type Service interface {
    // Signer returns a security.Signer for the specified key, which will
    // generally be referred to as a filename. Credentials
    // will generally be a passphrase for accessing the key.
    Signer(ctx context.Context, key string, credentials []byte) (security.Signer, error)

    // Close releases/closes all resources associated with the service instance.
    Close(ctx context.Context) error
}