Increase strength of hashes used for certificate signing

This does not increase of any packet because the size of certificate signature
only depends on the size of the certificate key.

With 512-bit hashes, it's still possible to use RSA keys as small as 768 bits.

demo/demo

@@ -172,7 +172,7 @@ if 1:
         " -set_serial 0x120010db80042 -days %u" % CA_DAYS, shell=True)
     with open('ca.crt') as f:
         ca = crypto.load_certificate(crypto.FILETYPE_PEM, f.read())
-    fingerprint = "sha1:" + hashlib.sha1(
+    fingerprint = "sha256:" + hashlib.sha256(
         crypto.dump_certificate(crypto.FILETYPE_ASN1, ca)).hexdigest()
     db_path = 'registry/registry.db'
     registry.screen('./py re6st-registry @registry/re6st-registry.conf'

re6st-conf

@@ -132,7 +132,7 @@ def main():
             create(key_path, key, 0600)
 
         req.set_pubkey(pkey)
-        req.sign(pkey, 'sha1')
+        req.sign(pkey, 'sha512')
         req = crypto.dump_certificate_request(crypto.FILETYPE_PEM, req)
 
         # First make sure we can open certificate file for writing,

re6st/registry.py

@@ -404,7 +404,7 @@ class RegistryServer(object):
         serial = 1 + self.getConfig('serial', 0)
         self.setConfig('serial', serial)
         cert.set_serial_number(serial)
-        cert.sign(self.cert.key, 'sha1')
+        cert.sign(self.cert.key, 'sha512')
         cert = crypto.dump_certificate(crypto.FILETYPE_PEM, cert)
         self.db.execute("UPDATE cert SET cert = ? WHERE prefix = ?",
                         (cert, client_prefix))

re6st/x509.py

@@ -138,10 +138,10 @@ class Cert(object):
         return r
 
     def verify(self, sign, data):
-        crypto.verify(self.ca, sign, data, 'sha1')
+        crypto.verify(self.ca, sign, data, 'sha512')
 
     def sign(self, data):
-        return crypto.sign(self.key, data, 'sha1')
+        return crypto.sign(self.key, data, 'sha512')
 
     def decrypt(self, data):
         p = openssl('rsautl', '-decrypt', '-inkey', self.key_path)
@@ -179,6 +179,11 @@ class Peer(object):
     - hello0 packets (0 & 1) are subject to DoS, because verifying a
       certificate uses much CPU. A solution would be to use TCP until the
       secret is exchanged and continue with UDP.
+
+    The fingerprint is only used to quickly know if peer's certificate has
+    changed. It must be short enough to not exceed packet size when using
+    certificates with 4096-bit keys. A weak algorithm is ok as long as there
+    is no accidental collision. So SHA-1 looks fine.
     """
     _hello = _last = 0
     _key = newHmacSecret()
@@ -233,7 +238,7 @@ class Peer(object):
         self._last = None
 
     def verify(self, sign, data):
-        crypto.verify(self.cert, sign, data, 'sha1')
+        crypto.verify(self.cert, sign, data, 'sha512')
 
     seqno_struct = struct.Struct("!L")