OPENSSL-ASN1PARSE(1SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

✨ NAME ✨

openssl-asn1parse — the ASN.1 parsing command, aka your little backstage pass into the DER/PEM guts 💅

🎀 SYNOPSIS 🎀

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

🦋 DESCRIPTION 🦋

Okay so this command is a diagnostic utility that can parse ASN.1 structures. That's the whole vibe.

And not only that — it can also be used to extract data from ASN.1 formatted data. Double threat. Main character energy fr fr. 👑

💖 OPTIONS 💖

  • -help — prints out a usage message. Your classic "bestie what do I even type" button.
  • -inform DER|PEM|B64 — the input format. Default is PEM, no cap. Peep openssl-format-options(1) for the details.
  • -in filename — the input file. If you don't say anything, the default is standard input.
  • -out 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 -strparse option.
  • -noout — don't output the parsed version of the input file. Like, we looked. We don't need to see.
  • -offset number — starting offset to begin parsing. Default is the start of the file.
  • -length number — number of bytes to parse. Default is until end of file.
  • -i — indents the output according to the "depth" of the structures. Make it pretty. ✨
  • -oid filename — a file containing additional OBJECT IDENTIFIERs (OIDs). The format of this file is described in the NOTES section below. 👀
  • -dump — dump unknown data in hex format.
  • -dlimit num — like -dump, but only the first num bytes are output.
  • -strparse offset — parse the contents octets of the ASN.1 object starting at offset. This option can be used multiple times to "drill down" into a nested structure. It's giving inception 🌀
  • -genstr string, -genconf file — generate encoded data based on string, file or both using ASN1_generate_nconf(3) format. If file only is present then the string is obtained from the default section using the name 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 -out option.
  • -strictpem — if this option is used then -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 name — attempt to decode and print the data as an ASN1_ITEM name. This can be used to print out the fields of any supported ASN.1 structure if the type is known. If the type is known, bestie. That's the catch. 😭

🌸 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. d=XX specifies the current depth. The depth is increased within the scope of any SET or SEQUENCE. hl=XX gives the header length (tag and length octets) of the current type. l=XX gives the length of the contents octets.

The -i option can be used to make the output more readable. 👑

Some knowledge of the ASN.1 structure is needed to interpret the output. Yeah. It's not giving hand-holding. Sorry bestie.

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 -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

💜 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 -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:

"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. Slay. ✨

🌈 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

💀 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). Not me reading that and feeling the pain fr. 🫠

🫶 SEE ALSO 🫶

openssl(1), ASN1_generate_nconf(3)

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 <https://www.openssl.org/source/license.html>.

k bye bestie 💖 mwah 🎀 — that was OPENSSL-ASN1PARSE(1SSL), from OpenSSL 4.0.3, dated 2026-09-29. Go parse something iconic. ✨💅🦋

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