Generating HMAC Signatures on the Command Line with OpenSSL

This post originally appeared on my personal website.

Proving authenticity of a message is important, even over transport methods such as HTTPS, as we may not be able to require full end-to-end encryption. One such method of producing a signature is using HMAC with a shared secret.

For instance, let us say that we want to use SHA256 as the hashing algorithm.

If using Java, we could write code similar to the below, leveraging the commons-codec project:

import org.apache.commons.codec.binary.Base64;
import org.apache.commons.codec.digest.HmacAlgorithms;
import org.apache.commons.codec.digest.HmacUtils;
// ...
String digest =
    new String(
        Base64.encodeBase64String(
            new HmacUtils(HmacAlgorithms.HMAC_SHA_256, "secret-key-here")
                .hmac("value-to-digest")));
// G73zFnFYggHRpmwuRFPgch6ctqEfyhZu33j5PQWYm+4=

However, this doesn't help when we want to script this from the command-line, and isn't as portable.

To do this we can utilise openssl:

$ echo -n "value-to-digest" | openssl dgst -sha256 -hmac "secret-key-here" -binary | openssl enc -base64 -A
// G73zFnFYggHRpmwuRFPgch6ctqEfyhZu33j5PQWYm+4=