2 files changed, 51 insertions(+), 2 deletions(-)

A examples/15.rs
M src/aes.rs

@@ 0,0 1,16 @@
+use std::error::Error;
+
+use cryptopals::aes::pkcs7_validation;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    println!(
+        "{}",
+        String::from_utf8(
+            pkcs7_validation(b"ICE ICE BABY\x04\x04\x04\x04".to_vec(), 16)
+                .expect("valid padding detected as invalid")
+        )
+        .unwrap()
+    );
+
+    Ok(())
+}

@@ 86,8 86,8 @@ pub fn cbc_decrypt(ciphertext: Vec<u8>, key: Vec<u8>, iv: Vec<u8>) -> Vec<u8> {
     plaintext
 }
 
-pub fn pkcs7(bytes: Vec<u8>, blocksize: u32) -> Vec<u8> {
-    let padding_size = blocksize - (bytes.len() as u32 % blocksize);
+pub fn pkcs7(bytes: Vec<u8>, block_size: usize) -> Vec<u8> {
+    let padding_size = block_size - (bytes.len() % block_size);
     let mut padded = bytes;
     for _ in 0..padding_size {
         padded.push(padding_size as u8);


@@ 96,6 96,28 @@ pub fn pkcs7(bytes: Vec<u8>, blocksize: u32) -> Vec<u8> {
     padded
 }
 
+pub fn pkcs7_validation<'a>(bytes: Vec<u8>, block_size: usize) -> Result<Vec<u8>, &'a str> {
+    if bytes.len() % block_size != 0 {
+        return Err("invalid length");
+    }
+
+    let final_block = bytes[bytes.len() - block_size..].to_vec();
+    let final_byte = final_block[final_block.len() - 1];
+
+    if final_byte > 16 {
+        return Err("invalid padding");
+    }
+
+    let padding_start = block_size - (final_byte as usize);
+    let supposed_padding = final_block[padding_start..].to_vec();
+
+    if !supposed_padding.iter().all(|&x| x == final_byte) {
+        return Err("invalid padding");
+    }
+
+    Ok(bytes[..bytes.len() - block_size + padding_start].to_vec())
+}
+
 pub fn random_key() -> Vec<u8> {
     let mut key = vec![0; 16];
     rand_bytes(&mut key).unwrap();


@@ 124,4 146,15 @@ mod tests {
 
         assert_eq!(plaintext, decrypted);
     }
+
+    #[test]
+    fn test_pkcs7_validation() {
+        assert_eq!(
+            pkcs7_validation(b"ICE ICE BABY\x04\x04\x04\x04".to_vec(), 16),
+            Ok(b"ICE ICE BABY".to_vec())
+        );
+
+        assert!(pkcs7_validation(b"ICE ICE BABY\x05\x05\x05\x05".to_vec(), 16).is_err());
+        assert!(pkcs7_validation(b"ICE ICE BABY\x01\x02\x03\x04".to_vec(), 16).is_err());
+    }
 }