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A Substitution-Permutation Network (SPN) is a cryptographic primitive that is used to provide confidentiality to digital data. It is a type of block cipher that works by iteratively applying a series of substitution and permutation operations to the input data.

The basic structure of an SPN consists of several rounds, with each round consisting of three main operations: substitution, permutation, and key mixing. In the substitution operation, a non-linear function is applied to a subset of the input data. The output of this operation is then passed through a permutation operation, which rearranges the order of the output bits. Finally, the output of the permutation operation is combined with a key using an XOR operation, and the resulting output is used as the input for the next round.

The number of rounds and the sizes of the substitution and permutation operations used in an SPN depend on the specific cryptographic application and the security requirements. However, in general, a larger number of rounds and larger substitution and permutation operations provide increased security.

SPNs are widely used in modern cryptographic protocols, including the Advanced Encryption Standard (AES) which is a widely used block cipher for encrypting digital data. SPNs have also been used in the design of hash functions, message authentication codes, and authenticated encryption schemes.