cbc_decr
ret = cbc_decr(key, ivec, data, num_data);
cbc_decr(krb5_key key, const krb5_data *ivec, krb5_crypto_iov *data,
ret = cbc_decr(key, ivec, data, num_data);
cbc_decr(krb5_key key, const krb5_data *ivec, krb5_crypto_iov *data,