One-Time Password (OTP)

Understand TOTP, HOTP, MFA use cases, and the security benefits and limitations of OTP authentication.

Last Updated date: July 2026

A one-time password (OTP) is a temporary, automatically generated code used to verify a user's identity for a single login session or transaction. Unlike a static password, an OTP expires after use or within a short time window, typically 30 to 240 seconds, and cannot be reused. This significantly reduces the risk of replay attacks and credential misuse.

At a Glance

Quick Summary
Field	Detail
Category	Authentication credential
Related to	MFA, 2FA, IAM, Zero Trust
Primary use	Second-factor verification for logins and transactions
Key benefit	Stolen static passwords alone cannot grant access

Why OTPs Matter in Identity Security

Static passwords remain one of the biggest attack surfaces in identity management. Credential stuffing, phishing, and password reuse attacks all depend on the same assumption: if attackers steal a password, they can use it.

OTPs disrupt that assumption. Even if a user's primary password is compromised, the attacker still needs access to a time-sensitive, device-linked code that cannot easily be predicted or reused.

For organizations operating under compliance frameworks like PSD2, HIPAA, or SOC 2, OTP-based multi-factor authentication (MFA) is often considered a baseline security requirement rather than an optional feature. Identity Governance platforms that enforce MFA policies at scale commonly rely on OTP mechanisms as a core authentication layer.

How a One-Time Password Works

OTP authentication follows a similar process regardless of how the code is delivered:

User initiates login The user enters their username and static password.
System generates the OTP A unique code is created using a shared secret along with either the current time or an event counter.
Code is delivered or displayed The OTP is sent through SMS, email, or an authenticator app such as Google Authenticator or Okta Verify.
User enters the OTP The code must be entered within the valid time window or before the counter changes.
System validates access The server verifies the OTP and immediately invalidates it after successful use.

The shared secret between the authentication server and the user's device is what makes OTP systems resistant to tampering. The secret itself is never transmitted across the network. Only the generated code is exchanged.

Two Types of OTP: TOTP vs. HOTP

OTPs are generated using one of two primary algorithms, and understanding the difference is important when designing an identity management strategy.

Time-Based OTP (TOTP)

TOTP generates a new code at fixed intervals, most commonly every 30 or 60 seconds. The code is derived from the current Unix timestamp and a shared secret. TOTP is the standard used by most modern authenticator apps and is defined in RFC 6238.

HMAC-Based OTP (HOTP)

HOTP uses an incrementing counter instead of time. A new code is generated whenever the user requests authentication. Since the code is not tied to a time window, it remains valid until used, which can introduce synchronization challenges between the client and server.

For most enterprise IAM deployments, TOTP is the preferred option because its short validity window reduces the risk of interception and misuse.

Where OTPs Are Used

OTPs are commonly used anywhere a single static password is not considered sufficient for authentication.

Consumer banking and fintech for transaction approvals and step-up authentication
Enterprise SSO and VPN access as a second authentication factor behind a corporate identity provider
Healthcare portals for HIPAA-compliant authentication of patients and clinicians
SaaS applications where identity governance platforms enforce MFA for both privileged and standard users
Password resets to verify identity before allowing credential changes

In regulated industries, OTP enforcement is often tied directly to access certification and lifecycle management workflows within Identity Governance platforms. This helps organizations apply MFA consistently across systems and user groups.

OTP vs. Static Password vs. Push Notification

OTP is one of several authentication methods commonly used in modern identity security.

Compared to static passwords, OTPs provide significantly stronger protection because they expire quickly, cannot be reused, and are tied to a specific device or authentication flow.

Compared to push notifications such as Duo Push or Microsoft Authenticator approvals, OTPs require users to manually enter the code. Push notifications create less friction but can introduce MFA fatigue, where users approve requests automatically without verifying legitimacy. OTPs reduce that particular risk because they require active user participation.

Method	Reusable	Expiry	Phishable	MFA Fatigue Risk
Static password	Yes	No	Yes	N/A
OTP (TOTP/HOTP)	No	Yes	Possible	Low
Push notification	No	Yes	Possible	High
Hardware token	No	Yes	Difficult	Low

Implementing OTP in an Enterprise Environment

Deploying OTP across an organization involves more than simply enabling MFA in an identity provider. A successful rollout depends on several operational and security decisions.

Choose the delivery channel Authenticator apps generally provide stronger security than SMS, while SMS may be easier for less technical users.
Select TOTP or HOTP TOTP is the standard choice for most enterprise IAM environments, while HOTP is more common in offline or hardware token scenarios.
Define enforcement policies Organizations must decide which users, applications, and risk levels require OTP protection. Identity Governance platforms can enforce these policies using roles and attributes.
Prepare account recovery workflows Users may lose access to their authentication device. Backup codes, verified recovery flows, and administrator-assisted resets should be planned before deployment.
Monitor and audit usage Logging OTP requests and failures can help detect phishing attempts, credential stuffing activity, or suspicious authentication behavior.

Known Limitations and Risks

OTPs strengthen authentication, but they are not a complete defense against every attack.

SMS OTP Vulnerabilities

SIM-swapping attacks can allow attackers to redirect a victim's phone number and intercept SMS-based OTPs. Because of this, organizations handling sensitive data often prefer app-based TOTP instead of SMS delivery.

Real-Time Phishing

Adversary-in-the-middle (AiTM) phishing attacks can capture OTPs and relay them to the target service before the code expires. In these cases, OTP protection can be bypassed in real time.

User Error and Friction

Time-sensitive codes can create usability challenges. Users may mistype codes or rush through authentication steps under pressure.

These limitations are why modern identity security strategies treat OTP as one layer within a broader zero trust architecture rather than a standalone security control.

Frequently Asked Questions

A one-time password is a temporary code used to verify a user's identity for a single login or transaction. It expires after use or within a short validity window, preventing reuse.

A regular password stays the same until the user changes it. An OTP is generated separately for each authentication attempt and becomes invalid after use or expiration.

OTPs are commonly delivered through SMS, email, or authenticator apps such as Google Authenticator or Okta Verify. App-based TOTP is generally considered more secure than SMS delivery.

Yes. Real-time phishing attacks using adversary-in-the-middle proxies can intercept and relay OTPs before they expire. This is one reason phishing-resistant authentication methods such as passkeys and hardware security keys are gaining adoption.

No. OTP is a type of authentication factor, while 2FA refers to the broader process of combining two separate authentication factors. OTP is one of the most common second factors used in 2FA systems.

Yes. Enterprise IAM and identity governance platforms can enforce OTP and MFA requirements based on user roles, applications, or risk levels to ensure consistent authentication policies across the organization.