I tried to sign and artifact response using this code, just switching the entityDescriptor to an artifactResponse and it worked fine. https://blog.samlsecurity.com/2012/11/signing-with-opensaml.html
<?xml version="1.0" encoding="UTF-8"?><saml2p:ArtifactResponse xmlns:saml2p="urn:oasis:names:tc:SAML:2.0:protocol" Version="2.0">
<ds:Signature xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
<ds:SignedInfo>
<ds:CanonicalizationMethod Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
<ds:SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
<ds:Reference URI="">
<ds:Transforms>
<ds:Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
<ds:Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#"/>
</ds:Transforms>
<ds:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<ds:DigestValue>ZkE02ZnvIqyd+FcfL6PaXNI88Co=</ds:DigestValue>
</ds:Reference>
</ds:SignedInfo>
<ds:SignatureValue>fDxfW06rbJEFu5nqmsGxwt6O53N8FWwmDOO0e3nUWh0in2TRYD9nj2927pnQZNL4Mk3KAcSWVETUuHX11XWL+MgcosfJd31TR0XEui/F+BbojQlXJRHfD2BfEO9cQCygFSyyOb9tE2FU5noqnx2b3vI5mToam3a135007mAN/t14Jm71EfvvCF9qL2wXI55R8uab0WGqXx1LYSrAjBZq455SH9AIQu+n8L+KaiOzfpjiL+h/5YJ/a+uyiLV6H06TsytowDTBSW67YW110fpoOsD5vgULrZOABmeK6NRZWpI8PK1M+/r6SO0DTEbUiSTYtHz9XBcqbnD9d7ZQ3oZpEQ==</ds:SignatureValue>
<ds:KeyInfo>
<ds:X509Data>
<ds:X509Certificate>MIIDaTCCAlGgAwIBAgIEeL6vczANBgkqhkiG9w0BAQsFADBlMQswCQYDVQQGEwJOTzENMAsGA1UE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</ds:X509Certificate>
</ds:X509Data>
</ds:KeyInfo>
</ds:Signature>
</saml2p:ArtifactResponse>
Try to skip the KeyInfo part. Your not getting any marshalling errors?