Sign and verify a file using OpenSSL command line tool. It exports the digital signature in Base64 format.

sign.sh

#!/bin/bash
# Sign a file with a private key using OpenSSL
# Encode the signature in Base64 format
#
# Usage: sign <file> <private_key>
#
# NOTE: to generate a public/private key use the following commands:
#
# openssl genrsa -aes128 -passout pass:<passphrase> -out private.pem 2048
# openssl rsa -in private.pem -passin pass:<passphrase> -pubout -out public.pem
#
# where <passphrase> is the passphrase to be used.

filename=$1
privatekey=$2

if [[ $# -lt 2 ]] ; then
  echo "Usage: sign <file> <private_key>"
  exit 1
fi

openssl dgst -sha256 -sign $privatekey -out /tmp/$filename.sha256 $filename
openssl base64 -in /tmp/$filename.sha256 -out signature.sha256
rm /tmp/$filename.sha256

verify.sh

#!/bin/bash
# Verify a file with a public key using OpenSSL
# Decode the signature from Base64 format
#
# Usage: verify <file> <signature> <public_key>
#
# NOTE: to generate a public/private key use the following commands:
#
# openssl genrsa -aes128 -passout pass:<passphrase> -out private.pem 2048
# openssl rsa -in private.pem -passin pass:<passphrase> -pubout -out public.pem
#
# where <passphrase> is the passphrase to be used.

filename=$1
signature=$2
publickey=$3

if [[ $# -lt 3 ]] ; then
  echo "Usage: verify <file> <signature> <public_key>"
  exit 1
fi

openssl base64 -d -in $signature -out /tmp/$filename.sha256
openssl dgst -sha256 -verify $publickey -signature /tmp/$filename.sha256 $filename
rm /tmp/$filename.sha256

Useful! Thanks. Although I wonder why transforming binary sig to base64 and then from base64? The base64 transformation can be skipped for the purposes of this example. Also might be worth noting that public key should be a PUBLIC KEY and not a certificate. In many cases you don't generate private/public key pairs but instead you have a private key and a certificate that might be signed by a public CA. So in this case you have to extract the public key from the certificate with: openssl x509 -in -pubkey -noout > public_key.pem

@ pshterev

x509 -in -pubkey -noout > public_key.pem

I did find this command example often in internet, but for my openssl version coming along with git this does not work, i get error x509: Unknown parameter >. But I can use just "-noout" to get a print in the console, or "-out public_key.pem" to get a file.

parameter >

Do you use bash ? cmd.exe ? or an exotic shell. cmd.exe and posix shell should support redirection to file (and doesn't interpret > as parameter). If you have a posix shell check the content of the variable IFS (it should contain at least space).

Here is a very nice blog: https://pagefault.blog/2019/04/22/how-to-sign-and-verify-using-openssl/.

See also official page: https://docs.openssl.org/3.1/man1/openssl-dgst/