code: func (c *Conversation) processSig(msg []byte) (err error) {
  	sigMsg := sig{}
  	err = sigMsg.deserialize(msg)
  	if err != nil {
  		return
  	}
  
  	theirMAC := sigMsg.macSig
  	encryptedSig := sigMsg.encryptedSig
  
  	if err := c.processEncryptedSig(encryptedSig, theirMAC, &c.ake.sigKey); err != nil {
  		return newOtrError("in signature message: " + err.Error())
  	}
  
  	return nil
  }
docs: // processSig = bob = x
  // Alice -- Signature -----------> Bob