CODEWITHSUNDEEP
perlasn.1
Kevin G.
Kevin G.

Reputation: 1455

Convert::ASN1 decode error

I must be missing something really obvious here. I can decode this sample bit of data using online tools like http://asn1-playground.oss.com/, but am having trouble with basic usage of Perl's Convert::ASN1. Any idea what I'm missing?

use strict;
use warnings;
use Convert::ASN1;
use feature 'say';

# example from:
# http://www.oss.com/asn1/resources/asn1-made-simple/introduction.html

my $hex_data = '3018800A4A6F686E20536D697468810A39383736353433323130';
my $bin_data = join '', pack 'H*', $hex_data;

Convert::ASN1::asn_dump($bin_data);
# prints:
#    0000   24: SEQUENCE {
#    0002   10:   [CONTEXT 0]
#    0004     :     4A 6F 68 6E 20 53 6D 69 74 68 __ __ __ __ __ __ John Smith
#    000E   10:   [CONTEXT 1]
#    0010     :     39 38 37 36 35 34 33 32 31 30 __ __ __ __ __ __ 9876543210
#    001A     : }    

my $asn = Convert::ASN1->new;
$asn->prepare(<<ASN1) or die $asn->error;    
    Contact ::= SEQUENCE {
        name VisibleString,
        phone NumericString
    }
ASN1

my $asn1_node = $asn->find('Contact') 
    or die $asn->error;

my $payload = $asn1_node->decode($bin_data) 
    or die "can't decode Contact: ".$asn1_node->error;
# prints:
#    can't decode Contact: decode error 80<=>1a 2 4 name

Supporting YaFred's answer below, this is where that 80 and 81 are in that encoded string:

SEQ length=24 ** l=10  J  o  h n   S m i t h  ** l=10  9 8 7 6 5 4 3 2 1 0
30  18        80 0A    4A 6F 686E20536D697468 81 0A    39383736353433323130

Upvotes: 1

Views: 1641

Answers (2)

YaFred
YaFred

Reputation: 10018

May be this is as easy as

$asn->prepare(<<ASN1) or die $asn->error; 
My-Module DEFINITIONS AUTOMATIC TAGS ::=
BEGIN   
        Contact ::= SEQUENCE {
            name VisibleString,
            phone NumericString
        }
END
    ASN1

It's a bit long to explain if you start with ASN.1 ...

You are not giving the tagging context (the type Contact should be part of a module). So, the tools are making choices ...

The hexa you show is the result of encoding with AUTOMATIC TAGS

The tags of the 2 strings are '80' (Context tag 0 = 1000 0000) and '81' (Context tag 1 = 1000 0001)

@xxfelixxx gets something different because the encoding was performed as EXPLICIT TAGS

The tags of the 2 strings are '1a' (universal tag for VisibleString) and '12' (universal tag for NumericString)

Upvotes: 1

xxfelixxx
xxfelixxx

Reputation: 6602

I'm not sure where you got your hex string from...but if you use the Convert::ASN1::encode method, you get a slightly different hex string which can get decoded correctly:

my $res = $asn->encode({ name => 'John Smith', phone => 9876543210 });
my $res_hex = unpack 'H*', $res;
print "res_hex after encode : $res_hex\n";
print "original hex_data    : " . lc($hex_data) . "\n";
print "\n";

my payload = $asn1_node->decode($res) or die $asn1_node->error;
use Data::Dumper;
print Dumper($payload);

output

res_hex after encode : 30181a0a4a6f686e20536d697468120a39383736353433323130
original hex_data    : 3018800a4a6f686e20536d697468810a39383736353433323130

$VAR1 = {
      'name' => 'John Smith',
      'phone' => '9876543210'
};

Upvotes: 0

Related Questions