Declare Serial Numbers as u64 in #6.1.1

Author: bitfl0wer

docs/Protocol Specifications/core.md

@@ -1081,7 +1081,7 @@ ID-Certs have the following structure:
 | --------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------ | -------------------------------------------------------- |
 | Correctly formatted Name attribute, according to [#6.1.1.1](#6111-polyproto-distinguished-name-pdn) | [polyproto Distinguished Name](#6111-polyproto-distinguished-name-pdn)                           | Issuer Name                                              |
 | Correctly formatted Name attribute, according to [#6.1.1.1](#6111-polyproto-distinguished-name-pdn) | [polyproto Distinguished Name](#6111-polyproto-distinguished-name-pdn)                           | Subject Name                                             |
-| A unique identifier for the certificate, used by the CA to identify this certificate.               | Must be unique across all certificates issued by a home server.                                  | Serial Number                                            |
+| A unique, numeric identifier for the certificate, used by the CA to identify this certificate.      | Must be unique across all certificates issued by a home server. 64-bit unsigned integer.         | Serial Number                                            |
 | The algorithm used to sign the certificate.                                                         |                                                                                                  | Certificate Signature Algorithm & Signature Algorithm ID |
 | The signature of the certificate, generated by using the home servers' private identity key.        |                                                                                                  | Certificate Signature                                    |
 | The expiry date of the certificate.                                                                 | Time must not be after expiry date of the home server's root certificate                         | Validity period: Not After                               |