Example of how to calculate an HMAC signature using command line utilities:
echo -n "/path/subscribe?account_id=my_account&club_id=club%20id&msisdn=27820000000×tamp=2012-05-29T15%3A00%3A12Z" | openssl dgst -sha256 -hmac "secret_value" -binary | base64
This is very handy! I stumbled upon this, and tried it out for a use-case I was working with. I discovered that the quotes around secret_value were also getting interpreted as part of the secret value.