ASN.1 parsing tool
openssl asn1parse [-inform PEM|DER] [-in filename] [-out filename] [-noout] [-offset number] [-length number] [-i] [-oid filename] [-strparse offset] [-genstr string] [-genconf file]
The asn1parse command is a diagnostic utility that can parse ASN.1 structures. It can also be used to extract data from ASN.1 formatted data.
the input format. DER is binary format and PEM (the default) is base64 encoded.
the input file, default is standard input
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.
don’t output the parsed version of the input file.
starting offset to begin parsing, default is start of file.
number of bytes to parse, default is until end of file.
indents the output according to the "depth" of the structures.
a file containing additional OBJECT IDENTIFIERs (OIDs). The format of this file is described in the NOTES section below.
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 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.
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.
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
If an OID is not part of OpenSSL’s internal table it will be represented in numerical form (for example 220.127.116.11). 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 white space. The final column is the rest of the line and is the "long name". asn1parse displays the long name. Example:
"18.104.22.168 shortName A long name"
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
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).