Question

Proof that One Time Pad (OTP) is perfectly secured under Ciphertext only attack (COA). Please explain briefly each step of the proof.

Answer 1

The One Time Pad ( OTP) is a encryption technique which cannot be cracked. In this technique a plain text is paired with a random secret key. Then each bit or the character is encrypted by combining it with the corresponding bit or character from the pad using the modular addition. The resultant cipher text is very impossible to decrypt if the below four condition were met:,

1) The key must be truly random

2) The key must be at least as long as the plaintext.

3) The key must never be used as whole.

4) The key should be completly secret.

The One Time Pad encryption is a binary additives stream cipher, where a stream of truly random keys is generated and then it combined with the plain text for encryption and with the cipher text for decryption by an XOR addition method.

True Random Key Generation

In One Time Pad Encryption , a truly random key must be employed. For Example , in mils electronic products, all keys are generated by the True Random Noise Source . This Source is incorporated into the hardware security token of each mils electronic application.

The Encryption Process

One Time Pad keys are used in pairs. One copy of the key is kept by each user and the keys are distributed securely prior to encryption. The confidentiality and authenticity of the One Time Pad keys are assured by continuous protection during their distribution and storage. This guarantees that outsiders will not be able to misuse the key (e.g. by copying or altering the key during distribution).

Steps

1. To encrypt plain text data, the sender uses a key string equal in length to the plain text. The key is used by mixing (XOR-ing) bit by bit, always a bit of the key with a bit of the plain text to create a bit of cipher text.
2. This cipher text is then sent to the recipient.
3. At the recipient’s end, the encoded message is mixed (XOR-ed) with the duplicate copy of the One Time Key and the plain text is restored.
4. Both sender’s and recipient’s keys are automatically destroyed after use, to ensure re-application of the same key is not possible