timestamp: add a local mode of timestamping using Django signing

docbow_project/docbow/timestamp.py

@@ -1,17 +1,35 @@
 import rfc3161
 import os.path
+import hashlib
+import datetime
 
 from django.utils import simplejson
+from django.core import signing
 
 __certificate_path = os.path.join(os.path.dirname(__file__), 'certum_certificate.crt')
 
 __timestamper = rfc3161.RemoteTimestamper('http://time.certum.pl',
         certificate=open(__certificate_path).read())
+__timestamper = None
 
 def timestamp(content):
     return __timestamper(data=content)
 
 def timestamp_json(json_dict):
+    if __timestamper:
+        return timestamp_json_rfc3161(json_dict)
+    else:
+        return timestamp_json_local(json_dict)
+
+def timestamp_json_local(json_dict):
+    s = simplejson.dumps(json_dict)
+    if s[-1] != '}':
+        raise ValueError("timestamp_json takes a dictionnary as argument: %s" % s)
+    signer = signing.Signer()
+    signed_string = signer.sign('{0}:{1}'.format(hashlib.sha1(s).hexdigest(), datetime.datetime.utcnow().isoformat()))
+    return s[:-1] + ',"timestamp": "%s"}' % signed_string
+
+def timestamp_json_rfc3161(json_dict):
     s = simplejson.dumps(json_dict)
     if s[-1] != '}':
         raise ValueError("timestamp_json takes a dictionnary as argument: %s" % s)
@@ -24,9 +42,22 @@ def timestamp_json(json_dict):
     else:
         return ValueError(error)
 
-def check_timestamp_json(content, certificate):
+def check_timestamp_json_rfc3161(content, certificate):
     content, tst = content.rsplit(',"timestamp": "', 1)
     content += '}'
     tst = tst[:-2].decode('base64')
     return rfc3161.check_timestamp(tst, certificate, data=content)
 
+def check_timestamp_json_local(content):
+    content, tst = content.rsplit(',"timestamp": "', 1)
+    content += '}'
+    tst = tst[:-2]
+    signer = signing.Signer()
+    try:
+        signed_string = signer.unsign(tst)
+    except signing.BadSignature:
+        return False
+    digest, tst = signed_string.split(':', 1)
+    if digest != hashlib.sha1(content).hexdigest():
+        return False
+    return datetime.datetime.strptime(tst, '%Y-%m-%dT%H:%M:%S.%f')