Will first derive a signing key from the secret key and current date, then
calculate the HMAC over given data. This process is modelled after Amazon's
_Message Signature Version 4_ but uses less key derivations as we don't have
more information at our hands.
During key derivation the strings li3,1 and li3,1_form are inserted. 1
denotes the version of our signature algorithm and should be raised when the
algorithm is changed. Derivation is needed to not reveal the secret key.
Note: As the current date (year, month, day) is used to increase key security by
limiting its lifetime, a possible sideeffect is that a signature doen't verify if it is
generated on day N and verified on day N+1.