Blind source separation (BSS) has been successfully applied to many fields such as communications and biomedical engineering. Its application for image encryption, however, remains largely unexplored. In this contribution, a novel BSS-based scheme is proposed for encrypting multiple images, in which the underdetermined BSS problem is fully exploited to achieve the image security. The necessary conditions for generating the key images required for this underdetermined system are presented. The sufficient conditions for constructing the underdetermined mixing matrix for encryption are then described. Extensive computer simulations, coupled with the performance analyses, demonstrate the high level of security of the proposed method.