How to sign and verify a signature on the command line

OpenSSL provides several subcommands for signing and verifying signatures. Let’s take a look:

• The deprecated RSA-specific openssl rsautl subcommand.
• The openssl dgst subcommand: This is usually used for message digest calculation but can also be used to sign the produced digests. This means that it cannot be used to sign PureEdDSA because that signature algorithm does not sign digests.
• The openssl pkeyutl subcommand: This subcommand can be used to sign with any signature algorithm supported by OpenSSL. Before OpenSSL 3.0, openssl pkeyutl did not support signing long inputs; the user had to make the message digest before signing. Since OpenSSL 3.0, openssl pkeyutl supports both “raw input,” as it is called in the documentation, and a message digest as input.

We are going to use the openssl pkeyutl subcommand for our examples. Its documentation can be found on the openssl-pkeyutl man page...