1 files changed, 40 insertions, 1 deletions
diff --git a/tools/certtools.py b/tools/certtools.py
index 48e7b41..13dad17 100644
--- a/tools/certtools.py
+++ b/tools/certtools.py
@@ -5,6 +5,8 @@ import urllib
 import urllib2
 import struct
 import sys
+import hashlib
+import ecdsa
 
 def get_cert_info(s):
     p = subprocess.Popen(
@@ -77,7 +79,9 @@ def unpack_tls_array(packed_data, length_len):
     padded_length[-length_len:] = packed_data[:length_len]
     (length,) = struct.unpack(">Q", "".join(padded_length))
     unpacked_data = packed_data[length_len:length_len+length]
-    assert len(unpacked_data) == length
+    assert len(unpacked_data) == length, \
+      "data is only %d bytes long, but length is %d bytes" % \
+      (len(unpacked_data), length)
     rest_data = packed_data[length_len+length:]
     return (unpacked_data, rest_data)
 
@@ -107,3 +111,38 @@ def decode_certificate_chain(packed_certchain):
         certs.append(cert)
         unpacked_certchain = rest
     return certs
+
+def decode_signature(signature):
+    (hash_alg, signature_alg) = struct.unpack(">bb", signature[0:2])
+    (unpacked_signature, rest) = unpack_tls_array(signature[2:], 2)
+    assert rest == ""
+    return (hash_alg, signature_alg, unpacked_signature)
+
+def check_signature(publickey, leafcert, sct):
+    calculated_logid = hashlib.sha256(publickey).digest()
+    received_logid = base64.decodestring(sct["id"])
+    assert calculated_logid == received_logid, \
+        "log id is incorrect:\n  should be %s\n        got %s" % \
+        (calculated_logid.encode("hex_codec"),
+         received_logid.encode("hex_codec"))
+
+    signature = base64.decodestring(sct["signature"])
+
+    version = struct.pack(">b", sct["sct_version"])
+    signature_type = struct.pack(">b", 0)
+    timestamp = struct.pack(">Q", sct["timestamp"])
+    entry_type = struct.pack(">H", 0)
+    signed_struct = version + signature_type + timestamp + \
+      entry_type + tls_array(leafcert, 3) + \
+      tls_array(base64.decodestring(sct["extensions"]), 2)
+
+    (hash_alg, signature_alg, unpacked_signature) = decode_signature(signature)
+    assert hash_alg == 4 # sha256
+    assert signature_alg == 3 # ecdsa
+
+    hash = hashlib.sha256()
+    hash.update(signed_struct)
+
+    vk = ecdsa.VerifyingKey.from_der(publickey)
+    vk.verify(unpacked_signature, signed_struct, hashfunc=hashlib.sha256,
+              sigdecode=ecdsa.util.sigdecode_der)