Notes - One-Time Pads

The following are notes from Introduction to Cryptography with Coding Theory.

• One-Time Pad is an unbreakable cryptosystem developed by Gilbert Vernam and Joseph Mauborgne 1918
• Key is a random sequence of 0,1 same length as the message and are added together with XOR aka exclusive or
• Example
• plaintext  00101001
• key         10101100
• ciphertext 10000101
• Decryption uses the same key, add teh key onto the ciphertext to return the plaintext
• If used on the alphabet, the key is a random sequence of shifts between 0 to 25
• unbreakable for ciphertext only attack
• if we only have a piece of plaintext, the random generation means we know nothing about the rest of the key
• Issues
• truly random generation
• trusted courier
• key is very long, dangerous to reuse
• Trivia
• hotline between washington and USSR was thought to be a one-time pad
• Variation of one time pad
• satellite produce and broadcast several random sequences of bits at a rate fast enough that no computer can store more than a very small faction of the outputs
• Alice wants to send message to bob
• use RSA to agree on a method to sample bits from the random bit streams, use these bits to generate a key for a one time pad
• Attacker Eve cannot decrypt because by the time she knows about the message the stream has disappeared

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