Solve challenge 20

data/set3/challenge20.txt

@@ -0,0 +1,60 @@
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src/set1.rs

@@ -106,36 +106,44 @@ fn challange5() {
     assert_eq!(xor_rep(input, key), hex_to_raw(output));
 }
 
-#[test]
-fn challange6() {
-    let bytes = read_base64("data/set1/challange6.txt");
-
+pub fn guess_rep_xor_key_size(block: &[u8]) -> usize {
     let mut key_size = 0;
     let mut min_hamming_distance = f64::MAX;
     // find the block length (key size) that produces the minimal
     // hamming distance for two consecutive blocks
     // * for resilience, I'm using 4x the block length
-    for l in 2..=40 {
+    for l in 2..=60 {
         // test 4 blocks of key_size
-        let hd = hamming_distance(&bytes[..4 * l], &bytes[4 * l..8 * l]);
+        let hd = hamming_distance(&block[..4 * l], &block[4 * l..8 * l]);
         let normalized_hd = f64::from(hd) / f64::from(l as u8);
         if normalized_hd < min_hamming_distance {
             min_hamming_distance = normalized_hd;
             key_size = l;
         }
     }
+    key_size
+}
 
-    let key: Vec<u8> = (0..key_size)
+// Returns the constructed key
+pub fn break_rep_xor(ciphertext: &[u8]) -> Vec<u8> {
+    let key_size = guess_rep_xor_key_size(&ciphertext);
+    (0..key_size)
         .map(|offset| {
-            bytes
+            ciphertext
                 .iter()
                 .skip(offset)
                 .step_by(key_size)
                 .map(|&x| x)
                 .collect::<Vec<u8>>()
         }) // group by xor-ed with same byte of key
         .map(|group| xor_cross_entropy_analysis(&group).0)
-        .collect();
+        .collect()
+}
+
+#[test]
+fn challange6() {
+    let bytes = read_base64("data/set1/challange6.txt");
+    let key = break_rep_xor(&bytes);
 
     // encryption is symmetric
     let raw = xor_rep(&bytes, &key);

src/set3.rs

@@ -2,8 +2,9 @@ use crate::{
     cbc::*,
     ctr::{aes128_ctr_decrypt, aes128_ctr_encrypt},
     ecb::*,
-    set1::xor_cross_entropy_analysis,
+    set1::{break_rep_xor, xor_cross_entropy_analysis},
     utils::{conversions::base64_to_raw, io::*},
+    xor::xor_rep,
 };
 use lazy_static::lazy_static;
 use rand::{Rng, SeedableRng};
@@ -156,7 +157,10 @@ fn challange18() {
 fn challange19() {
     let plaintexts = read_base64_lines("data/set3/challenge19.txt");
     let key = rand::rngs::StdRng::from_seed([57; 32]).gen();
-    let ciphertexts: Vec<_> = plaintexts.iter().map(|p| aes128_ctr_encrypt(&p, &key, 0, 0)).collect();
+    let ciphertexts: Vec<_> = plaintexts
+        .iter()
+        .map(|p| aes128_ctr_encrypt(&p, &key, 0, 0))
+        .collect();
 
     let mut decoded = vec![Vec::new(); ciphertexts.len()];
     let len = ciphertexts.iter().map(|c| c.len()).max().unwrap();
@@ -177,3 +181,29 @@ fn challange19() {
     assert!(String::from_utf8_lossy(&decoded[34]).contains("yet I number him in the song"));
     // doesn't decode all the way correctly but gets the bulk of the job done
 }
+
+#[test]
+fn challange20() {
+    let plaintexts = read_base64_lines("data/set3/challenge20.txt");
+    let key = rand::rngs::StdRng::from_seed([57; 32]).gen();
+    let ciphertexts: Vec<_> = plaintexts
+        .iter()
+        .map(|p| aes128_ctr_encrypt(&p, &key, 0, 0))
+        .collect();
+
+    let len = ciphertexts.iter().map(|c| c.len()).min().unwrap();
+    let mut long_cipher: Vec<u8> = Vec::with_capacity(len * ciphertexts.len());
+    for ciphertext in ciphertexts {
+        long_cipher.extend(&ciphertext[..len]);
+    }
+
+    let key = break_rep_xor(&long_cipher);
+    let long_plain = xor_rep(&long_cipher, &key);
+    let decoded: Vec<_> = long_plain.chunks_exact(len).map(|x| x.to_vec()).collect();
+
+    assert_eq!(plaintexts.len(), decoded.len());
+    assert!(String::from_utf8_lossy(&decoded[5]).contains("when I come your warned"));
+    assert!(String::from_utf8_lossy(&decoded[34])
+        .contains("wake ya with hundreds of thousands of volts"));
+    // doesn't decode all the way correctly but gets the bulk of the job done
+}