What is the difference between BER and DER?

BER allows several valid encodings of the same value; DER picks exactly one, which is what makes digital signatures over ASN.1 reproducible. Certificates and signed structures always use DER.

How do I look up an OID?

This decoder resolves the common ones inline. OIDs are hierarchical: 1.2.840.113549 is RSA Security, 2.5.4 is X.500 attribute types, so even an unresolved OID's prefix tells you which family it belongs to.

Why does 0x30 appear everywhere?

0x30 is SEQUENCE — the constructed type that wraps ordered fields. Almost every ASN.1 structure starts with one, which is why a valid DER file usually begins with 30 82.

ASN.1 & OID Decoder

Walk an ASN.1 DER structure tag by tag and resolve its object identifiers.

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What is the ASN.1 & OID Decoder?

An ASN.1 decoder parses DER-encoded data into its tag-length-value tree, labelling each element's type and resolving object identifiers (OIDs) to their registered names.

ASN.1 is the schema language, DER the strict binary encoding, and together they underpin most of security infrastructure: X.509 certificates, PKCS containers, LDAP, SNMP, Kerberos and EMV smartcards all speak it. The encoding is a nested tag-length-value tree — an identifier byte naming the type, a length, then the content, which may itself be more TLV elements.

The tags you meet constantly are SEQUENCE (0x30), SET (0x31), INTEGER (0x02), BIT STRING (0x03), OCTET STRING (0x04), NULL (0x05), OBJECT IDENTIFIER (0x06) and the string and time types. OIDs are the vocabulary layer: 1.2.840.113549.1.1.11 means sha256WithRSAEncryption, 2.5.4.3 means commonName. Resolving them turns an opaque number tree into something you can read.

This is the tool you drop to when a higher-level parser refuses a file. If a certificate decoder says 'malformed', walking the raw ASN.1 shows you exactly which element is truncated or mis-tagged.

ASN.1 & OID Decoder at a glance

Encoding: DER — the canonical, unambiguous subset of BER
Structure: Tag-Length-Value, arbitrarily nested
Common tags: 0x30 SEQUENCE · 0x02 INTEGER · 0x06 OID
Used by: X.509, PKCS, LDAP, SNMP, Kerberos, EMV

How to use the ASN.1 & OID Decoder

1Paste DER bytes as hex, or a Base64/PEM block — the armor is stripped for you.
2Read the tree: each node shows its tag, length and decoded value.
3Check resolved OID names to identify algorithms and attribute types.

When you would reach for it

Diagnosing a certificate or key file that every higher-level parser rejects.
Identifying an unknown algorithm or extension from its OID.
Learning how X.509 and PKCS structures are actually laid out on the wire.

ASN.1 & OID Decoder — frequently asked questions

Last reviewed 25 July 2026 · questions or corrections: contact@decoder.tools