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Also known as: OTP
An encryption scheme that XORs plaintext with a truly random, single-use key achieving perfect secrecy when used correctly.
DESCRIPTION
The one-time pad uses a random key at least as long as the message, applied once via XOR. When keys are truly random, never reused, and kept secret, it offers information-theoretic security. Practical challenges include generating and distributing large random keys and preventing reuse. OTPs inspired modern stream ciphers that approximate ideal properties with pseudorandomness and authentication.
ALTERNATIVE DEFINITIONS
Wikipedia
The one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a one-time pre-shared key that is the same size as, or longer than, the message being sent.
States perfect secrecy and key length requirement.
NIST Glossary
An encryption technique in which a random key the same length as the plaintext is used only once to encrypt the message, providing theoretically unbreakable security.
Emphasizes one-time use and theoretical security.
RELATED TERMS
Vernam Cipher Unbreakable Encryption Shannon Entropy Key Distribution Randomness Binary XOR ChaCha20 Rijndael Keystream
COMPETING TERMS
Stream Cipher Symmetric Block Cipher Algorithms
KNOWLEDGE AREAS
TARGET AUDIENCE
Cybersecurity Engineer Security Architect Chief Information Security Officer Software Developer
EXTERNAL SOURCES
- Wikipedia - One-time pad
- NIST SP 800-90A - Random number generation guidance
- Bruce Schneier - Book discussing OTP and stream ciphers
TOP QUESTIONS
Why is the One-time Pad considered perfectly secure?
It achieves Shannon’s perfect secrecy when keys are truly random, used once, and kept secret, making ciphertext statistically independent of plaintext.
Category: Overview
What is the biggest practical issue with One-time Pad?
Securely generating, distributing, and storing long random keys is challenging at scale.
Category: Risks
What happens if a One-time Pad key is reused?
Keystream reuse enables attackers to recover information by XORing ciphertexts, devastating security.
Category: Mistakes
Is One-time Pad used in modern systems?
Rarely; modern stream ciphers and AEAD modes offer practical security with manageable keys.
Category: Opinion
How should random keys be generated for One-time Pad?
Use high-quality entropy sources and validated RNGs; avoid pseudorandom generators for OTP keys.
Category: Best practices
What is an alternative to One-time Pad?
Use AES-CTR or ChaCha20 with robust key management and authentication (e.g., GCM or Poly1305).
Category: Alternatives
What is a striking analogy for One-time Pad?
It’s like a single-use, perfectly camouflaged blanket: reuse leaves recognizable patterns.
Category: Example
What standards relate to randomness for One-time Pad?
NIST SP 800-90 series provides RNG guidance and validation criteria.
Category: Overview
Can One-time Pad be combined with authentication?
Yes, add an independent MAC, but OTP alone provides only confidentiality.
Category: Requirements