Implement detached signatures

Could not get signxml to do detached signatures like proXSign®, so do
it manually.

The protocol doesn’t seem to support using other canonicalization and
signature methods.

1 files changed, 55 insertions, 14 deletions

diff --git a/fauxsign.py b/fauxsign.py
index c2c2349..7fa346b 100755
--- a/fauxsign.py
+++ b/fauxsign.py
@@ -1,23 +1,65 @@
 #!/usr/bin/env python3
 
 import argparse
+import base64
 import http.server
 import json
 import pathlib
 import ssl
+import subprocess
 import urllib.parse
 
 import lxml
 import signxml
 
-version = '2.2.9.276'
+version = '2.2.13.38'
 identifier = 'f8e5f470-bcff-4c50-8fd6-ccfa2fea12d6'
 
-def sign(xml, key, cert):
-    original = lxml.etree.fromstring(xml)
-    signed = signxml.XMLSigner().sign(original, key=key, cert=cert)
-    return ('<?xml version="1.0" encoding="UTF-8" standalone="no" ?>' +
-            lxml.etree.tostring(signed, encoding='unicode'))
+# XML templates for detached signatures
+detached_digest = '''\
+<ds:SignedInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">\
+<ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"></ds:CanonicalizationMethod>\
+<ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"></ds:SignatureMethod>\
+<ds:Reference URI="{reference}">\
+<ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"></ds:DigestMethod>\
+<ds:DigestValue>{digest}</ds:DigestValue>\
+</ds:Reference>\
+</ds:SignedInfo>'''
+
+detached_response = '''\
+<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">\
+<ds:SignedInfo>\
+<ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>\
+<ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>\
+<ds:Reference URI="{reference}">\
+<ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>\
+<ds:DigestValue>{digest}</ds:DigestValue>\
+</ds:Reference>\
+</ds:SignedInfo>\
+<ds:SignatureValue>{signature}</ds:SignatureValue>\
+<ds:KeyInfo>\
+<ds:X509Data><ds:X509Certificate>{certificate}</ds:X509Certificate></ds:X509Data>\
+</ds:KeyInfo>\
+</ds:Signature>'''
+
+def sign(data, key, cert):
+    if 'SIG_TYPE_DETACHED' in data['options']:
+        # might be possible to use signxml for detached signatures also
+        reference, digest = data['URIId'][0].split(',')
+        signature = subprocess.run(
+            ['openssl', 'dgst', '-sha1', '-sign', key], capture_output=True,
+            input=detached_digest.format(reference=reference, digest=digest).encode()
+        ).stdout
+        return detached_response.format(
+            reference=reference, digest=digest,
+            signature=base64.b64encode(signature).decode(),
+            certificate=open(cert).read() # strip header and footer
+                .split('-----BEGIN CERTIFICATE-----', 1)[1]
+                .split('-----END CERTIFICATE-----', 1)[0])
+    else:
+        original = lxml.etree.fromstring(data['bytes'][0].removeprefix('XML:').encode())
+        signed = signxml.XMLSigner().sign(original, key=open(key).read(), cert=open(cert).read())
+        return lxml.etree.tostring(signed, encoding='unicode')
 
 class Handler(http.server.BaseHTTPRequestHandler):
     def reply(self, data):
@@ -43,15 +85,14 @@ class Handler(http.server.BaseHTTPRequestHandler):
             case '/signXML':
                 length = int(self.headers['content-length'])
                 data = json.loads(self.rfile.read(length).decode())
-                xml = data['bytes'][0].removeprefix('XML:').encode()
-                print(f'{self.headers.get("origin", "unknown")} wants to sign:\n{xml}\nConfirm?', end=' ')
+                print(f'{self.headers.get("origin", "unknown")} wants to sign:\n{data}\nConfirm?', end=' ')
                 if input() in ('y', 'yes'):
-                    signed = sign(xml, key=self.server.user_key, cert=self.server.user_cert)
+                    signed = sign(data, key=self.server.user_key, cert=self.server.user_cert)
                     self.reply({
                         'error': 1,
                         'errorMessage': '',
                         'filename': '',
-                        'result': signed,
+                        'result': '<?xml version="1.0" encoding="UTF-8" standalone="no" ?>' + signed,
                         'signatures': [],
                         'timestamps': []
                     })
@@ -60,8 +101,8 @@ class Handler(http.server.BaseHTTPRequestHandler):
 
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description='Fake the proXSign® application.')
-    parser.add_argument('-k', '--user-key', type=open, required=True, help='key file')
-    parser.add_argument('-c', '--user-cert', type=open, required=True, help='certificate file')
+    parser.add_argument('-k', '--user-key', type=pathlib.Path, required=True, help='key file')
+    parser.add_argument('-c', '--user-cert', type=pathlib.Path, required=True, help='certificate file')
     parser.add_argument('-K', '--app-key', type=pathlib.Path, required=True, help='app key file')
     parser.add_argument('-C', '--app-cert', type=pathlib.Path, required=True, help='app certificate file')
     parser.add_argument('-p', '--port', type=int, default=14972, help='port to listen on')
@@ -72,7 +113,7 @@ if __name__ == '__main__':
     tls_context.load_cert_chain(keyfile=args.app_key, certfile=args.app_cert)
 
     httpd = http.server.HTTPServer(('localhost', args.port), Handler)
-    httpd.user_key = args.user_key.read()
-    httpd.user_cert = args.user_cert.read()
+    httpd.user_key = args.user_key
+    httpd.user_cert = args.user_cert
     httpd.socket = tls_context.wrap_socket(httpd.socket, server_side=True)
     httpd.serve_forever()