# ASN.1 and PEM

The Abstract Syntax Notation One (ASN.1) is a language used to describe data structures. There are some pre-defined base data types like integers or sequences and an author of a protocol or file format can then define custom data types using the basic ones.

### DER Encoding

The [ITU-T standard X.680](https://www.itu.int/rec/T-REC-X.680/) defines different encodings for data specified as ASN.1. For X.509-related data, the most important encoding is DER, because there is only one way to encode a type; therefore, a hash of the binary DER representation of the type will always have the same value, which is important for example when signing ASN.1-encoded data.

### PEM Encoding

For many, but not all X.509-related file types, you can either store the file binary in DER-encoding or apply an additional [PEM-encoding](https://datatracker.ietf.org/doc/html/rfc7468) on top of the DER encoding. PEM uses only ASCII characters and can therefore be copy and pasted easily in a clipboard or, some decades ago when this was still relevant, send via email.

### Tools

If you have an ASN.1-encoded file and either you do not know which type it is or you have no application  handling this specific type, you can still decode the raw ASN.1 structure and see what it shows. On Windows, the built-in tool certutil can do this with the command `certutil -decode`.


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