README.asn1

ASN1
====

Abstract Syntax Notation One (or ASN1) is a standard by ITU-T and ISO/IEC
and used as a description language for defining data structure in
an independent manner.
Any data described in ASN1 notation can be serialized (or encoded) and
de-serialized (or decoded) with well-defined encoding rules.

A combination of ASN1 compiler and ASN1 decoder library function will
provide a function interface for parsing encoded binary into specific
data structure:
1) define data structure in a text file (*.asn1)
2) define "action" routines for specific "tags" defined in (1)
3) generate bytecode as a C file (*.asn1.[ch]) from *.asn1 file
   with ASN1 compiler (tools/asn1_compiler)
4) call a ASN1 decoder (asn1_ber_decoder()) with bytecode and data

Usage of ASN1 compiler
----------------------
  asn1_compiler [-v] [-d] <grammar-file> <c-file> <hdr-file>

  <grammar-file>:	ASN1 input file
  <c-file>:		generated C file
  <hdr-file>:		generated include file

Usage of ASN1 decoder
---------------------
  int asn1_ber_decoder(const struct asn1_decoder *decoder, void *context,
		       const unsigned char *data, size_t datalen);

  @decoder:		bytecode binary
  @context:		context for decoder
  @data:		data to be parsed
  @datalen:		size of data


As of writing this, ASN1 compiler and decoder are used to implement
X509 certificate parser, pcks7 message parser and RSA public key parser
for UEFI secure boot.