What is a one-time password (OTP)?

A one-time password (OTP) is a secure authentication code that works just once and for a short duration to verify a user’s identity before expiring, eliminating the risk of password reuse across multiple accounts.

Understanding the power of dynamic authentication

A one-time password (OTP) is a dynamically generated string of characters or numbers that authenticates a user for a single login attempt or transaction. Systems generate one-time passwords using sophisticated algorithms that factor in various security elements, such as time-based data, device fingerprints, or transaction context. From enterprise OTP deployment in large organizations to OTP-based security for a remote workforce, one-time passwords offer flexible implementation options.

OTPs are a widely used component of two-factor authentication (2FA) or multi-factor authentication (MFA), providing an additional security layer beyond traditional passwords. Organizations commonly use them in passwordless authentication flows and in adaptive authentication systems that adjust security requirements based on risk levels.

OTP vs. static password

Understanding how one-time passwords compare to permanent or static passwords helps organizations make informed security decisions:

• Security: OTPs provide stronger protection; attackers can guess or crack static passwords
• Reusability: OTPs work only once; users can reuse static passwords across sessions
• Vulnerability to theft: Stolen OTPs expire quickly; stolen static passwords remain valid until changed
• Expiration: OTPs expire in seconds to minutes; static passwords may remain unchanged for months

When to use OTPs

• Banking and financial transactions: Verify wire transfers, payment approvals, and account changes with an additional authentication layer
• Enterprise VPN access: Secure remote connections to corporate networks and sensitive internal systems
• Account recovery: Confirm user identity during password resets and account verification processes
• E-commerce checkout: Authenticate high-value purchases and card-not-present transactions
• Remote workforce authentication: Protect distributed teams accessing cloud applications and collaboration tools
• Government and citizen services: Authenticate access to public service platforms, tax portals, and social security accounts
• Electronic health records: Secure access to patient portals, prescription systems, and sensitive electronic health and medical records

How does OTP authentication work?

The OTP authentication process creates one-time passwords and validates them using shared secrets between an OTP app and an authentication server. Some systems may also use a secret link sent by email as an alternative delivery method.

Authentication systems typically rely on three independent factors:

• Knowledge: Information the user knows (passwords, PINs)
• Possession: Something the user has (authenticator apps, FIDO2 security keys, mobile devices, including OTPs)
• Biometric: Unique characteristics of the user (fingerprints, facial recognition, behavioral patterns, continuous authentication signals)

Security teams commonly distribute OTPs using tokens and push notifications to leverage users' existing devices.

How long is an OTP valid?

Most OTPs expire within 30–60 seconds for time-based one-time password (TOTP) implementations. This short validity window is critical for security because it minimizes the time attackers have to intercept and use a stolen code.

Types of OTPs

Time-based one-time password (TOTP)

TOTPs are the most widely used type of OTP, functioning like a synchronized digital lock between your device and the authentication server. 

Time-based OTPs feature:

• Shared secret: The device and server maintain a shared cryptographic key
• Time synchronization: Both parties use precise timestamps
• Algorithm processing: The system applies SHA-1 or SHA-256 hashing functions to combine the secret and the current times
• Code generation: Produces a temporary (typically 6-digit) code
• Validation window: Codes remain valid for 30–60 seconds, with servers typically accepting codes from adjacent time windows to account for minor clock synchronization issues

HMAC-based one-time password (HOTP)

While less common than TOTPs in modern implementations, HOTPs use an incrementing counter instead of time. 

HMAC-based OTPs feature:

• Shared secret: The device and server maintain a shared cryptographic key
• Counter synchronization: Both parties track an incrementing counter value
• Algorithm processing: The system applies HMAC-SHA-1 hashing functions to combine the secret and counter
• Code generation: Produces a temporary (typically six-digit) code
• Look-ahead window: The server maintains a window to handle missed codes and prevent synchronization issues

TOTP vs. HOTP

What are the benefits of one-time passwords (OTPs)?

OTPs offer several advantages for organizations implementing strong authentication:

Enhanced security through dynamic generation

Unlike traditional passwords, OTPs resist replay attacks and protect against bad actors who might intercept authentication data during transmission. However, OTPs do not inherently prevent real-time man-in-the-middle (MITM) attacks if an attacker intercepts the OTP through phishing.

Additional security benefits of OTPs:

• Advanced algorithm protection: OTPs use cryptographic pseudo-random number generators (PRNGs) rather than “true” randomness. This provides security from the combination of secure PRNGs and cryptographic algorithms (such as HMAC-SHA1 and HMAC-SHA256). These algorithms typically integrate multiple dynamic factors, such as timestamps and device identifiers.
• Time-limited exposure: With validity periods limited to seconds, attackers have a narrow window to exploit stolen credentials. This constraint is particularly effective against automated attack tools.
• Password reuse mitigation: Even when credential stuffing attacks expose compromised passwords across multiple services, OTPs can prevent account takeovers by requiring an additional authentication factor.
• Rate limiting and adaptive security: Many OTP implementations use adaptive security measures, such as dynamically adjusting validation windows and applying incremental delays based on patterns of failed attempts.

Compliance and risk management

According to NIST Special Publication 800-63B Digital Identity Guidelines, OTPs, when implemented as part of an MFA system, can help organizations meet Authenticator Assurance Level 2 (AAL2) requirements. However, OTPs alone do not meet AAL3, which requires hardware-based authentication.

Key compliance benefits:

• Meeting MFA requirements for regulatory compliance
• Supporting Zero Trust architecture implementation
• Aligning with regulations such as GDPR and PSD2 that require strong authentication
• Providing audit trails for authentication attempts

Integration and adoption benefits

While OTPs offer protection, their success depends on seamless implementation and user adoption. 

Authentication solutions that incorporate OTPs provide:

• Streamlined integration: Organizations can leverage OTP generator APIs and validation services via standardized protocols such as OATH TOTP/HOTP, which provide REST APIs and SDKs for mobile and web applications.
• User-friendly implementation: Smartphones and authenticator apps are ubiquitous, making OTP adoption familiar to most users.
• Flexible deployment options: Based on security requirements and user preferences, organizations can choose from multiple delivery methods, enable phased rollouts, and accommodate varying user technical comfort levels.
• Cost-effective security: Compared with traditional hardware tokens or complex biometric systems, OTP solutions often offer a more cost-effective way to implement MFA. Many solutions leverage devices users already own, reducing deployment costs.

OTP delivery methods

Hard tokens

Hardware tokens are physical devices that generate OTP codes. They are highly secure but require device management:

Security keys (FIDO2)

Security keys offer advanced features:

• Built-in support for biometric authentication
• NFC capabilities for mobile device compatibility
• Multi-protocol support (FIDO2, U2F, TOTP)
• Physical presence verification

Smart cards

Enterprise-grade smart cards provide:

• Integration with physical access control systems
• Support for multiple authentication methods
• Offline authentication capabilities
• Hardware security module protection

Soft tokens

Software-based OTP solutions:

Mobile authenticator apps

Authenticator applications generate OTP codes on-device. They’re preferred over SMS due to SIM-swapping risks and offer enhanced security features:

• End-to-end encrypted push notifications
• Offline code generation capabilities
• Secure backup and recovery options
• Cross-platform synchronization
• Biometric protection for app access

Browser-based solutions

Developments in browser authentication:

• Native WebAuthn support in modern browsers
• Biometric authentication integration
• No additional hardware requirements
• Phishing-resistant design

Which OTP authentication methods are best?

Not all authentication methods are equal. Implementing MFA improves on using passwords alone, but each authentication factor offers different degrees of protection.

Download our Factor Assurance Datasheet for an overview of the security assurance levels associated with different authentication factors, including their advantages and disadvantages.

Authentication methods comparison table

SMS authentication: Convenience at a security cost

While SMS remains a widely used method for OTP delivery due to its familiarity, it presents significant security vulnerabilities:

• SIM swapping and social engineering: Threat actors can convince carriers to transfer a phone number to a new SIM card they control, gaining access to all SMS-based OTPs. This attack vector has become increasingly sophisticated, with malicious actors exploiting carrier customer service processes.
• Account takeover via web portals: Many carriers provide web portals where users can view SMS messages. If attackers compromise portal accounts through weak passwords or credential-stuffing attacks, they can intercept OTP codes without controlling the physical device.
• Device synchronization risks: Syncing messages across multiple devices expands the attack surface. When users forward or sync SMS messages to tablets, computers, or cloud services, each additional endpoint becomes a potential vulnerability.
• Phishing vulnerability: Social engineering attacks can trick users into revealing their primary credentials and SMS OTPs. Unlike modern methods, SMS OTPs do not protect against real-time adversary-in-the-middle (AITM) phishing attacks.

Hardware security keys: Strong security with trade-offs

Hardware security keys represent a significant security upgrade over SMS-based OTPs, offering several advantages:

• Phishing resistance: Security keys use asymmetric encryption algorithms that ensure the device never transmits authentication data
• Offline capability: Many tokens can generate codes without network connectivity
• Physical security: Hardware control introduces another layer of protection

However, hardware tokens present additional challenges:

• Device management: Requires distribution, replacement, and recovery procedures
• Cost considerations: Hardware purchases add an expense per user
• Compatibility issues: Not all devices support physical security keys, particularly in mobile environments
• User experience (UX): Additional hardware can be inconvenient for users to carry and manage

Authenticator apps: The modern standard

Mobile authenticator apps have emerged as the preferred solution for most organizations, offering an optimal balance of security and usability:

• Device binding: The system ties authentication to specific devices rather than phone numbers, which eliminates SIM-swapping risks
• Offline operation: Apps can generate codes without internet connectivity
• Enhanced security: Short-lived codes and encrypted push notifications reduce exploitation risks
• Biometric integration: Support for fingerprint and facial recognition adds an extra security layer
• Cost-effective: Many providers offer free solutions or include them with existing identity platforms

WebAuthn: The future of authentication

Representing the latest evolution in authentication technology, WebAuthn offers unique advantages:

• Native browser support: Built-in support within popular browsers (Chrome, Safari, Firefox, and Edge)
• Platform integration: Existing device security features like Touch ID, Face ID, and Windows Hello
• Phishing prevention: Public key cryptography makes it virtually impossible to intercept or replay authentication attempts
• Streamlined UX: Users can verify their Identity using familiar biometric gestures
• FIDO2 compliance: Follows industry standards for strong authentication

Implementation best practices

Organizations should transition toward passwordless authentication while maintaining OTPs as a secondary security layer. Modern authentication strategies should incorporate emerging standards like passkeys, WebAuthn, and phishing-resistant MFA.

When choosing authentication methods, organizations should:

• Layer authentication methods: Use authenticator apps as primary and WebAuthn, where supported
• Maintain backups: Keep SMS as a fallback option with additional security controls
• Consider the context: Adjust security requirements based on risk levels and user needs
• Plan for evolution: Design systems to accommodate emerging authentication standards

Security requirements

Code generation

• Minimum 6-digit codes (8 digits recommended for high-security applications)
• Cryptographic random number generation
• 30–120 second validity based on risk assessment
• Rate limiting on generation and validation attempts

Delivery method security

• End-to-end encryption
• Multiple delivery channel support
• Secure channel verification
• Automated monitoring for unusual patterns

Enterprise implementation

Organizations deploying OTPs at scale should consider the following:

High availability

• Load-balanced authentication servers
• Geographic distribution
• Real-time monitoring and alerting
• Automated failover mechanisms

Integration architecture

When planning OTP integration with Active Directory or cloud services like Microsoft Entra ID and AWS IAM, organizations should consider:

• Identity provider compatibility
• API gateway security controls
• Directory service synchronization
• Comprehensive audit logging

Secure, seamless authentication with adaptive MFA

While OTPs provide a vital layer of defense against credential theft, modern security challenges require a more holistic approach. Okta Adaptive MFA elevates the security of standard OTPs by analyzing contextual signals—like device health, location, and network—to prompt for authentication only when anomalous behavior is detected, making enterprise access simple and secure.

Ready to seamlessly secure your workforce? Discover how Okta Workforce Identity can strengthen your security posture today.

Frequently asked questions