What decoding a certificate actually involves

A PEM file is just Base64 wrapped around a binary structure encoded in ASN.1 DER — a self-describing, tag-length-value binary format used throughout X.509 and cryptography in general. Reading a certificate means walking that DER structure: finding the tag that says "this next chunk is a SEQUENCE", then the one that says "this is the validity period, encoded as two UTCTime or GeneralizedTime values", and so on down to the actual bytes. This tool implements that walk directly — a small ASN.1/DER parser — rather than calling out to a server or a native crypto library, which is also why it works entirely offline and never transmits the certificate anywhere.

Parsing structure is not the same as verifying trust

Decoding a certificate tells you what claims it makes — subject, issuer, validity window, SANs, algorithms — but says nothing about whether those claims should be trusted. Verifying trust requires checking the signature against the issuer's public key, walking the chain up to a trusted root, and checking revocation status (CRL/OCSP) — none of which a static decode can do, and none of which this tool attempts. For that live-connection verification, this site's SSL Certificate Checker tool actually connects to a host and validates the certificate chain it presents; use this decoder for a certificate or CSR you already have in hand and want to read the contents of.