From 956d75ab4eaaad271a3e82ffc821f420fb9cbb66 Mon Sep 17 00:00:00 2001
From: Felix Martin <mail@felixm.de>
Date: Tue, 28 Jun 2022 15:56:31 -0400
Subject: [PATCH] Finish challenge 16 and problem set 2.

---
 src/bytes.rs  | 19 ++++++++++++++++++-
 src/parser.rs |  8 +++++++-
 src/set2.rs   | 34 +++++++++++++++++++++++++++++++++-
 3 files changed, 58 insertions(+), 3 deletions(-)

diff --git a/src/bytes.rs b/src/bytes.rs
index 021c275..ec74ada 100644
--- a/src/bytes.rs
+++ b/src/bytes.rs
@@ -1,6 +1,6 @@
 #![allow(dead_code)]
 use rand::Rng;
-use std::fmt::Write; // need to import this trait
+use std::fmt::Write;
 
 #[derive(PartialEq, PartialOrd, Debug)]
 pub struct Bytes(pub Vec<u8>);
@@ -117,6 +117,23 @@ impl Bytes {
         return true;
     }
 
+    pub fn remove_pkcs7(&mut self, block_size: usize) -> () {
+        if !self.has_valid_pkcs7(block_size) {
+            return;
+        }
+        let Bytes(v) = self;
+        let pad_byte_count = v[v.len() - 1];
+        for _ in 0..(pad_byte_count as usize) {
+            v.pop();
+        }
+    }
+
+    pub fn flip_bit(&mut self, byte_index: usize, bit_index: usize) -> () {
+        let Bytes(v) = self;
+        let flip_mask: u8 = 0b1 << bit_index;
+        v[byte_index] ^= flip_mask;
+    }
+
     pub fn xor(Bytes(a): &Bytes, Bytes(b): &Bytes) -> Bytes {
         Bytes(
             Iterator::zip(a.iter(), b.iter())
diff --git a/src/parser.rs b/src/parser.rs
index 1df0516..a6de9dd 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -5,6 +5,7 @@ pub enum Token {
     Identifier(String),
     Equal,
     Ampersand,
+    Semicolon,
 }
 
 pub type Tokens = Vec<Token>;
@@ -19,6 +20,9 @@ pub fn parse_key_value(text: &str) -> HashMap<String, String> {
             [Token::Identifier(key), Token::Equal, Token::Identifier(value), Token::Ampersand] => {
                 result.insert(key.to_string(), value.to_string());
             }
+            [Token::Identifier(key), Token::Equal, Token::Identifier(value), Token::Semicolon] => {
+                result.insert(key.to_string(), value.to_string());
+            }
             [Token::Identifier(key), Token::Equal, Token::Identifier(value)] => {
                 result.insert(key.to_string(), value.to_string());
             }
@@ -40,6 +44,8 @@ fn scan(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
         tokens.push(Token::Ampersand);
     } else if c == '=' {
         tokens.push(Token::Equal);
+    } else if c == ';' {
+        tokens.push(Token::Semicolon);
     } else {
         panic!("Unexpected char '{}' at index {}", c, ix);
     }
@@ -62,4 +68,4 @@ fn scan_identifier(code: &str, mut ix: usize, mut tokens: Tokens) -> Tokens {
     scan(code, ix, tokens)
 }
 
-const SPECIAL_CHARS: &[char] = &['.', '@'];
+const SPECIAL_CHARS: &[char] = &['.', '@', '%'];
diff --git a/src/set2.rs b/src/set2.rs
index 9855e6b..cc28324 100644
--- a/src/set2.rs
+++ b/src/set2.rs
@@ -411,5 +411,37 @@ pub fn challenge15() {
 }
 
 pub fn challenge16() {
-    println!("[xxxx] Challenge 16: TBD");
+    fn encrypt(input: &str, key: &Bytes, iv: &Bytes) -> Bytes {
+        let mut r = String::new();
+        for c in input.chars() {
+            if c == ';' || c == '=' {
+                panic!("encrypt: invalid char {}", c);
+            }
+        }
+        r.push_str("comment1=cooking%20MCs;userdata=");
+        r.push_str(input);
+        r.push_str(";comment2=%20like%20a%20pound%20of%20bacon");
+        let mut cleartext = Bytes(r.as_bytes().to_vec());
+        cleartext.pad_pkcs7(16);
+        cbc::encrypt(&key, &iv, &cleartext)
+    }
+
+    let iv = Bytes::random(16);
+    let key = Bytes::random(16);
+
+    // 0               16              32              48              64
+    // 0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f0..34..78..bc..f
+    // comment1=cooking%20MCs;userdata=xxx=x;admin=true;comment2=%20like%20a%20pound%20of%20bacon
+    // flip_bit(2, '9') = '='; flip_bit(1, '9') = ';'
+    let mut cipher = encrypt("xxx9x9admin9true", &key, &iv);
+    cipher.flip_bit(19, 2); // results in flipping same bit in byte of the following block
+    cipher.flip_bit(21, 1);
+    cipher.flip_bit(27, 2);
+    let mut cleartext = cbc::decrypt(&key, &iv, &cipher);
+    cleartext.remove_pkcs7(16);
+    let cleartext_stripped = Bytes(cleartext.0[32..].to_vec());
+    let dict = parser::parse_key_value(&cleartext_stripped.to_utf8());
+    let admin_status = dict.get("admin").unwrap();
+    assert_eq!(admin_status, "true");
+    println!("[okay] Challenge 16: admin={}", admin_status);
 }