@Override
    public String getId() {
        return this.principal != null ? this.principal.getId() : UNKNOWN_ID;
    }
Read the contents of the source into a byte array.

@param source the byte array source
@return the byte[] read from the source or null
support/cas-server-support-spnego/src/main/java/org/apereo/cas/support/spnego/authentication/principal/SpnegoCredential.java