UC and EUC Weak Bit-Commitments Using Seal-Once Tamper-Evidence

Based on tamper-evident devices, i.e., a type of distinguishable, sealed envelopes, we put forward weak bit-commitment protocols which are UC-secure. These commitments are weak in that it is legitimate that a party could cheat. Unlike in several similar lines of work, in our case, the party is not obliged to cheat, but he has ability to cheat if and when needed. The empowered party is the sender, i.e., the protocols are also sender-strong. We motivate the construction of such primitives at both theoretical and practical levels. Such protocols complete the picture of existent receiver-strong weak bit-commitments based on tamper-evidence. We also show that existent receiver-strong protocols of the kind are not EUC-secure, i.e., they are only UC-secure. Further, we put forward a second formalisation of tamper-evident distinguishable envelopes which renders those protocols and the protocols herein EUC-secure. We finally draw most implication-relations between the tamper-evident devices, our weak sender-strong commitments, the existent weak receiver-strong commitments, as well as standard commitments. The mechanisms at the foundation of these primitives are lightweight and the protocols yielded are end-to-end humanly verifiable.