What is a One Time Pad?
A one time pad, also known as a “Vernam cipher” or “perfect cipher,” is an encryption technique that provides absolute security when used correctly. It works by using a random key that is as long as the message itself. This key is used only once, hence the name “one-time pad.”
Think to yourself…
What benefits would having the key as long as the message have?
What limitations could this pose?
Key characteristics?
Perfect Security: When used with a truly random and secret key, a one-time pad provides perfect security. This means that even with unlimited computational power, an attacker cannot determine the original message. This is because the key is as long as the message, making it statistically impossible to crack.
Key Length: The key must be as long as the message, which can be a limitation for practical use. Sharing long, random keys securely can be challenging.
Key Reusability: The key should never be reused for another message. Reusing a one-time pad key weakens the security because it allows patterns to emerge that attackers might exploit.
Key Security: Protecting the secrecy and integrity of the key is crucial. If the key is compromised, all security is lost.
Limited Practical Use: One-time pads are rarely used in modern communication due to their limitations, especially in securely distributing and managing long, random keys. Instead, modern encryption methods like public-key cryptography and symmetric-key algorithms are more practical for most purposes.
How does it work?
Key Generation: To encrypt a message using a one-time pad, you need a random key that is as long as the message. This key can be generated using truly random processes, such as a hardware random number generator, to ensure that it’s unpredictable and unique for each use.
Key Distribution: The key must be securely shared between the sender and the receiver. This is a significant challenge because if the key is compromised or intercepted, the security of the system is lost. In practice, this is one of the main limitations of one-time pads.
Encryption: To encrypt a message, the sender combines each character of the plaintext (the message) with the corresponding character in the key using modular addition. This process creates the ciphertext. The key is never reused for another message.
Decryption: To decrypt the message, the receiver uses the same one-time pad key and subtracts it from the ciphertext using modular subtraction to recover the original plaintext.
Have a go at a Vernam Cipher here.
Is it effective?
A one-time pad is a theoretically perfect encryption method that relies on using a random and secret key that is as long as the message. While it provides unbreakable security, it has practical limitations that make it less suitable for most everyday encryption needs.