The error message from the import-secret-key operation on the credential-store and secret-key-credential-store resources should be more helpful when importing invalid key (truncated in this case)

[standalone@localhost:9990 /] /subsystem=elytron/credential-store=a:import-secret-key(alias=a2,key=RUxZA)
{
    "outcome" => "failed",
    "failure-description" => "COM00501: Expected padding",
    "rolled-back" => true
}

The same message appears when loading a file with a corrupted key.