A one-time password (OTP) is an automatically generated numeric or alphanumeric string of characters that authenticates a user for a single transaction or session. It adds an additional layer of security against unauthorized access and is more secure than a static password, especially one that is user-created, because they can be weak and/or reused across multiple accounts.

OTPs are typically sent to a user’s device via SMS, email, or an authentication app and expire after a single use or short time period.

How one-time passwords work

One-time passwords (OTPs) provide a unique, temporary code for accessing a digital system such as a computer or online website. Simply put, it’s a password valid for only one login session or transaction, on a computer system or other digital device. 

Here’s a typical usage scenario that explains how they work:

  1. First, the user provides standard credentials (like a username and a traditional password).
  2. The system then checks these details. 
  3. If they’re accurate, it sends a unique code, often consisting of letters, numbers, or a combination of both, to a device that is associated with the user (like a smartphone).
  4. The user enters this code on the website or system.
  5. The system checks if the received code matches the sent code.
  6. If it matches, the user gains access to the system or authorizes the transaction.

The core principles used in generating OTPs are:

  1. Time-synchronization between the authentication server and the client for generating OTPs (TOTP).
  2. Mathematical algorithms that generate OTPs using a secret key and an incrementing counter (HOTP).
  3. OTPs sent through a separate communication channel like SMS, email, or push notifications (mOTP).

This dynamic nature of one-time passwords gives them their strength. Even if a potential attacker gains the one-time password, they won’t be able to use it in the future when it will have either been used or expired. This provides an additional layer of security, making One-Time Passwords a popular choice for two-factor and multi-factor authentication systems.

Use cases and examples of one-time passwords

One-time passwords play a crucial role in maintaining secure user authentication in various sectors. Here are just some use cases for OTPs:

  • Online Banking: OTPs are often used for online banking transactions to prevent fraudulent activities. Whenever a user initiates a transaction like a funds transfer, the bank sends an OTP to the registered mobile number which must be entered to complete the transaction.
  • E-Commerce: On e-commerce sites, during the checkout process or while changing sensitive details like passwords, OTPs might be used as an extra layer of security to confirm the user’s identity. 
  • Two-Factor Authentication: Many applications use OTPs as two-factor authentication (2FA). After entering the username and password, users receive an OTP via email or SMS, which they must enter to access the application. This enhances security by requiring evidence from two separate categories of credentials.
  • Password Reset: If a user forgets their password, an OTP can be sent to their registered email address or mobile number. This OTP can authenticate the user and allow them to reset their password.
  • Device Verification: When logging in from an unrecognized device, services may send an OTP to verify the user. This helps prevent unauthorized access to user accounts.
  • Access Control Systems: In certain high-security environments (like data centers or secure labs), OTPs can be used for physical access control. This ensures that only authorized personnel can enter these environments.
  • Confirming Transactions: For high-value financial transactions or significant account changes, OTPs can confirm the user’s identity and validate the change.

In conclusion, OTPs enhance security by preventing unauthorized access and use in both digital and physical environments. While they’re not foolproof, when used wisely in tandem with other security measures, one-time passwords can significantly minimize cybersecurity risks.

Types of one-time passwords

Expanding upon one-time passwords, there are also time-based one-time passwords (TOTPs) and HMAC-based one-time passwords (HOTPs).


HMAC-based one-time passwords are based on Hash-based Message Authentication Codes (HMAC). It defines an algorithm for authentication that can be used for identity confirmations, transaction validation, and similar security purposes.

HOTP passwords are event-based, so every time a password is used, a counter on the server and client increments, and this counter must be synchronized on both sides. The server-side counter is incremented when the password is accepted and the client-side, when it’s generated. Passwords generated by the HOTP algorithm are valid until used, meaning they do not have an expiration time.


Time-based one-time passwords have a time dependency so passwords are automatically changed at a set interval (typically 30 to 60 seconds). This essentially means that even if an attacker manages to intercept a TOTP password, they will only have a limited time to use it before it expires. The operation of TOTP relies on the client and server staying relatively synchronized in time.

Both HOTP and TOTP are open standards for generating one-time passwords, providing a strong level of security due to the uniqueness of each password they generate. However, they aren’t perfect. Both can be susceptible to various attacks if not properly supplemented with other cybersecurity measures. For example, phishing can be particularly effective because although the password can only be used once, it can be used immediately, which is often enough for an attacker. 

Nevertheless, TOTP and HOTP are significant improvements over static password systems and should be considered essential elements in an organization’s multi-factor authentication strategy.

Strengths of one-time passwords

  • Enhanced Security: One-time passwords (OTPs) provide an additional layer of security, by making it nearly impossible for attackers to guess or predict the password. Using an OTP significantly reduces the risk of unauthorized access to a user’s confidential information.
  • Protection against Replay Attacks: OTPs are only valid for a single login session or transaction. This means that even if an attacker manages to intercept the OTP, they cannot reuse it in a subsequent replay attack.
  • User Convenience: Since OTPs are usually sent to users through their registered mobile phones or emails, there’s no need for them to memorize complex, strong passwords. This enhances the user experience and reduces the load on support teams dealing with forgotten passwords.
  • Mitigation of Password Reuse and Sharing: The dynamic nature of OTPs eliminates the issues of password reuse across multiple platforms and sharing of passwords, common security risks associated with static passwords.
  • Reduced Risk of Brute Force Attacks: Brute force attacks, which attempt to gain access by trying all possible password combinations, are rendered ineffective against OTPs due to their transient nature.
  • Ensuring Transaction Verification: OTPs are often used for verifying crucial transactions, such as online purchases or banking operations, providing users with a secure way to confirm their identities.

Weaknesses of one-time passwords

  • Interception: OTPs are usually sent via SMS or email, making them vulnerable to interception. Skilled hackers might find ways to divert OTP messages, particularly in the case of SMS-based OTPs. 
  • Phishing Attacks: The OTP methodology can be exploited through phishing, where users are tricked into entering their OTPs on a malicious website, thereby granting attackers access.
  • Human Error: Users can inadvertently expose their OTPs, either by leaving them visible on their screens, or sharing them unknowingly with malicious individuals.
  • Time-Sensitive: In general, OTPs are time-sensitive, meaning they expire after a certain period. This creates a sense of urgency that can lead to mistakes and can be exploited by attackers.
  • Dependence on Device: If the device used to receive the OTP is lost, stolen, or runs out of power, the user is effectively locked out until they can regain access to the device.
  • Man-in-the-Middle Attacks: Though rare, sophisticated hackers can execute man-in-the-middle attacks wherein they insert themselves into the communication channel between two parties, potentially intercepting and altering the OTP.
  • SIM Swapping: Some attackers could convince the service provider that they are the actual customer, in an attack called SIM swapping. They could receive the OTP instead of the actual user, thus gaining unauthorized access.
  • Inconvenience: OTPs can be seen as inconvenient by some users due to the need for an extra step in the authentication process or because they require a separate device, such as a mobile phone, which must be readily available.

One-time passwords & multi-factor authentication

One-Time Passwords (OTPs) in multi-factor authentication serve as an additional layer of protection in the user authentication process. 

Multi-factor authentication (MFA) typically combines at least two of the following things: something you know (like a password), something you have (like a smart card or a mobile device), and something you are (like a biometric feature). OTPs are commonly used in the “something you have” category since they are usually sent to a device owned by the user.

Here’s how OTPs help boost multi-factor authentication.

Enhanced Security

OTPs offer a dynamic form of authentication, unlike static, user-created passwords that remain unchanged until manually reset. The time-sensitive nature of an OTP allows it to provide an additional layer of security as it becomes invalid after a short period or as soon as it’s used.

Reduced Vulnerability

In case an OTP is intercepted or accidentally exposed, the risk is minimal as it lacks longevity. A one-time password that has been used or has expired would be useless to malicious actors looking to gain unauthorized access.

User Verification

When an OTP is sent via SMS or email to a user, the multi-factor authentication process also verifies that the user has physical possession of the device registered for the OTP delivery. This adds an extra layer of certainty about the user’s identity.

Mitigates the risk of common attacks

OTPs can serve to prevent common attacks such as keylogging, credential stuffing, and brute force attacks, as the password is unique for each session and doesn’t rely on the user’s input.

Easy integration and wide acceptance

OTPs can be easily integrated into almost any system or platform. They aren’t disruptive to user experience as they are widely accepted as part of improved security measures. 

To conclude, while not foolproof, the use of OTPs as part of a multi-factor authentication strategy adds a significant, and often necessary, obstacle against unauthorized access. By making it that much harder for cybercriminals to succeed, OTPs play a crucial role in comprehensive cybersecurity strategies.

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