=pod

=head1 NAME

openssl-asn1parse - ASN.1 parsing command

=head1 SYNOPSIS

B<openssl> B<asn1parse>
[B<-help>]
[B<-inform> B<DER>|B<PEM>|B<B64>]
[B<-in> I<filename>]
[B<-out> I<filename>]
[B<-noout>]
[B<-offset> I<number>]
[B<-length> I<number>]
[B<-i>]
[B<-oid> I<filename>]
[B<-dump>]
[B<-dlimit> I<num>]
[B<-strparse> I<offset>]
[B<-genstr> I<string>]
[B<-genconf> I<file>]
[B<-strictpem>]
[B<-item> I<name>]

=head1 DESCRIPTION

This command is a diagnostic utility that can parse ASN.1 structures.
It can also be used to extract data from ASN.1 formatted data.

=head1 OPTIONS

=over 4

=item B<-help>

Print out a usage message.

=item B<-inform> B<DER>|B<PEM>|B<B64>

The input format; the default is B<PEM>.
See L<openssl-format-options(1)> for details.

=item B<-in> I<filename>

The input file, default is standard input.

=item B<-out> I<filename>

Output file to place the DER encoded data into. If this
option is not present then no data will be output. This is most useful when
combined with the B<-strparse> option.

=item B<-noout>

Don't output the parsed version of the input file.

=item B<-offset> I<number>

Starting offset to begin parsing, default is start of file.

=item B<-length> I<number>

Number of bytes to parse, default is until end of file.

=item B<-i>

Indents the output according to the "depth" of the structures.

=item B<-oid> I<filename>

A file containing additional OBJECT IDENTIFIERs (OIDs). The format of this
file is described in the NOTES section below.

=item B<-dump>

Dump unknown data in hex format.

=item B<-dlimit> I<num>

Like B<-dump>, but only the first B<num> bytes are output.

=item B<-strparse> I<offset>

Parse the contents octets of the ASN.1 object starting at B<offset>. This
option can be used multiple times to "drill down" into a nested structure.

=item B<-genstr> I<string>, B<-genconf> I<file>

Generate encoded data based on I<string>, I<file> or both using
L<ASN1_generate_nconf(3)> format. If I<file> only is
present then the string is obtained from the default section using the name
B<asn1>. The encoded data is passed through the ASN1 parser and printed out as
though it came from a file, the contents can thus be examined and written to a
file using the B<-out> option.

=item B<-strictpem>

If this option is used then B<-inform> will be ignored. Without this option any
data in a PEM format input file will be treated as being base64 encoded and
processed whether it has the normal PEM BEGIN and END markers or not. This
option will ignore any data prior to the start of the BEGIN marker, or after an
END marker in a PEM file.

=item B<-item> I<name>

Attempt to decode and print the data as an B<ASN1_ITEM> I<name>. This can be
used to print out the fields of any supported ASN.1 structure if the type is
known.

=back

=head2 Output

The output will typically contain lines like this:

  0:d=0  hl=4 l= 681 cons: SEQUENCE

.....

  229:d=3  hl=3 l= 141 prim: BIT STRING
  373:d=2  hl=3 l= 162 cons: cont [ 3 ]
  376:d=3  hl=3 l= 159 cons: SEQUENCE
  379:d=4  hl=2 l=  29 cons: SEQUENCE
  381:d=5  hl=2 l=   3 prim: OBJECT            :X509v3 Subject Key Identifier
  386:d=5  hl=2 l=  22 prim: OCTET STRING
  410:d=4  hl=2 l= 112 cons: SEQUENCE
  412:d=5  hl=2 l=   3 prim: OBJECT            :X509v3 Authority Key Identifier
  417:d=5  hl=2 l= 105 prim: OCTET STRING
  524:d=4  hl=2 l=  12 cons: SEQUENCE

.....

This example is part of a self-signed certificate. Each line starts with the
offset in decimal. C<d=XX> specifies the current depth. The depth is increased
within the scope of any SET or SEQUENCE. C<hl=XX> gives the header length
(tag and length octets) of the current type. C<l=XX> gives the length of
the contents octets.

The B<-i> option can be used to make the output more readable.

Some knowledge of the ASN.1 structure is needed to interpret the output.

In this example the BIT STRING at offset 229 is the certificate public key.
The contents octets of this will contain the public key information. This can
be examined using the option C<-strparse 229> to yield:

    0:d=0  hl=3 l= 137 cons: SEQUENCE
    3:d=1  hl=3 l= 129 prim: INTEGER           :E5D21E1F5C8D208EA7A2166C7FAF9F6BDF2059669C60876DDB70840F1A5AAFA59699FE471F379F1DD6A487E7D5409AB6A88D4A9746E24B91D8CF55DB3521015460C8EDE44EE8A4189F7A7BE77D6CD3A9AF2696F486855CF58BF0EDF2B4068058C7A947F52548DDF7E15E96B385F86422BEA9064A3EE9E1158A56E4A6F47E5897
  135:d=1  hl=2 l=   3 prim: INTEGER           :010001

=head1 NOTES

If an OID is not part of OpenSSL's internal table it will be represented in
numerical form (for example 1.2.3.4). The file passed to the B<-oid> option
allows additional OIDs to be included. Each line consists of three columns,
the first column is the OID in numerical format and should be followed by white
space. The second column is the "short name" which is a single word followed
by whitespace. The final column is the rest of the line and is the
"long name". Example:

C<1.2.3.4       shortName       A long name>

For any OID with an associated short and long name, this command will display
the long name.

=head1 EXAMPLES

Parse a file:

 openssl asn1parse -in file.pem

Parse a DER file:

 openssl asn1parse -inform DER -in file.der

Generate a simple UTF8String:

 openssl asn1parse -genstr 'UTF8:Hello World'

Generate and write out a UTF8String, don't print parsed output:

 openssl asn1parse -genstr 'UTF8:Hello World' -noout -out utf8.der

Generate using a config file:

 openssl asn1parse -genconf asn1.cnf -noout -out asn1.der

Example config file:

 asn1=SEQUENCE:seq_sect

 [seq_sect]

 field1=BOOL:TRUE
 field2=EXP:0, UTF8:some random string


=head1 BUGS

There should be options to change the format of output lines. The output of some
ASN.1 types is not well handled (if at all).

=head1 SEE ALSO

L<openssl(1)>,
L<ASN1_generate_nconf(3)>

=head1 COPYRIGHT

Copyright 2000-2026 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
