AQA A’Level Encryption - Vernam cipher

Summary

In this video, we explore the Vernon cipher, also known as the one-time pad, a method of encryption that guarantees perfect security when used correctly. Introduced by Gilbert Vernon in 1917, the cipher relies on a key that is as long as the plaintext, truly random, and used only once. While offering unparalleled security, its impracticality for widespread use arises from the need for secure physical key exchange. The video demonstrates how this method works through an XOR operation on binary values, explaining why the one-time pad remains a theoretical model of unbreakable encryption.

Takeaways

• 😀 The Vernon cipher, also known as the one-time pad, was invented by Gilbert Vernam in 1917.
• 😀 The one-time pad is considered a cipher with perfect security, but only under specific conditions.
• 😀 For the one-time pad to offer perfect security, the encryption key must be at least as long as the plaintext message.
• 😀 The one-time pad key must be truly random to ensure security.
• 😀 The one-time pad must only be used once and then securely destroyed after encryption.
• 😀 A major limitation of the Vernon cipher is the need for secure, in-person exchange of the encryption key, which is impractical over long distances.
• 😀 Sending the key electronically requires an additional encryption method to prevent interception, making it less secure than physical exchange.
• 😀 The Vernon cipher offers perfect security because it relies on a random key, whereas other encryption methods are based on computational security.
• 😀 All other forms of encryption can theoretically be cracked, unlike the Vernon cipher when conditions are met.
• 😀 In the example shown, the message 'Dave' is encrypted using the one-time pad, where an XOR operation is applied to each character.
• 😀 The recipient can decrypt the message by applying the same one-time pad and XORing the encrypted message to reveal the original plaintext.

Q & A

What is the Vernon cipher and who invented it?

The Vernon cipher, also known as the one-time pad, is a cryptographic encryption method invented by Gilbert Vernon in 1917. It is known for offering perfect security when used under certain conditions.

What are the key conditions for the Vernon cipher to provide perfect security?

For the Vernon cipher to provide perfect security, the encryption key (the one-time pad) must meet three conditions: it must be at least as long as the plaintext message, it must be truly random, and it must be used only once before being destroyed.

Why is it important for the key to be used only once in the Vernon cipher?

The key must be used only once because if the key is reused, it can allow attackers to potentially crack the encryption. Reusing the key undermines the cipher’s perfect security.

Why is the Vernon cipher not commonly used for everyday encryption?

The Vernon cipher is not commonly used because securely sharing the one-time pad key requires physical exchange, which is impractical, especially for long-distance communication or online transmissions.

What would happen if the one-time pad key were transmitted electronically?

If the one-time pad key were transmitted electronically, it would require additional encryption methods to ensure its security, which could potentially make the system vulnerable to attacks, defeating the purpose of the Vernon cipher.

What is the XOR operation and how is it used in the Vernon cipher?

The XOR (exclusive OR) operation is a binary operation used in the Vernon cipher. It compares two bits, returning 1 if the bits are different and 0 if they are the same. In the cipher, XOR is used to combine the binary values of the plaintext message and the one-time pad key, producing the encrypted text.

How does the Vernon cipher offer perfect security compared to other encryption methods?

The Vernon cipher offers perfect security because, under the right conditions, the encrypted message cannot be decrypted without the exact same one-time pad. Unlike other encryption methods, which rely on computational security and can potentially be cracked with enough computational power, the Vernon cipher cannot be cracked if the conditions are met.

What is the process of encrypting a message using the Vernon cipher?

To encrypt a message using the Vernon cipher, you first convert the plaintext into binary using ASCII values. Then, you apply the XOR operation between each binary character of the plaintext and the corresponding binary character of the one-time pad key. The result is the encrypted message.

What would someone need to do to decrypt a message encrypted with the Vernon cipher?

To decrypt a message encrypted with the Vernon cipher, the recipient needs to apply the same one-time pad key to the encrypted message and perform the XOR operation again. This will reveal the original plaintext message.

Why is the Vernon cipher considered unbreakable under ideal conditions?

The Vernon cipher is considered unbreakable because, when the key is truly random, as long as it is at least as long as the message and used only once, there is no pattern to exploit. The ciphertext has no statistical structure to reveal any information about the plaintext, making it impossible to decrypt without the key.