PRSONA/prsona/src/proof.cpp

#include "proof.hpp"

/* Altered from answer at
 * https://stackoverflow.com/questions/51144505/generate-sha-3-hash-in-c-using-openssl-library
 */

// Convert the bytes to a single integer, then make that a Scalar
Scalar bytes_to_scalar(const std::vector<uint8_t>& bytes)
{
    std::stringstream stream;
    for (uint8_t b : bytes)
    {
        stream << std::setw(2)
            << std::setfill('0')
            << std::hex
            << static_cast<int>(b);
    }
    mpz_class value;
    value.set_str(stream.str(), 16);

    return Scalar(value);
}

// Random Oracle (i.e. SHA3_256)
Scalar oracle(const std::string& input)
{
    uint32_t digest_length = SHA256_DIGEST_LENGTH;
    const EVP_MD* algorithm = EVP_sha3_256();
    uint8_t* digest = static_cast<uint8_t*>(OPENSSL_malloc(digest_length));
    
    EVP_MD_CTX* context = EVP_MD_CTX_new();
    EVP_DigestInit_ex(context, algorithm, NULL);
    EVP_DigestUpdate(context, input.c_str(), input.size());
    EVP_DigestFinal_ex(context, digest, &digest_length);
    EVP_MD_CTX_destroy(context);

    std::vector<uint8_t> digestBytes(digest, digest + digest_length);
    Scalar output = bytes_to_scalar(digestBytes);
    OPENSSL_free(digest);

    return output;
}