Multi-signatures have seen renewed interest due to their application to blockchains, e.g., BIP 340 (one of the Bitcoin improvement proposals), which has triggered the proposals of several new schemes with improved efficiency. However, many previous works have a “loose” security reduction (a large gap between the difficulty of the security assumption and breaking the scheme) or depend on strong idealized assumptions such as the algebraic group model (AGM). This makes the achieved level of security uncertain when instantiated in groups typically used in practice, and it becomes unclear for developers how secure a given scheme is for a given choice of security parameters. Thus, this leads to the question “what kind of schemes can we construct that achieves tight security based on standard assumptions?”. In this paper, we show a simple two-round tightly-secure pairing-based multi-signature scheme based on the computation Diffie-Hellman problem in the random oracle model. This proposal is the first two-round multi-signature scheme that achieves tight security based on a computational assumption and supports key aggregation. Furthermore, our scheme reduce the signature bit size by 19% compared with the shortest existing tightly-secure DDH-based multi-signature scheme. Moreover, we implemented our scheme in C++ and confirmed that it is efficient in practice; to complete the verification takes less than 1[ms] with a total (computational) signing time of 13[ms] for under 100 signers. The source code of the implementation is published as OSS.