This paper presents new cryptographic protocols for a stand-alone simulatable two-party parallel coin-flipping (into a well) and a universally composable commitment scheme, with near optimal asymptotic communication rate, in the static and computational malicious model. The approach, denoted expand-mask-hash, uses in both protocols a pseudo-random generator (PRG) and a collision-resistant hash function (CR-Hash) to combine separate extractable commitments and equivocable commitments (associated with short bit-strings) into a unified extractable-and-equivocable property amplified to a larger target length, amortizing the cost of base commitments. The new stand-alone coin-flipping protocol is based on a simple augmentation of the traditional coin-flipping template. To the knowledge of the author, it is the first proposal shown to simultaneously be two-side-simulatable and have an asymptotic (as the target length increases) communication rate converging to two bits per flipped coin and computation rate per party converging to that of PRG-generating and CR-hashing a bit-string with the target length. The new universally composable commitment scheme has efficiency comparable to very recent state-of-the-art constructions – namely asymptotic communication rate as close to 1 as desired, for each phase (commit and open) – while following a distinct design approach. Notably it does not require explicit use of oblivious transfer and it uses an erasure encoding instead of stronger error correction codes.
