What is OTP?

One-Time Password (OTP) is a single-use authentication code that a user submits to prove their identity, generated for one login or transaction and invalid after use or after a short expiry window. Versa uses OTP as a multifactor authentication step for SASE client registration and login.

Key takeaways

• OTP is a single-use code that supplements or replaces a static password.
• Delivery methods include SMS, email, and app-generated time-based codes.
• TOTP and HOTP are the two algorithmic OTP standards in wide use.
• Short validity windows limit the damage from phishing or credential theft.
• Versa supports SMS/email OTP and TOTP for SASE client authentication.

On this page: Why it matters · How it works · Types · OTP & Versa · FAQ

Why is OTP important?

Static passwords are reusable by design, which is exactly what makes them dangerous: once a password leaks through a breach or a phishing page, an attacker can replay it until it is changed. OTP breaks that pattern. Because each code is valid for a single use or a short time window, a captured OTP is worthless to an attacker moments after the legitimate user consumes it, and credential-stuffing attacks fail outright against an OTP-protected login.

That property makes OTP one of the most widely deployed multifactor authentication mechanisms, spanning banking, cloud services, and enterprise remote access. It is cheap to deploy – no hardware token required when delivered by SMS, email, or an app – and it raises the cost of account takeover. In Zero Trust architectures, where every access request is verified rather than trusted by default, OTP is a standard step-up factor layered on top of a password or certificate.

What is OTP?

An OTP system works one of two ways. In the out-of-band model, a server generates a random code at login time and delivers it over a separate channel – typically SMS or email – that the user relays back into the login form. In the algorithmic model, the client and server share a secret established during setup, and each independently computes the same code from that secret, with no code transmitted over the network until the user types it in.

In both models, the server validates the submitted code against the expected value and accepts it only once, only within its validity window, and only for the account it was issued to. After that, the code is discarded – a new attempt requires a newly generated code, which is what makes replay attacks ineffective against OTP-protected accounts.

Types of OTP

Two industry standards define the algorithmic model. HOTP (HMAC-based One-Time Password) derives each code from a shared secret and an incrementing counter, so the code changes only when a new one is requested – useful for hardware tokens with no clock. TOTP (Time-based One-Time Password) derives each code from the shared secret and the current time instead, so codes rotate automatically every 30 seconds to a few minutes; this is the model behind most authenticator apps, provisioned by scanning a QR code once.

Delivery-based OTP is a third pattern rather than a generation algorithm: the server creates the code and pushes it over SMS or email at login, trading the authenticator app for dependence on a working phone number or inbox. It remains common because it needs no app install, though it is more exposed to SIM-swap and mailbox-compromise attacks than a locally generated code.

OTP & Versa

The Versa SASE client supports OTP as one of its multifactor authentication factors, alongside TOTP, x509 certificates, LDAP, RADIUS, and SAML. During registration and login, a user completing two-factor authentication enters the OTP delivered to their email or through a mobile message server; once validated, registration completes. Administrators enable this by configuring an authenticator profile on the secure access portal policy – SMS or email OTP requires SMTP settings on the profile, while TOTP requires a time-based authenticator type and NTP enabled on the VOS device so the server and the user’s app stay in sync.

Both Versa Director and Concerto support configuring OTP and TOTP for secure access portals, and a separate OTP mechanism lets administrators generate a short-lived code from the Versa Tamper Protection Generation tool to authorize disabling client tamper protection.

Frequently asked questions

What is the difference between OTP and TOTP?

OTP is the general category: any single-use authentication code, however it is generated or delivered. TOTP is a specific OTP standard that derives the code from a shared secret and the current time, so it regenerates automatically every 30 seconds to a few minutes.

How long is an OTP valid?

It depends on the implementation, but algorithmic OTPs are typically valid for 30 seconds to a few minutes before they expire and a new code is required. Delivery-based OTPs sent by SMS or email often carry a similar short window.

Is OTP the same as two-factor authentication?

No. Two-factor authentication (2FA) is the broader concept of requiring two different proof-of-identity factors. OTP is commonly used as the second factor in a 2FA flow, paired with a static password or certificate as the first, though 2FA can also use a hardware key or biometric.

How does Versa deliver OTPs to SASE client users?

Versa delivers OTPs by SMS or email for standard two-factor authentication, or as a TOTP generated by an authenticator app after the user scans a QR code during registration. Both methods are configured through an authenticator profile in Versa Director.

What happens if an OTP expires before I submit it?

The code becomes invalid and the login or registration attempt fails. The user must request or generate a new code – for TOTP, the authenticator app produces the next one automatically; for SMS or email OTP, the portal resends a fresh code.

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