CAP75 proposes adding two native host functions to Soroban for Poseidon and Poseidon 2 permutations, enabling efficient zero-knowledge proof operations on BLS and BN254 fields without expensive contract-side implementations.
CAP75 introduces native Soroban host functions for Poseidon and Poseidon 2 permutations rather than full hash functions. The proposal addresses the need for efficient cryptographic operations in zero-knowledge applications by exposing the internal permutation primitive instead of the sponge interface. This approach reduces complexity, avoids state management issues, and allows developers to implement various sponge-based applications (hashing, random streams, MACs) at the contract level. Performance testing shows the 3x3 matrix case requires approximately 500k CPU instructions and the 5x5 case around 1M instructions, well below the target ceiling for supporting 20 hash operations per contract. The implementation leverages existing field arithmetic and will include preset parameters for common use cases like CIRCOM and NOIR, with advanced users able to access a hazmat interface for custom parameters.