ASN.1 Decoder
Open DER, BER or PEM data (certificates, keys, CSRs) as a tree with tag names, offsets and object identifier names. Runs in your browser.
Input
Stays in your browserShortcuts: Ctrl+Enter run · Ctrl+Shift+C copy result · / search (use ⌘ on Mac)
Next steps
Done here? Use the Send to… menu on the result to carry it on to Certificate Decoder or CSR Decoder.
Free Online ASN.1 Decoder
See what is inside a certificate, key or other ASN.1 data: every element with its type, position and value, and OIDs shown by name. It gives developers a quick browser-based way to solve a focused asn.1 decoder task before moving the result into a project.
How to use ASN.1 Decoder
Paste PEM, hex or Base64, or open a file. Each line shows the type, the number of elements, the offset (@), header length and length.
Common mistakes
- Pasting a certificate that is not PEM or Base64 (for example with extra text around it): paste only the block.
- Expecting an encrypted key to open: encrypted content is shown as bytes.
Frequently asked questions
What is ASN.1?
A way to describe structured data that is used in certificates, keys, LDAP and many protocols. DER and BER are the binary encodings.
What do the numbers after @ mean?
@ is the byte offset where the element starts. hdr is the length of its tag and length bytes, and len is the length of its value.
How are OIDs named?
About 70 common object identifiers (algorithms, name fields, certificate extensions) are named. Others are shown as dotted numbers.
Is my data uploaded?
No. Compression runs in your browser and nothing you paste or open is sent to a server.
How do I use this asn.1 decoder tool?
Use the interactive controls above; processing stays in your browser and the result is intended as a practical starting point for development work.
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