Adding Key Information

<Signature Id="..."> 

  <SignedInfo>

    ... 

  </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, …