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.gitignore

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+*.pyc

README

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+xmldsig
+Sign and verify digital signatures in native Python with RSA
+
+Copyright © 2010 Andrew D. Yates
+All Rights Reserved
+
+Working Version 0
+

__init__.py

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+#!/usr/bin/python2.5
+# -*- coding: utf-8 -*-
+# Copyright © 2010 Andrew D. Yates
+# All Rights Reserved
+"""XMLDSig: Sign and Verify XML digital cryptographic signatures.
+
+xmldsig is a minimal implementation of bytestring cryptographic
+digital signatures which I have written to handle the Google
+Application Single Sign On service in Security Assertion Markup
+Language. (Google Apps, SSO, SAML respectively). 
+
+XMLDSIG is a ridiculous but necessary protocol for identification and
+authentication in computer systems. Why is XMLDSIG ridiculous? The
+defining feature of XML is that its exact bytestring of the message is
+arbitrary. The defining feature of a cryptographic signature is that
+the exact bytestring of the message is critical. Only by what I can
+imagine to be some "practical man's" increasingly pathetic sophistry
+to assert himself over the ideas which he derides as "of no practical
+importance" could two such elegant ideas be so crudely forced together
+and then ---with so much energy--- propped, pushed, and defended.
+
+In this module, all XML must be in Bytestring XML Format:
+
+Bytestring XML Format
+=====================
+* XML is a utf-8 encoded bytestring.
+* XML namespaces must explicitly define all xmlns prefix names
+* XML is in minimum whitespace representation.
+* <Signature> always signs the entire xml string
+* signed XML must be in "Canonicalization" (c14n) form
+  see: http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments
+* <Signature> is always enveloped as the first child of root
+  see: http://www.w3.org/2000/09/xmldsig#enveloped-signature
+
+Note that whitespace, character case, and encoding are significant in
+Bytestring XML: e.g. "<b>text</b>" is not the same as "<b> text</b>".
+
+References
+==========
+* [DI]
+  http://www.di-mgt.com.au/xmldsig.html
+  Signing an XML document using XMLDSIG
+* [RFC 2437]
+  http://www.ietf.org/rfc/rfc2437.txt
+  PKCS #1: RSA Cryptography Specifications
+* [RFC 3275]
+  http://www.ietf.org/rfc/rfc3275.txt
+  (Extensible Markup Language) XML-Signature Syntax and Processing
+* [RSA-SHA1]
+  http://www.w3.org/TR/2008/REC-xmldsig-core-20080610/#sec-PKCS1
+  XML Signature Syntax and Processing (Second Edition)
+  Section: 6.4.2 PKCS1 (RSA-SHA1)
+"""
+
+import hashlib
+import re
+# change to string literal
+import os.path
+
+# change to python string insertion
+from google.appengine.ext.webapp import template
+
+
+RX_ROOT = re.compile('<[^> ]+ ?([^>]*)>')
+RX_NS = re.compile('xmlns:[^> ]+')
+RX_SIGNATURE = re.compile('<Signature.*?</Signature>')
+RX_SIGNED_INFO = re.compile('<SignedInfo.*?</SignedInfo>')
+RX_SIG_VALUE = re.compile('<SignatureValue[^>]*>([^>]+)</SignatureValue>')
+
+PATH = os.path.dirname(os.path.abspath(__file__))
+SIGNED_INFO = os.path.join(PATH, "templates/signed_info.xml")
+SIGNATURE = os.path.join(PATH, "templates/signature.xml")
+
+# SHA1 digest with ASN.1 BER SHA1 algorithm designator prefix [RSA-SHA1]
+PREFIX = '\x30\x21\x30\x09\x06\x05\x2B\x0E\x03\x02\x1A\x05\x00\x04\x14'
+
+PTN_SIGNEDINFO_XML = \
+'<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#"{% if xmlns_attr %} {% endif %}{{ xmlns_attr }}><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"></CanonicalizationMethod><SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"></SignatureMethod><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"></Transform></Transforms><DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></DigestMethod><DigestValue>{{ digest_value }}</DigestValue></Reference></SignedInfo>'
+
+PTN_SIGNATURE_XML = \
+'<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">{{ signed_info_xml }}<SignatureValue>{{ signature_value }}</SignatureValue><KeyInfo><KeyValue><RSAKeyValue><Modulus>{{ modulus }}</Modulus><Exponent>{{ exponent }}</Exponent></RSAKeyValue></KeyValue></KeyInfo></Signature>'
+
+b64e = lambda s: s.encode('base64').strip() 
+b64d = lambda s: s.decode('base64').strip() 
+
+
+def sign(xml, f_private, modulus, exponent):
+  """Return xmldsig XML string from xml_string of XML.
+
+  Args:
+    xml: str of bytestring xml to sign
+    f_private: func 
+    modulus: str
+    exponent: str
+  Returns:
+    str: signed bytestring xml
+  """
+  signed_info_xml = _signed_info(xml)
+  signed = _signed_value(signed_info_xml, len(modulus))
+  signature_value = f_private(signed)
+
+  signature_cxt = {
+    'signed_info_xml': signed_info_xml,
+    'signature_value': b64e(signature_value),
+    'modulus': b64e(modulus),
+    'exponent': b64e(exponent),
+  }
+  signature_xml = template.render(SIGNATURE, signature_cxt)
+
+  # insert xmldsig after first '>' in message
+  signed_xml = xml.replace('>', '>'+signature_xml, 1)
+  return signed_xml
+
+
+def verify(xml, f_public, modulus):
+  """Return if <Signature> in `xml` matches `xml` with `key`.
+  
+  Args:
+    xml: str of XML with xmldsig <Signature> element
+    f_public: func
+    modulus: str
+  Returns:
+    bool: signature for `xml` is valid
+  """
+  signature = RX_SIGNATURE.search(xml).group(0)
+  unsigned_xml = xml.replace(signature, '')
+
+  # retreive the signed value in `xml` using `key`
+  signature_value = RX_SIG_VALUE.search(signature).group(1)
+  expected = f_public(b64d(signature_value))
+
+  # compute the actual signed value for `xml`
+  signed_info_xml = _signed_info(unsigned_xml)
+  actual = _signed_value(signed_info_xml, len(modulus))
+
+  return expected == actual
+
+
+def _digest(data):
+  """SHA1 hash digest of message data.
+  
+  Implements RFC2437, 9.2.1 EMSA-PKCS1-v1_5, Step 1. for "Hash = SHA1"
+  
+  Args:
+    data: str of bytes to digest
+  Returns:
+    str: of bytes of digest from `data`
+  """
+  hasher = hashlib.sha1()
+  hasher.update(data)
+  return hasher.digest()
+
+
+def _get_xmlns_prefixes(xml):
+  """Return string of root namespace prefix attributes in given order.
+  
+  Args:
+    xml: str of bytestring xml
+  Returns:
+    str: [xmlns:prefix="uri"] list ordered as in `xml`
+  """
+  root_attr = RX_ROOT.match(xml).group(1)
+  ns_attrs = [a for a in root_attr.split(' ') if RX_NS.match(a)]
+  return ' '.join(ns_attrs)
+
+
+def _signed_info(xml):
+  """Return <SignedInfo> for bytestring xml.
+
+  Args:
+    xml: str of bytestring
+  Returns:
+    str: xml bytestring of <SignedInfo> computed from `xml`
+  """
+  signed_info_cxt = {
+    'xmlns_attr': _get_xmlns_prefixes(xml),
+    'digest_value': b64e(_digest(xml)),
+  }
+  signed_info_xml = template.render(SIGNED_INFO, signed_info_cxt)
+  return signed_info_xml
+
+
+def _signed_value(data, key_size):
+  """Return unencrypted rsa-sha1 signature value `padded_digest` from `data`.
+  
+  The resulting signed value will be in the form:
+  (01 | FF* | 00 | prefix | digest) [RSA-SHA1]
+  where "digest" is of the generated c14n xml for <SignedInfo>.
+  
+  Args:
+    data: str of bytes to sign
+    key_size: int of number of bytes in crytographic key
+  Returns:
+    str: rsa-sha1 signature value of `data`
+  """
+  asn_digest = PREFIX + _digest(data)
+
+  # Pad to "one octet shorter than the RSA modulus" [RSA-SHA1]
+  padded_size = key_size - 1
+  pad_size = padded_size - len(asn_digest) - 2
+  pad = '\x01' + '\xFF' * pad_size + '\x00'
+  padded_digest = pad + asn_digest
+
+  return padded_digest
+