ASN.1 Decoder
Decode PEM, hex or base64 DER data — certificates, keys, CRLs — into an interactive ASN.1 tree with OID names, hex view and byte highlighting, fully in your browser.
What is ASN.1?
ASN.1 (Abstract Syntax Notation One) is a standard notation for describing data structures, widely used in cryptography and telecom. X.509 certificates, private keys, CRLs, OCSP responses and LDAP payloads are all encoded with ASN.1 rules, so an ASN.1 decoder lets you see the raw structure behind them.
What is the difference between DER and PEM?
DER is the binary encoding of an ASN.1 structure. PEM is the same bytes wrapped in a text envelope: a "-----BEGIN ...-----" header, base64 lines and an "-----END ...-----" footer. This tool detects both automatically — paste PEM text, hex or base64, or upload a binary .der file.
Which files can I upload?
Any file containing DER data: .der, .pem, .crt, .cer, .key, .pub, .csr, .p7b and similar. PEM inside a file takes priority; otherwise the file is read as raw binary DER. Nothing is uploaded anywhere — the file is read by your browser locally.
How do I read the tree?
Each line shows the tag name (SEQUENCE, INTEGER, OID...), the byte offset and total length, and a value preview. Click the chevron to fold or unfold a node; SEQUENCE and SET nodes contain nested children. [n] marks context-specific tags such as certificate version [0] or extensions [3].
What are the OID names shown next to numbers?
Object Identifiers are dotted number chains that name algorithms and fields (2.5.4.3, 1.2.840.113549.1.1.11...). The tool includes a built-in table of the most common ones — commonName, rsaEncryption, sha256WithRSAEncryption, basicConstraints and more — and shows the friendly name next to the dotted value.
How does the hex view work?
The right pane shows the decoded bytes as a classic hex dump with offsets and ASCII. Click any tree node and its exact byte range is highlighted: header bytes (tag and length) get one style, content bytes another, so you can see precisely where each structure lives in the file.
Why do I get "not valid DER data"?
The tool could not parse any ASN.1 structure from your input after trying PEM, hex and base64 interpretations. Common causes: a truncated PEM block, copying only part of a certificate, or stray characters in hex input. Check that the base64 lines between BEGIN and END are complete.
Can it decode encrypted private keys?
It shows the outer PKCS#8/PKCS#12 structure including the encryption algorithm OID, but the inner fields remain an opaque OCTET STRING because they need your passphrase. The structure and algorithm are still visible and highlighted in the hex view.
Is my certificate or key sent to a server?
No. The decoder and the entire ASN.1 library run locally in your browser. Certificates, keys, CRLs and uploaded files never leave your device, which makes it safe even for private-key structures — though you should still avoid pasting production secrets anywhere out of habit.
When would I use this instead of OpenSSL?
Whenever you need a quick visual answer: inspecting a certificate chain, checking SANs or validity dates, explaining certificate structure to a colleague, or teaching ASN.1. It is the browser equivalent of "openssl asn1parse" with a linked hex view and friendlier OID names.
How to Use ASN.1 Decoder
This ASN.1 decoder turns DER-encoded data — X.509 certificates, private keys, CRLs, OCSP responses — into an interactive tree you can explore. Paste PEM text, hex or base64, or upload a file; the input format is detected automatically (PEM first). The tree shows every tag with its byte offset, length and a value preview, common OIDs get friendly names, and clicking a node highlights its exact byte range in the hex view. Everything runs locally in your browser.
- Paste your data into the input box, press Load example to try a sample certificate, or use Upload file for .der/.pem/.crt/.key files.
- Click Decode. The tool tries PEM, hex and base64 interpretations until one parses as DER, then shows the detected format and byte count.
- Browse the ASN.1 tree: each line has the tag name (SEQUENCE, INTEGER, OID…), the @offset · size and a preview. Values shown in [brackets] are context-specific tags.
- Click the chevron next to a node to fold or unfold its children; use Expand all / Collapse all for the whole tree, or Copy tree to get a plain-text version.
- Click any node to highlight its bytes in the hex view — header bytes (tag + length) and content bytes are styled differently — and check OID rows for names like commonName or sha256WithRSAEncryption.
If decoding fails, the PEM block is usually truncated: make sure you copied from -----BEGIN to -----END inclusive. Encrypted private keys decode down to the encryption algorithm OID, but the inner bytes stay opaque without the passphrase.
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