code: func (v *validator) ValidateWithSignature(ks *keystore.Keystore, sig io.ReadSeeker) (*openpgp.Entity, error) {
  	if ks == nil {
  		return nil, nil
  	}
  	if _, err := v.image.Seek(0, 0); err != nil {
  		return nil, errwrap.Wrap(errors.New("error seeking ACI file"), err)
  	}
  	if _, err := sig.Seek(0, 0); err != nil {
  		return nil, errwrap.Wrap(errors.New("error seeking signature file"), err)
  	}
  	entity, err := ks.CheckSignature(v.ImageName(), v.image, sig)
  	if err == pgperrors.ErrUnknownIssuer {
  		log.Print("If you expected the signing key to change, try running:")
  		log.Print("    rkt trust --prefix <image>")
  	}
  	if err != nil {
  		return nil, err
  	}
  	return entity, nil
  }
docs: // ValidateWithSignature verifies the image against a given signature
  // file.