This paper proposes an enhanced translation of Security Protocol Language (SPL) in high-level Petri nets in order to allow to prove automatically, using model-checking techniques, the authentication property of Needham-Schroeder- Lowe (NSL) protocol. The proposed approach generates finite nets and goes this way beyond the limitation which was imposed by the previous semantics due to the treatment of the replication operator. In order to reach this goal, we modify the way attacks are modelled. Due to fact that the presented approach focuses on the treatment of the protocol environment, it may be successfully reused for automated verification of properties of other security protocols.