger floods of OTP sends toward premium-rate number ranges, draining your messaging budget without ever touching a real user. See SMS pumping fraud prevention.

Because SMS codes can be intercepted, security teams treat SMS OTP as one layer, not the whole defence. NIST classes phone-network delivery of codes as restricted, which means allowed with extra safeguards, and advises checking risk signals such as a SIM change or number porting before sending a code that way. Layer these controls around it:

• Short expiry windows for high-value actions.
• Single-use enforcement: invalidate the code on first entry.
• Rate limits per phone number and per IP address to block brute-force guessing.
• SIM-change and number-porting checks before authorising high-value transactions.
• Dynamic linking on payments: bind the code to the specific amount and beneficiary, so an intercepted code cannot be reused elsewhere.

Done this way, SMS OTP stays a dependable second factor for the widest possible audience. For fixes to the errors teams hit while building this, see common OTP API errors and fixes.

How businesses send and verify OTP codes in Ghana, Nigeria, South Africa and Tanzania

Knowing what an OTP code is solves half the problem. The other half is delivery: a code that never arrives is a sign-up that never finishes and a payment that never clears.

SMS works on any phone that can receive a text, including feature phones that cannot run an authenticator app. Those phones are still common.

In Tanzania, the Tanzania Communications Regulatory Authority counted about 58.4 million feature phones on mobile networks in the quarter ending June 2026, against 31.3 million smartphones. Those are device counts, not people, since one person can carry several phones.

In Ghana, the 2021 Population and Housing Census found that one in ten people aged 12 and over owned only a non-smart phone.

Which channels can deliver an OTP code in each country?

A text can still fail to land, so the question for your business is what happens next. With Arkesel Phone Number Verification, you generate, deliver and verify OTP codes through one API. Each request names the channel, SMS or voice, and which channels you can use depends on the customer’s country.

What happens if an SMS OTP code does not arrive?

With Arkesel, nothing switches channel on its own. If a text does not arrive, no voice call or USSD step follows by itself, so give every customer a clear way to request a fresh code. For Ghanaian numbers, you can choose to resend by voice in your own app logic, for example after an SMS delivery failure.

In Ghana, a customer who was sent a code by SMS but never received it can dial a USSD code to see it on their screen. This works for Ghanaian numbers only. The current dial code lives in the Arkesel developer documentation, so point your support copy there rather than hard-coding it.

Setting up OTP verification in your app

On the build side, your app asks the OTP API to generate a code, with a length between 6 and 15 digits and an expiry between 1 and 10 minutes, then sends the digits the customer types back to be verified. The developer documentation carries the current generate and verify calls.

To weigh your options first, compare OTP API providers for 2026. When you are ready to build, start with the OTP API integration guide.

You are billed per OTP sent, not per successful verification, which is one more reason to rate-limit resends. For current rates, see current pricing.

OTP FAQ

What does OTP mean?

OTP stands for one-time password: a single-use code that confirms a user’s identity for one login or transaction, then expires.

Is an OTP the same as two-factor authentication (2FA)?

Not quite. An OTP is a type of factor; 2FA is the practice of requiring two factors. An OTP is often the second factor in a 2FA setup, but 2FA can also use a biometric or a security key.

How long is an OTP valid?

It depends on the type. Authenticator-app codes change on a time step, with 30 seconds as the recommended default in the TOTP specification. For codes sent by SMS or voice, NIST’s guideline treats 10 minutes as the outer limit, and shorter windows give an attacker less time to reuse a code.

Is a 4-digit OTP secure?

A 4-digit OTP has only 10,000 possible values, against 1,000,000 for six digits, and both the HOTP specification and NIST set six digits as the minimum. If you must use a short code, pair it with strict attempt limits and a very short expiry.

Is SMS OTP safe?

SMS OTP is widely used and far stronger than a password alone, though it carries SIM-swap and interception risk. Treat it as one layer, and add short expiry, rate limits and SIM-change checks around it.

What is the difference between HOTP and TOTP?

HOTP codes advance with an event counter; TOTP codes advance with the clock, so each one expires on its own when its time step ends.

Conclusion

An OTP code is a single-use, short-lived number that proves a user is who they claim to be. Make it at least six digits, keep its life short, and plan for the moment a text does not arrive.

Add OTP verification to your app with Arkesel Phone Number Verification, read the developer documentation, and start free.

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