Cryptography challenges

List of challenges

• Convert hex to base64
• Fixed XOR
• Single-byte XOR cipher
• Detect single-character XOR
• Implement repeating-key XOR
• Break repeating-key XOR
• AES in ECB mode
• Detect AES in ECB mode
• Implement PKCS#7 padding
• Implement CBC mode
• The CBC padding oracle
• Implement CTR mode
• Break fixed-nonce CTR mode using substitutions
• Recover the key from repeating-nonce CTR
• Implement the MT19937 Mersenne Twister RNG
• Break MT19937 seed
• Clone an MT19937 RNG
• Create the MT19937 stream cipher
• Break the MT19937 stream cipher
• AES key recovery from nonce
• Implement RSA
• Implement an E=3 RSA broadcast attack
• Implement Diffie-Hellman
• Implement a MITM attack on Diffie-Hellman
• Implement an authenticated key exchange
• Break SRP with a zero key
• Implement simple DSA signing
• Break a bad DSA implementation
• Implement elliptic curve operations

Convert hex to base64

The string

49276d206b696c6c696e6720796f757220627261696e206c696b65206120706f69736f6e6f7573206d757368726f6f6d

Should produce :

SSdtIGtpbGxpbmcgeW91ciBicmFpbiBsaWtlIGEgcG9pc29ub3VzIG11c2hyb29t

Code

use base64::*;
use hex::FromHex;
use anyhow::*;
use prelude::BASE64_STANDARD;


fn hexto64(hex: &str) -> Result<String, Error>{

    let byte = hex::decode(hex)?;

    let result = BASE64_STANDARD.encode(byte);

    Ok(result)
}

Fixed XOR

Write a function that takes two equal-length buffers and produces their XOR combination.

If your function works properly, then when you feed it the string:

1c0111001f010100061a024b53535009181c

... after hex decoding, and when XOR'd against:

686974207468652062756c6c277320657965

... should produce:

746865206b696420646f6e277420706c6179

Code

use base64::*;
use hex::FromHex;
use anyhow::*;

fn fixed_xor(input1: &str, input2: &str) -> Result<String, Error>{

    let mut byte1 = hex::decode(input1)?;
    let byte2 = hex::decode(input2)?;

    for i in 0..byte1.len(){
        byte1[i] ^= byte2[i];
    }

    let result = hex::encode(byte1);
    Ok(result)
}

Single-byte XOR cipher

The hex encoded string:

1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736

... has been XOR'd against a single character. Find the key, decrypt the message.

You can do this by hand. But don't: write code to do it for you.

How? Devise some method for "scoring" a piece of English plaintext. Character frequency is a good metric. Evaluate each output and choose the one with the best score.