What characters are valid for an SSL certificate serial number?

Question

question from:https://stackoverflow.com/questions/66060831/what-characters-are-valid-for-an-ssl-certificate-serial-number

Answer 1

From RFC 5280 section 4.1.2.2 Serial Number:

4.1.2.2. Serial Number

The serial number MUST be a positive integer assigned by the CA to each certificate. It MUST be unique for each certificate issued by a given CA (i.e., the issuer name and serial number identify a unique certificate). CAs MUST force the serialNumber to be a non-negative integer.

To answer your question; since it's an integer, valid characters are 0-9. Of course, for display purposes you can encode this integer however you want (decimal, hex, base64, etc), but in the actual certificate file it's going to be saved as ASN.1 raw bytes and other software consuming your certificate will parse that as an integer and throw errors if it's not in the right range.