One-Time Password vs. One-Time Pad: What’s the Difference?

Last updated on March 13, 2025

One-Time Pad is an encryption method that uses a random key of the same length as the message to encrypt each character or bit individually, whereas One-Time Password is an authentication method that uses a short and temporary key to authenticate a user or encrypt a message.

In this article, we will compare and contrast One-Time Pad and One-Time Password in terms of security, usability, efficiency, and scalability. We will also discuss some of their applications and challenges in real-world scenarios.

tl;dr: One-Time Password vs. Pad

Unlike the key in One-Time Pad, which is used only once and then discarded, the key in One-Time Password is generated by an algorithm or a device and changes every time or after a certain period. One-Time Pad is the only encryption method that is proven to be unbreakable, meaning that there is no way to decrypt the message without knowing the key. In contrast, One-Time Password is breakable, but it offers a high level of security against attacks that try to reuse or intercept the key. Nonetheless, the One-Time Pad also has some practical limitations, such as the difficulty of generating and distributing truly random keys. For practical reasons, it is the One-Time Password that is widely used in applications that require strong authentication, such as online banking and email.

One-Time Password (OTP) vs. One-Time Pad (OTP): Key Differences and Best Use Cases Explained

Key Differences:

• Purpose and Functionality: A One-Time Password (OTP) is a temporary code used for authentication, typically generated by an algorithm or device, and changes with each use or after a set period. In contrast, a One-Time Pad is an encryption method that uses a random key of the same length as the message, providing theoretically unbreakable encryption when used correctly.​
• Security Level: One-Time Pads offer perfect secrecy under specific conditions, making them unbreakable. However, they are impractical for widespread use due to key management challenges. OTPs, while not unbreakable, provide a high level of security in authentication processes and are widely used in applications like online banking and email.​

Best Use Cases:

• One-Time Passwords (OTPs): Ideal for scenarios requiring secure user authentication, such as logging into online services, accessing sensitive information, or performing financial transactions.​
• One-Time Pads: Suitable for highly sensitive communications where absolute secrecy is paramount, often in diplomatic or military contexts, despite their logistical challenges.

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One-Time Pad: The Unbreakable Cipher

One-Time Pad is an encryption method that uses a random key of the same length as the message to encrypt each character or bit individually. The key is used only once and then discarded. One-Time Pad is the only encryption method that is proven to be unbreakable. This means that there is no way to decrypt the message without knowing the key.

One-Time Pad was first described by Frank Miller in 1882 and then reinvented by Gilbert Vernam and Joseph Mauborgne in 1917. It is based on modular addition or XOR operation, which combines the plaintext and the key in a way that produces a random ciphertext. For example, if the plaintext is HELLO and the key is MONEY, the ciphertext is TSYPM.

One-Time Pad Conditions for Perfect Secrecy

One-Time Pad has four conditions for perfect secrecy:

1. The key must be at least as long as the plaintext.
2. The key must be random and independent of the plaintext.
3. The key must never be reused in whole or in part.
4. The communicating parties must keep the key completely secret.

If these conditions are met, the One-Time Pad is 100% secure. It does not have any statistical relation with the plaintext, so it does not reveal any information about it. It also does not have any weaknesses that cryptanalysis can exploit. Any ciphertext can be decrypted into any plaintext with equal probability, so there is no way to tell which one is correct.

One-Time Pad has been used for critical diplomatic and military communication, especially during World War II and the Cold War era. Also, spies and secret agents used it to exchange messages. However, the One-Time Pad has some practical limitations, such as the difficulty of generating and distributing truly random keys. It also requires a lot of storage space and synchronization between the sender and the receiver. Moreover, it does not provide any authentication or integrity protection for the messages. These downsides make it not very practical to use nowadays.

One-Time Password: The Practical Solution

One-Time Password is an authentication method that uses a short and temporary key to encrypt a message or authenticate a user. The key is generated by an algorithm and changes every time or after a certain period. One-Time Password is not unbreakable, but it offers a high level of security against attacks that try to reuse or intercept the key.

One-Time Passwords can be based on two different mechanisms: Hash-Based One-Time Password (HOTP) and Time-Based One-Time Password (TOTP). HOTP uses a counter as the moving factor, while TOTP uses time as the moving factor. Both mechanisms use an HMAC function to combine the key and the factor to produce a password.

One-Time Password has several advantages over static passwords:

• It reduces the risk of forgotten passwords and password resets.
• It mitigates replay attacks and brute force attacks.
• It enhances Multi-Factor Authentication (MFA) by adding another layer of verification.
• It improves user convenience and experience by simplifying the login process.

One-Time Password is widely used in applications that require strong authentication, such as online banking, email, e-commerce, and social media. It can be delivered to users via various channels, such as SMS and authenticator apps. However, One-Time Password also has some challenges, such as:

• Depends on the availability and reliability of the delivery channel.
• Event-based OTP requires synchronization and coordination between the sender and the receiver.
• Does not provide any encryption or integrity protection for the messages.
• May be vulnerable to phishing or social engineering attacks.

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Enhancing Security with Multi-Factor Authentication (MFA) Using One-Time Passwords

Integrating One-Time Passwords into a Multi-Factor Authentication framework significantly bolsters security. By requiring users to provide both something they know (e.g., a password) and something they have (e.g., an OTP generated by a hardware token like YubiKey OTP or an authenticator app like Rublon Authenticator), organizations can effectively mitigate unauthorized access.​

One-Time Password vs. One-Time Pad: What’s the Difference?

Having different purposes, One-Time Pad and One-Time Password have different strengths and weaknesses. Let’s compare them in terms of five criteria: security, usability, efficiency, and scalability.

Difference #1: Security

One-Time Pad is the most secure encryption method, as it provides perfect secrecy and immunity to cryptanalysis. However, it also has strict requirements for key generation, distribution, usage, and storage. If these requirements are not met, the One-Time Pad can be compromised or broken. One-Time Password is not as secure as One-Time Pad, as it can be vulnerable to phishing, social engineering, and brute force attacks. However, it also provides a high level of security against replay attacks and password theft. It also enhances multi-factor authentication by adding another layer of verification.

Difference #2: Usability

One-Time Pad is not very user-friendly, as it requires a lot of manual work and coordination between the sender and the receiver. It also requires a large amount of storage space and synchronization for the keys. In contrast to the One-Time Pad, the One-Time Password is more user-friendly, as it simplifies the login process and reduces the risk of forgotten passwords. It also offers various delivery channels for passwords, such as SMS, email, and authenticator apps like Google Authenticator, Microsoft Authenticator, and Rublon Authenticator.

Difference #3: Efficiency

One-Time Pad is not very efficient. It requires a large amount of computational resources and bandwidth to generate and transmit the keys. Further, it has a low throughput rate, as the key length must match the message length. In stark contrast, One-Time Password is more efficient, as it requires less computational resources and bandwidth to generate and transmit the passwords. It also has a higher throughput rate, as the password length is usually shorter than the message length.

Difference #4: Scalability

One-Time Pad is not very scalable, as it requires a large number of keys for each communication session or transaction. It also has a high maintenance cost, as the keys must be securely stored and disposed of after use. On the other hand, One-Time Password is more scalable, as it requires fewer keys for each communication session or transaction. It also has a low maintenance cost, as the keys are automatically generated and expired after use.

One-Time Password vs. One-Time Pad: Comparison Table

Criteria	One-Time Password	One-Time Pad
Main purpose	Authentication	Encryption
Key generation	Pseudorandom	Truly random (if done correctly)
Key length	Short and variable	Same as the message length
Key usage	Multiple times or once per period	Only once
Key distribution	Various channels (SMS, email, app, etc.)	Manual or secure channel
Encryption method	Hash-based or time-based	Modular addition or XOR
Security level	High, but breakable	Highest, unbreakable
Practicality	High, user-friendly, and convenient	Low, user-unfriendly, and tedious

Potential Cyberattacks Targeting One-Time Passwords and One-Time Pads

Understanding the vulnerabilities associated with each method is crucial:​

• One-Time Passwords: Susceptible to phishing attacks, man-in-the-middle attacks, and SIM swapping, where attackers intercept or redirect OTPs to gain unauthorized access.
• One-Time Pads: While theoretically unbreakable, practical implementation challenges such as key generation, distribution, and management can introduce vulnerabilities. Reusing keys or failing to maintain key secrecy compromises security.

Using One-Time Passwords in Modern Multi-Factor Authentication Systems

For small to medium-sized businesses (SMBs) and enterprises aiming to enhance security protocols, integrating One-Time Passwords (OTPs) into Multi-Factor Authentication (MFA) systems offers a practical and effective solution. OTPs provide an additional verification layer, ensuring that even if primary credentials are compromised, unauthorized access is mitigated.​

Advantages of OTP-Based MFA for Organizations:

• Enhanced Security: OTPs are dynamic and expire after a single use, reducing the risk of credential-based attacks.​
• Cost-Effective Implementation: Utilizing software-based OTP generators, such as mobile authenticator apps, minimizes the need for additional hardware, making it a budget-friendly option for SMBs.​
• User Convenience: Employees can receive OTPs through various channels, including mobile apps, SMS, or email, facilitating seamless integration into existing workflows.

Considerations for Critical Infrastructure:

While OTPs are suitable for everyday applications, securing critical infrastructure demands more robust, phishing-resistant authentication methods:​

• Phishing Risks: OTPs delivered via SMS or email are susceptible to interception through phishing attacks or SIM swapping. ​
• Advanced Threats: Sophisticated adversaries may exploit vulnerabilities in OTP delivery mechanisms, compromising sensitive systems.

Adopting Phishing-Resistant Authentication Methods:

To safeguard critical assets, organizations should consider implementing stronger authentication solutions:​

• FIDO Security Keys: WebAuthn/U2F security keys offer robust protection against phishing and man-in-the-middle attacks. These hardware authenticators utilize public-key cryptography, ensuring that private keys remain securely on the device.​
• FIDO Passkeys: Passkeys can be either software-bound or hardware-bound. They can enable passwordless login experiences and are inherently phishing-resistant. Security keys and passkeys are different authentication methods, even though both leverage the FIDO standard.

Strategic Implementation of OTP MFA – Expert Recommendations:

• Assess Security Requirements: Identify systems and data that require elevated protection to determine appropriate authentication methods.​
• Implement Layered Security: Combine OTP-based MFA for general applications with phishing-resistant methods for critical systems, ensuring a balanced security posture.​
• Educate Employees: Conduct regular training sessions to raise awareness about phishing tactics and the importance of adhering to security protocols.​
• Stay Informed: Monitor emerging threats and update authentication strategies accordingly to address evolving security challenges.

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One-Time Password vs. Pad: Conclusion

In conclusion, One-Time Pad and One-Time Password both aim to improve your security. However, they do it in different ways. While One-Time Password secures user authentication, One-Time Pad ensures secure encryption. One-Time Pad is a secure authentication method, but it also has many practical limitations. One-Time Pad is not very practical in the real world, but it is theoretically unbreakable. Implementing it poses many security challenges, though. All in all, use a One-Time Password for authentication and a One-Time Pad for encryption.