basic functionalty

modules/Crypto/Util/Padding.py

@@ -0,0 +1,103 @@
+#
+# -*- coding: utf-8 -*-
+#
+#  Util/Padding.py :  Functions to manage padding
+#
+# ===================================================================
+# The contents of this file are dedicated to the public domain.  To
+# the extent that dedication to the public domain is not available,
+# everyone is granted a worldwide, perpetual, royalty-free,
+# non-exclusive license to exercise all rights associated with the
+# contents of this file for any purpose whatsoever.
+# No rights are reserved.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+# ===================================================================
+
+""" Functions to manage padding
+
+This module provides minimal support for adding and removing standard padding
+from data.
+"""
+
+__all__ = [ 'PaddingError', 'pad', 'unpad' ]
+
+from Crypto.Util.py3compat import *
+
+class PaddingError(ValueError):
+    """Exception raised when padding is incorrect and cannot be removed."""
+    pass
+
+def pad(data_to_pad, block_size, style='pkcs7'):
+    """Apply standard padding.
+    
+    :Parameters:
+      data_to_pad : byte string
+        The data that needs to be padded.
+      block_size : integer
+        The block boundary to use for padding. The output length is guaranteed
+        to be a multiple of ``block_size``.
+      style : string
+        Padding algorithm. It can be *'pkcs7'* (default), *'iso7816'* or *'x923'*.
+    :Return:
+      The original data with the appropriate padding added at the end.
+    """
+
+    padding_len = block_size-len(data_to_pad)%block_size
+    if style == 'pkcs7':
+        padding = bchr(padding_len)*padding_len
+    elif style == 'x923':
+        padding = bchr(0)*(padding_len-1) + bchr(padding_len)
+    elif style == 'iso7816':
+        padding = bchr(128) + bchr(0)*(padding_len-1)
+    else:
+        raise ValueError("Unknown padding style")
+    return data_to_pad + padding
+
+def unpad(padded_data, block_size, style='pkcs7'):
+    """Remove standard padding.
+   
+    :Parameters:
+      padded_data : byte string
+        A piece of data with padding that needs to be stripped.
+      block_size : integer
+        The block boundary to use for padding. The input length
+        must be a multiple of ``block_size``.
+      style : string
+        Padding algorithm. It can be *'pkcs7'* (default), *'iso7816'* or *'x923'*.
+    :Return:
+        Data without padding.
+    :Raises PaddingError:
+        if the padding is incorrect.
+    """
+
+    pdata_len = len(padded_data)
+    if pdata_len % block_size:
+        raise PaddingError("Input data is not padded")
+    if style in ('pkcs7', 'x923'):
+        padding_len = bord(padded_data[-1])
+        if padding_len<1 or padding_len>min(block_size, pdata_len):
+            raise PaddingError("Padding is incorrect.")
+        if style == 'pkcs7':
+            if padded_data[-padding_len:]!=bchr(padding_len)*padding_len:
+                raise PaddingError("PKCS#7 padding is incorrect.")
+        else:
+            if padded_data[-padding_len:-1]!=bchr(0)*(padding_len-1):
+                raise PaddingError("ANSI X.923 padding is incorrect.")
+    elif style == 'iso7816':
+        padding_len = pdata_len - padded_data.rfind(bchr(128))
+        if padding_len<1 or padding_len>min(block_size, pdata_len):
+            raise PaddingError("Padding is incorrect.")
+        if padding_len>1 and padded_data[1-padding_len:]!=bchr(0)*(padding_len-1):
+            raise PaddingError("ISO 7816-4 padding is incorrect.")
+    else:
+        raise ValueError("Unknown padding style")
+    return padded_data[:-padding_len]
+