X.509 certificate parser and decoder

A clear answer. Here’s how.

Reads a certificate and lays out what it says; it does not decide whether to trust it. The input can be PEM (one block or a chain of up to 10), the bare Base64 body, the DER bytes as hex, or a .pem, .crt, .cer or .der file (a binary DER file or PEM text; a chosen file replaces the pasted text). The reader is a small ASN.1 DER parser (ITU-T X.690) that accepts definite lengths only and refuses indefinite lengths, nesting deeper than 20 levels, more than 40,000 elements and truncated data, each with a plain message. It decodes the RFC 5280 structure: the version; the serial number in hex and decimal; the signature algorithm, with RSASSA-PSS parameters; the issuer and subject names as an RFC 4514 style string (last name part first) and as a table of attributes, each with the string type it was encoded as (UTF8String, PrintableString, IA5String, TeletexString read as ISO 8859-1, BMPString as UTF-16, UniversalString as UTF-32); the validity (a UTCTime year of 50 to 99 means 1950 to 1999 and 00 to 49 means 2000 to 2049, as RFC 5280 section 4.1.2.5 says); the public key (RSA modulus size and exponent; elliptic-curve keys by named curve, including P-256, P-384, P-521 and secp256k1, with the point form and length; Ed25519, Ed448, X25519 and X448; DSA; ML-DSA, ML-KEM and SLH-DSA by name; any other key by its OID); and the extensions with their critical flags: subject and issuer alternative names (DNS, IP, email, URI, directory and other names), key usage, extended key usage, basic constraints, subject and authority key identifiers, CRL distribution points, authority information access, certificate policies, name constraints, TLS feature and the number of embedded signed certificate timestamps. An extension the tool does not decode is listed with its OID, critical flag and size. The fingerprints are SHA-1 and SHA-256 of the DER bytes, as OpenSSL prints them, and the SHA-256 of the key info is the value used for key pinning. The check date asks whether the certificate is valid on a day you pick, counting the whole day in UTC; the tool never reads the clock. The observations are factual checks and each names its rule: a SHA-1, MD5 or MD2 signature; an RSA key under 2048 bits (CA/Browser Forum Baseline Requirements section 6.1.5); a TLS server certificate valid for longer than section 6.3.2 allows for its not-before date (398 days before 2026-03-15, 200 days until 2027-03-15, 100 days until 2029-03-15, 47 days after, as in version 2.3.1 of 4 October 2026); a missing subject alternative name; an issuer equal to the subject; and breaches of RFC 5280 (a serial number over 20 bytes, zero or negative, two signature algorithms that differ, CA basic constraints that are not critical, keyCertSign without the CA flag). This tool does not verify the signature, build or check a chain, contact an OCSP or CRL server, decide trust, match a host name or check that a key point lies on its curve. A private key pasted by mistake is refused and never shown back.