Adding Key Information

<Signature Id="..."> 
  <SignedInfo> 
    ... 
    <Reference URI="..."> 
      ...
    </Reference> 
  </SignedInfo> 
  <SignatureValue>MC0CFFrVLtRlk=...</SignatureValue> 
  <KeyInfo> 
    <KeyValue>
      <PGPData>
        <PGPKeyID>FGHJ</PGPKeyID> 
        <PGPKeyPacket>BKJHasdfJKN</PGPKeyPacket> 
      </PGPData> 
    </KeyValue> 
  </KeyInfo> 
</Signature>

The example is for PGP; there are variants for DSA, RSA, X509, ...