Use HMAC-256 rather than SHA1 for secret digests

    HMAC is thought more resitant to hash extension or oracle attacks.  To
    remedy this, use an HMAC with a constant key to fulfill the role of a
    digest.
    
    A constant (non-secret) key is used, "AttrVault", as a debugging aid so
    one can quickly verify if some bytes are likely originated from
    AttrVault.