Parsed locally — your data is never uploaded.
About ASN.1 & DER Decoder & Viewer Online
ASN.1 (Abstract Syntax Notation One) is the encoding behind X.509 certificates, private keys, CSRs, and most of public-key cryptography. Its DER and BER serialisations pack everything into a nested tree of tags, lengths, and values — compact for machines, but opaque to read by hand.
This free ASN.1 decoder expands any DER blob into a navigable structure tree. Paste a PEM block, raw Base64, or a hex dump and instantly see each SEQUENCE, INTEGER, OBJECT IDENTIFIER, and string laid out with its tag, length, and decoded value — including friendly names for common OIDs like sha256WithRSAEncryption.
Everything runs in your browser. Certificates and keys are parsed entirely on your device and never leave it, so it is safe to inspect private material.
Features
- Accepts PEM, Base64, or hex — auto-detected
- Collapsible tag/length/value tree for nested structures
- Friendly names for common OIDs (signature algorithms, curves, X.500 attributes)
- Decodes INTEGER, BIT STRING, OCTET STRING, OID, UTF8String, UTCTime and more
- 100% offline — your data never leaves the browser
How to use
- Paste a certificate, key, or DER blob (PEM, Base64, or hex) into the input pane.
- The structure tree appears on the right, with each node showing its tag, length, and value.
- Expand or collapse SEQUENCE and SET nodes to drill into nested structures.
- Copy any decoded value or hex with the copy button on its row.
Frequently asked questions
What is the difference between DER and BER?
BER (Basic Encoding Rules) is the flexible base encoding; DER (Distinguished Encoding Rules) is a strict subset with a single canonical form, used for certificates and signatures. This decoder reads both — DER is just BER with the ambiguity removed.
Can I decode a PEM certificate or private key?
Yes. Paste the whole PEM block including the -----BEGIN----- / -----END----- lines; the Base64 body is extracted and parsed automatically. You can also paste just the Base64 or a hex dump.
Is it safe to paste a private key here?
Yes. Parsing happens entirely in your browser using a local JavaScript library — nothing is uploaded to any server. That said, treat production keys as secrets and only paste them on a machine you trust.
Why are some values shown as raw hex?
Large integers, BIT STRING and OCTET STRING contents, and unknown tags are shown as hex because there is no canonical text form. Constructed types like SEQUENCE expand into child nodes instead of a single value.
What do the OID names mean?
An OBJECT IDENTIFIER is a dotted number like 1.2.840.113549.1.1.11. The decoder maps well-known OIDs to readable names — that example is sha256WithRSAEncryption — so you can recognise signature algorithms, curves, and certificate fields at a glance.
Related tools
Everything runs locally in your browser — your input is never uploaded.