**compute salted hmac

** now compute login digest using nonce

** our example variables
username: "alice"
password: "secret"
userSalt: "6s6Q5Rn0xZP0LPf89bNdv+65EmMUrTsey2fIhim/wKU="
nonce: "3da210bdb1163d0d41d3c516314cbd6e"
hmac: "z9NILqJ3QHSG5+GlDnXsV9txjgo="
digest: "B2B3mIzE/+dqcqOJJ/ejSGXRKvE="