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Real World Crypto 101

My notes when reading Real-World Cryptography

Hash function convert from input to digest

• Pre-image resistance: Given digest, can not find input
• Second pre-image resistance: Given input, digest, can not find another input produce same digest. Small change to input make digest big change.
• Collision resistance: Can not find 2 input produce same digest.

MAC aka Message Authentication Code produce from key, message to authentication tag.

• A send B message with MAC (generate from message and A key).
• B double check message with MAC (generate from receive message and B key).
• A and B use same key.

sequenceDiagram
    participant alice
    participant bob

    alice ->> bob: send alice, mac(secret_key_alice, alice)
    bob ->> bob: compare mac(secret_key_alice, alice) with mac(secret_key_bob, alice)

• Prevent forgery: without secret_key, can not generate MAC even if knowing a lot of alice and mac(secret_key, alice),
• Prevent collisions: keep MAC long enough (256-bit),
• Replay attacks: send transaction 2 times with perfectly MAC and u know why -> instead of mac(secret_key, alice), use counter as mac(secret_key, counter, alice).
• Verify must be done in constant time: if not, probaly return error the moment the bytes differ -> attacker recreate byte by byte by measuring how long -> timing attacks

Constant time comparision:

for i := 0; i < len(x); i++ {
    // Use XOR instead of compare x[i] == y[i]
    // If x[i] == y[i] -> XOR is 1
    // Otherwise XOR is 0
    v |= x[i] ^ y[i]
}

// v == 1 means all XOR is 1 means x == y

Use for:

• Integrity: because MAC ensure no one can tamper with message without noticing

sequenceDiagram
    participant alice
    participant bob

    alice ->> bob: send username, password
    bob -->> alice: return alice|mac(secret_key, alice)
    alice ->> bob: send alice|mac(secret_key, alice)
    bob -->> alice: return OK
    alice ->> bob: send bob|mac(secret_key, alice)
    bob -->> alice: return ERROR

HMAC is MAC using hash

AES

Currently (2023) the world using AES-128 which take a key 128 bits == 16 bytes/

• Take a variable-length key
• Take plaintext of 128 bits
• Give ciphertext of 128 bits

AES is kind of cipher, handle fixed-size plaintext so we called block cipher.