question: private String extractSignatureId ( UseKeyType useKey , SecurityHeaderType header ) { assert header != null ; String signatureId = null ; if ( useKey != null ) { signatureId = useKey . getSig ( ) ; if ( header . getSignature ( ) == null || ! signatureId . equalsIgnoreCase ( header . getSignature ( ) . getId ( ) ) ) { throw new InvalidSecurityHeaderException ( "SignatureId from UseKey doesn't match signature from the SecurityHeader" ) ; } } return signatureId ; }
target:  Extracts the signature id from the usekey element and verify that the signature id is valid