How to check a public RSA key file

Question

Inside a shell script I want verify public RSA file. All I want to do is that find a way to check this file is a genuine public key file, nothing else.

Can I ask experts here what are the ways I can verify this input file to check this is a genuine public key file , not a regular file.

I will be using this public key file in future to validate an incoming encrypt gzip file but that is out of scope for now.

All I want is validate input file to check its genuine RSA public key file not an ordinary file.please note that I do not have any other files with me (eg : private key) .

e.g.: if the file is ‘public.pem’ I just want check inside that it’s a genuine RSA public key file not just a file with texts or file is not corrupted . I’m already checking that file is not zero sized and md5 .

other possible checks I found check file got text ‘BEGIN PUBLIC KEY’ and ‘END PUBLIC KEY’ Also found this command in google , Is there a better way to do this using openssl

‘openssl rsa -noout -text -inform PEM -in pubkey.pem -pubin’

Thanks

Answer 1

It's possible to use any public key format parser, including openssl or even parse key yourself as the format is not that difficult.

Command line tools set a non-zero exit code, when parsing fails:

openssl rsa -inform PEM -pubin -in pubkey.pem -noout &> /dev/null
if [ $? != 0 ] ; then
    echo "this was definitely not a RSA public key in PEM format"
    exit 1
fi

Just to check any public key:

openssl pkey -inform PEM -pubin -in pubkey.pem -noout &> /dev/null
if [ $? != 0 ] ; then
    echo "this was definitely not a public key in PEM format"
    exit 1
fi