Decode ASN.1/DER encoded data into a human-readable tree structure for certificate and key inspection.
Takes Base64 or hex input in DER form - the binary encoding behind X.509 certificates, RSA and EC keys, CSRs, and CMS messages - and parses its tag-length-value structure into an indented tree, resolving object identifiers such as 1.2.840.113549.1.1.1 to names like rsaEncryption. Decoding exposes structure, not meaning: a certificate's signature value appears as an opaque BIT STRING, and a pasted public key turns out to be two integers wrapped inside a BIT STRING rather than a certificate. The usual surprise is that DER is deterministic but not self-explanatory - identical structures mean different things depending on where they appear.
A concrete input and expected output from the current implementation.
Input
MA0GCSqGSIb3DQEBAQUA
Expected output
SEQUENCE (len 13, 2 elements) - OBJECT IDENTIFIER 1.2.840.113549.1.1.1 (rsaEncryption) - NULL
That base64 decodes to the 15 DER bytes 30 0D 06 09 2A 86 48 86 F7 0D 01 01 01 05 00: a SEQUENCE holding the rsaEncryption OID (1.2.840.113549.1.1.1) and an empty NULL parameter. This exact blob is the AlgorithmIdentifier that prefixes every RSA public key in SubjectPublicKeyInfo form, which is why it recurs in PEM key files.
Every DER element is three parts: a tag byte naming the type (0x30 SEQUENCE, 0x02 INTEGER, 0x06 OBJECT IDENTIFIER, 0x03 BIT STRING, 0x04 OCTET STRING, 0x05 NULL), a length, and the value. Lengths under 128 bytes use a single byte; longer values switch to the long form with a byte count. Constructed types such as SEQUENCE are decoded recursively, so nesting in the data becomes depth in the tree.
Object identifiers decode as big-endian base-128 components with the first two arcs folded into one byte: 1.2.840.113549.1.1.1 becomes 2A 86 48 86 F7 0D 01 01 01. Known OIDs map to names such as rsaEncryption; unknown ones stay dotted. INTEGERs are big-endian and signed, so values with the high bit set carry a leading zero byte. BIT STRINGs begin with an unused-bits count, and time fields use two-digit UTCTime or four-digit GeneralizedTime years.
Because DER stores many values as opaque blobs by design: a certificate signature is a BIT STRING of raw bytes, and extensions like AuthorityKeyIdentifier are themselves DER nested inside an OCTET STRING. The outer encoding is exactly what hides the inner structure, so the tree shows the wrapper and its hex payload. That is the format's intent, not a decoder shortcoming.
Yes. PEM is just base64 armor around the same DER bytes: the BEGIN line, headers, base64 body, and END line are all stripped before decoding. What the tool will not do is name the document type - the same bytes could be a certificate, a CSR, or a key, and the tree reports structure while you supply the interpretation.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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