White-Box Cryptography: A Time-Security Trade-Off for the SPNbox Family

White-box cryptography aims to ensure the security of cryptographic algorithms in an untrusted environment where the adversary has full access to their implementations. Typical applications are DRM, Pay Tv boxes, and smartphones. A number of white-box implementations for standard cryptographic algorithms—e.g., AES and DES—have been published in the literature. Unfortunately, such implementations are subjected to algebraic attacks, side channel attacks, etc. and thus researchers developed new ciphers—e.g., SPACE and the SPNbox family—with a dedicated design approach for white-box implementations. In this chapter, we focus on the SPNbox family. Our aim is to modify the small internal block cipher used in SPNbox in order to increase the number of bits of the key used in each round. This approach provides us the possibility to reduce the number of rounds of about 25%, making the algorithm faster than the previous one.