This book constitutes the refereed proceedings of the 6th International Workshop on Lightweight Cryptography for Security and Privacy, LightSec 2025, held in Istanbul, Turkey, during September 1&2, 2025. The 18 full papers presented were carefully reviewed and selected from 31 submissions. The papers were organized into the following topical sections: Post-Quantum Cryptography; Efficient Implementation of Post-Quantum Cryptography; Advanced Cryptographic Algorithms: Fully Homomorphic Encryption, Zero-Knowledge; Hardware and Architecture Security; and Security, Cryptanalysis and Attacks.