The majority of real-world applications of digital signatures use timestamping to ensure non-repudiation in face of possible key revocations. This observation led Buldas, Laanoja, and Truu to a server-assisted digital signature scheme built around cryptographic timestamping. In this paper, we report on the machine-checked proofs of existential unforgeability under the chosen-message attack (EUF-CMA) of some variations of BLT digital signature scheme. The proofs are developed and verified using the EasyCrypt framework, which provides interactive theorem proving supported by the state-of-the-art SMT solvers.