This has been possible for a long time using a combination of OpenSSL and ssh.
To sign: openssl dgst -sha512 -sign ~/.ssh/id_rsa file > file.sig
To verify, needs converting the public key (who.pub) to something OpenSSL can grok:
ssh-keygen -e -f /tmp/who.pub -m pkcs8 > /tmp/who.openssl.pub
Then verify: openssl dgst -sha512 -verify /tmp/who.openssl.pub -signature file.sig file
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