find_derived_key
cached_key = find_derived_key(usage, DK_CKSUM_KEY_BYTE,
cached_key = find_derived_key(usage, DK_ENCR_KEY_BYTE, key);
cached_key = find_derived_key(usage, DK_HASH_KEY_BYTE, key);
krb5_keyblock *find_derived_key(krb5_keyusage, uchar_t, krb5_keyblock *);