code: func (c *Conversation) sigMessage() ([]byte, error) {
  	c.ake.keys.ourKeyID++
  
  	encryptedSig, err := c.generateEncryptedSignature(&c.ake.sigKey)
  	if err != nil {
  		return nil, err
  	}
  
  	macSig := sumHMAC(c.ake.sigKey.m2, encryptedSig, c.version)
  	sigMsg := sig{
  		encryptedSig: encryptedSig,
  		macSig:       macSig,
  	}
  
  	return sigMsg.serialize(c.version), nil
  }
docs: // sigMessage = alice = y
  // Alice -- Signature -----------> Bob