Adding Public Key Certificate to a SOAP Request

I am trying to add a digital certificate that only contains the public key to the header of the request, but I get this error in the "error log". I guess that in the configuration it expects a private key but in this case I don't want to send it, I just want the public one.

The error:

org.apache.ws.security.WSSecurityException: Signature creation failed

Caused by: org.apache.ws.security.WSSecurityException: Cannot find key for alias: [certificado]

at org.apache.ws.security.components.crypto.Merlin.getPrivateKey(Merlin.java:703)

at org.apache.ws.security.message.WSSecSignature.computeSignature(WSSecSignature.java:504)