One-Time Pad vs Meet-in-the-Middle

TL;DR

A correctly implemented one-time pad can prevent a meet-in-the-middle (MITM) attack, but only if the key is truly random, as long as the message, and used only once. If any of these conditions are broken, MITM attacks become possible.

What is a Meet-in-the-Middle Attack?

Imagine Alice wants to send Bob a secret message encrypted with two different keys. An attacker intercepts both ciphertexts. They try to decrypt each ciphertext using all possible keys for each encryption scheme. The goal is to find a common plaintext value – the ‘meeting point’ in the middle. If successful, they’ve broken the encryption.

How One-Time Pads Work

A one-time pad (OTP) uses a key that is:

• Truly Random: Generated from a source of high entropy (e.g., hardware random number generator).
• As Long As The Message: The key must be the same length as the plaintext message.
• Used Only Once: Crucially, the key is never reused for any other message.

Encryption and decryption are typically done using XOR (exclusive OR) operation.

Can a One-Time Pad Prevent MITM?

1. Perfect Secrecy: A properly used OTP provides perfect secrecy. This means the ciphertext reveals absolutely no information about the plaintext.
2. MITM Requires Knowledge of Keys: For a MITM attack to succeed, the attacker needs to know (or guess) parts of both keys involved in the double encryption. With a true OTP, this is impossible.
3. Randomness is Key: If the key isn’t truly random, patterns emerge that an attacker can exploit. This weakens the security and makes MITM attacks feasible.

Step-by-Step Protection Against MITM with OTP

1. Generate a Random Key: Use a cryptographically secure random number generator (CSPRNG).

   openssl rand -base64 32

   This generates a 32-byte key in base64 format.

2. Encrypt the Message: XOR the plaintext message with the key.
3. Decryption: XOR the ciphertext with the same key to recover the original message.
4. Key Disposal: Immediately and securely destroy the key after use. Do not store it for later reuse!

Why OTPs Fail (and MITM Succeeds)

• Key Reuse: If you reuse a key, an attacker can XOR two ciphertexts encrypted with that same key to eliminate the key and reveal information about the plaintexts. This is a classic vulnerability exploited in many attacks.
• Non-Random Keys: Predictable keys (e.g., based on dates or easily guessable patterns) allow attackers to reduce the search space for the correct key, making MITM attacks practical.
• Compromised Key Exchange: If an attacker intercepts the key during exchange, they can decrypt messages and launch a MITM attack. Secure key distribution is essential.

Example (Simplified)

Let’s say Alice wants to send Bob the message “Hello”.

1. Key Generation: A random key, for example, “XORPD”
2. Encryption: XOR “Hello” with “XORPD”. (This is a simplified illustration; actual implementation uses binary representation).
3. Decryption: Bob XORs the ciphertext with “XORPD” to get “Hello”.

If the key were reused, an attacker intercepting two ciphertexts could perform XOR operations and potentially recover plaintext.

cyber security Considerations

While OTPs offer strong theoretical security, they are impractical for most real-world applications due to the challenges of secure key management. More practical encryption algorithms (like AES) provide a good balance between security and usability. However, understanding the principles behind OTPs is crucial for appreciating the importance of randomness, key uniqueness, and proper implementation in any cyber security system.