Generic asn.1 parser

Sean Hinde Sean.Hinde@REDACTED
Wed Apr 11 17:28:30 CEST 2001


I felt the need for an asn.1 parser which would decode generic asn.1 data
into a tree structure. Here's the result in case anyone else might have felt
this need!

It seems to work fine for a whole bunch of c7 MAP messages (which include
nested indefinite length constructor tags). I guess if it deals with those
it should be in a reasonably working state :)

The output format is a bit arbritrary.

Don't sue me if it causes your business to fail etc. Please let me know if
you improve it

I use atoms quite a bit - does anyone have a good feel for the performance
impact of this over using integers for matching and tagging?


%% Decode asn1 coded binary into parse tree
%% Handles indefinite length if re-assembly has already been
%% done - should be relatively easy to allow for segmented though
%% as we keep a count of unrequited indefinite length
%% constructor tags.
asn1_decode(Bin) ->
    asn1_decode(Bin, 0).
asn1_decode(<<>>, 0) ->
asn1_decode(Bin, N0) ->
    {Class, Form, Tag, Rest, N} = get_tag(Bin, N0),
    case tag_type(Class, Form, Tag) of
	indefinite_end ->
	    asn1_decode(Rest, N);
	Constructor when Constructor == set;
			     Constructor == seq;
			     Constructor == constructor ->
	    case get_length(Rest) of
		{indefinite, Rest1} ->
		    [{{Constructor, indef, Class, Tag}, asn1_decode(Rest1,
		{Len, Rest1} ->
		    {Data, Rest2} = get_content(Len, Rest1),
		    [{{Constructor, Class, Tag}, asn1_decode(Data, 0)}|
		     asn1_decode(Rest2, N)]
	tag ->
	    {Len, Rest1} = get_length(Rest),				   
	    {Data, Rest2} = get_content(Len, Rest1),
	    [{{tag, fmt_class(Class), Tag}, Data}|asn1_decode(Rest2, N)]

%% Get tag data. 0:1, 0:15 gets around compiler
%% bug as I haven't updated my PC yet..
get_tag(<<0:1, 0:15, Rest/binary>>, 0) ->
get_tag(<<0:1, 0:15, Rest/binary>>, N) ->
    {indefinite_end, 0, 0, Rest, N-1};
get_tag(<<Class:2, Form:1, Tag:5, Rest/binary>>, N) ->
    {Tag1, Rest1} = get_tag1(Tag, Rest),
    {Class, Form, Tag1, Rest1, N}.

%% Handle extension parts of the tag field
get_tag1(31, <<0:1, Tag:7, Rest/binary>>) ->
    {Tag, Rest};
get_tag1(31, <<1:1, Msb:7, _:1, Lsb:7, Rest/binary>>) ->
    {Msb*128+Lsb, Rest};
get_tag1(Tag, Rest) ->
    {Tag, Rest}.
% Do short and long definite length forms
% And *now*... indefinite length!
get_length(<<0:1, Len:7, Rest/binary>>) ->
    {Len, Rest};
get_length(<<1:1, 0:7, Rest/binary>>) ->
    {indefinite, Rest};
get_length(<<1:1, Len_len:7, Rest/binary>>) ->
    <<Len:Len_len/unit:8, Rest1/binary>> = Rest,
    {Len, Rest1}.

% Get actual content of field
get_content(Len, Rest) ->
    <<Data:Len/binary, Rest1/binary>> = Rest,
    {Data, Rest1}.
% tag_type(Class, Form, Tag) -> tag|seq|set|constructor
tag_type(indefinite_end, _, _) -> indefinite_end;
tag_type(Class, 0, Tag) -> tag;
tag_type(0, 1, 16)       -> seq;
tag_type(0, 1, 17)       -> set;
tag_type(Class, 1, Els) -> constructor.

fmt_class(0) -> univ;
fmt_class(1) -> app;
fmt_class(2) -> context;
fmt_class(3) -> priv.

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