• Abbreviated as OTP • First description was presented by Frank Miller in 1882 • Regiestered as a patent by Vernam in 1917 • One of the simplest cryptographic schemes • Under certain conditions, it satisfies the perfect secrecy requirements. • Was historically used by the German diplomats pre-WWII and by KGB spies during the cold war.
Use xor operation, i.e., xor the plaintext/ciphertext with the key. All operations are performed using the binary representation.
Encryption: plaintext ⨁ key → ciphertext Decryption: ciphertext ⨁ key → plaintext
Encrypt the plaintext 'A' using the key 'p' 'A' has the ASCII value of 112, and 'p' has the ASCII value of 65
Decryption:
• Concept developed by Claude Shannon using information theory. • Proved that OTP is "information-theoretically secure", mathematically speaking.
<aside> 💡 If a ciphertext provides no information about the corresponding plaintext, except length, it satisfies the perfect secrecy property.
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This means regardless of how much ciphertext you have, there is no statistical difference, and breaking the cipher is as difficult.
In other terms, the ciphertext can be equally likely transmitted to any plaintext. P[e(x1,k) = y] = P[e(x2,k) = y]
where x is plaintext, y is ciphertext, k is key, e is encryption function and P denotes probability.
<aside> 💡 Any cryptographic scheme that satisfies the above property is considered to have Perfect Secrecy.
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For instance, if the ciphertext SGM was generated by a perfectly secure scheme, then it could have been generated from the key HIT, SIT, RUN, RAT, HEN, SIN, or any other three letter word with equal probabilities.