OPENSSL-ASN1PARSE(1ossl)

📦
version 3.5.9 · 8 versions
, in alpine 3.23, alpine 3.24

💅 OPENSSL-ASN1PARSE — ASN.1 parsing command ✨

okay so bestie, this is openssl-asn1parse from the 1ossl manual fam 🎀 — a whole diagnostic utility moment. It's giving "peak inside your ASN.1 structures" energy, and honestly? She also eats when it comes to extracting data out of ASN.1 formatted data. Let's get into it 🦋

🌸 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 ✨

So basically this command is a diagnostic utility that can parse ASN.1 structures 👑. That's her main character job. She can also be used to extract data from ASN.1 formatted data — versatile icon, we love to see it 💖

🎀 OPTIONS 🎀

  • -help — prints out a usage message. cute, helpful, does what it says 🫶
  • -inform DER|PEM|B64 — the input format, and the default is PEM. lowkey important. See openssl-format-options(1) for the deets 📖
  • -in filename — the input file. if you don't say anything, the default is standard input 🌈
  • -out filename — output file where the DER encoded data goes. If this option isn't present, then literally no data will be output (yes, really). This is most useful when you combine it with the -strparse option 🔥
  • -noout — don't output the parsed version of the input file. sometimes you just don't wanna see her 💀
  • -offset number — starting offset to begin parsing. default is the start of the file ⭐
  • -length number — number of bytes to parse. default is until the end of the file 🦋
  • -i — indents the output according to the "depth" of the structures. makes it readable, slay 📐
  • -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. hex girl summer 🔢
  • -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 🕳️🕳️
  • -genstr string, -genconf file — generate encoded data based on the string, the file, or both, using ASN1_generate_nconf(3) format. If only the file 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 — so the contents can 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 👑

🌟 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, iykyk 🔐 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 ✨

ngl, some knowledge of the ASN.1 structure is needed to interpret the output — this is not a vibes-only situation, 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 isn't 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 lets you include additional OIDs. Each line consists of three columns:

  1. the first column is the OID in numerical format and should be followed by white space
  2. the second column is the "short name", which is a single word followed by whitespace
  3. 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 👑

💖 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 😭 and the output of some ASN.1 types is not well handled (if at all). fr fr, she's trying her best 🙈

🌈 SEE ALSO 🌈

openssl(1), ASN1_generate_nconf(3)

Copyright 2000-2023 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>.

okay that's the whole page bestie, go parse something iconic 💅✨ this was version 3.5.9, dated 2026-09-29, from the OPENSSL-ASN1PARSE(1ossl) manual — mwah, ily, bye 🫶💜

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