finish 9

set2/challenge9.py

@@ -0,0 +1,35 @@
+"""
+PKCS#7 padding is a byte-padding scheme that pads the 
+input data to a specific block length by appending N 
+bytes of value N, where N is the number of bytes required 
+to reach the desired block size.
+"""
+
+
+def pkcs7_pad(data: bytes, block_size: int) -> bytes:
+    """Apply PKCS#7 padding to the given data."""
+    padding_size = block_size - (len(data) % block_size)
+    padding = bytes([padding_size] * padding_size)
+    return data + padding
+
+def pkcs7_unpad(data: bytes, block_size: int) -> bytes:
+    """Remove PKCS#7 padding from the given data."""
+    padding_size = data[-1]
+    if padding_size > block_size or padding_size == 0:
+        raise ValueError('Invalid padding')
+    padding = bytes([padding_size] * padding_size)
+    if data[-padding_size:] != padding:
+        raise ValueError('Invalid padding')
+    return data[:-padding_size]
+
+# Example usage:
+data = b"Hello, world??"
+block_size = 16
+
+# Pad the data
+padded_data = pkcs7_pad(data, block_size)
+print(padded_data)
+
+# Unpad the data
+unpadded_data = pkcs7_unpad(padded_data, block_size)
+print(unpadded_data)