Implement challenge 15 pkcs7 validation.

src/bytes.rs

@@ -101,6 +101,22 @@ impl Bytes {
         }
     }
 
+    pub fn has_valid_pkcs7(&mut self, block_size: usize) -> bool {
+        if self.len() > 0 && self.len() % block_size != 0 {
+            return false;
+        }
+        let last_block_index = self.len() / block_size - 1;
+        let last_block = self.get_block(last_block_index, block_size).0;
+        let pad_byte_count = last_block[block_size - 1];
+        for i in 0..(pad_byte_count as usize) {
+            let byte = last_block[block_size - 1 - i];
+            if byte != pad_byte_count {
+                return false;
+            }
+        }
+        return true;
+    }
+
     pub fn xor(Bytes(a): &Bytes, Bytes(b): &Bytes) -> Bytes {
         Bytes(
             Iterator::zip(a.iter(), b.iter())