X.509 decoder from PEM

Decodes X.509 PKI certificate presented in PEM format.

In cryptography, X.509 is an International Telecommunication Union (ITU) standard defining the format of public key certificates. X.509 certificates are used in many Internet protocols, including TLS/SSL, which is the basis for HTTPS, the secure protocol for browsing the web. They are also used in offline applications, like electronic signatures.¹

The structure foreseen by the standards is expressed in a formal language, Abstract Syntax Notation One (ASN.1).

The structure of an X.509 v3 digital certificate is as follows:

    Certificate
        Version Number
        Serial Number
        Signature Algorithm ID
        Issuer Name
        Validity period
            Not Before
            Not After
        Subject name
        Subject Public Key Info
            Public Key Algorithm
            Subject Public Key
        Issuer Unique Identifier (optional)
        Subject Unique Identifier (optional)
        Extensions (optional)
            ...
    Certificate Signature Algorithm
    Certificate Signature

The Extensions field, if present, is a sequence of one or more certificate extensions. Each extension has its own unique ID, expressed as object identifier (OID), which is a set of values, together with either a critical or non-critical indication. A certificate-using system must reject the certificate if it encounters a critical extension that it does not recognize, or a critical extension that contains information that it cannot process. A non-critical extension may be ignored if it is not recognized, but must be processed if it is recognized.

PEM

Privacy-Enhanced Mail (PEM) is a de facto file format for storing and sending cryptographic keys, certificates, and other data, based on a set of 1993 IETF standards defining "privacy-enhanced mail." While the original standards were never broadly adopted and were supplanted by PGP and S/MIME, the textual encoding they defined became very popular. The PEM format was eventually formalized by the IETF in RFC 7468.²

Sources:
[1] en.wikipedia.org/wiki/X.509
[2] en.wikipedia.org/wiki/Privacy-Enhanced_Mail