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# Introduction

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The `asn1` application provides the following:

- An ASN.1 compiler for Erlang, which generates encode and decode functions to be used by Erlang programs sending and receiving ASN.1-specified data.
- Runtime functions used by the generated code.
- Support for the following encoding rules:
  - Basic Encoding Rules (BER)
  - Distinguished Encoding Rules (DER), a specialized form of BER that is used in security-conscious applications
  - Packed Encoding Rules (PER), both the aligned and unaligned variant
  - JSON Encoding Rules (JER)

## Scope

This application covers all features of ASN.1 up to the 1997 edition of the specification. In the 2002 edition, new features were introduced. The following features of the 2002 edition are fully or partly supported:

- Decimal notation (for example, `"1.5e3`) for REAL values. The NR1, NR2, and NR3 formats as explained in ISO 6093 are supported.
- The `RELATIVE-OID` type for relative object identifiers is fully supported.
- The subtype constraint (`CONTAINING`/`ENCODED BY`) to constrain the content of an octet string or a bit string is parsed when compiling, but no further action is taken. This constraint is not a PER-visible constraint.
- The subtype constraint by regular expressions (`PATTERN`) for character string types is parsed when compiling, but no further action is taken. This constraint is not a PER-visible constraint.
- Multiple-line comments as in C, `/* ... */`, are supported.

## Prerequisites

It is assumed that the reader is familiar with the Erlang programming language, concepts of OTP, and is familiar with the ASN.1 notation. The ASN.1 notation is documented in the standard definition X.680, which is the primary text. It can also be helpful, but not necessary, to read the standard definitions X.681, X.682, X.683, X.690, and X.691.

A good book explaining those reference texts is Dubuisson: ASN.1 - Communication Between Heterogeneous Systems, is free to download at http://www.oss.com/asn1/dubuisson.html.

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