sample code for https://github.com/openssl/openssl/issues/20286

test_25519.cc

#include <string>
#include <string_view>

#include <openssl/err.h>
#include <openssl/evp.h>
#include <openssl/x509.h>

constexpr size_t k25519PrivateKeyBytes = 32;
constexpr size_t k25519PublicKeyBytes = 32;
char private_key[k25519PrivateKeyBytes];

char *foo(std::string_view private_key)
{
  constexpr const char private_prefix[] = {
    0x30, 0x2e,          // SEQUENCE(3 elem) (46 bytes)
    0x02, 0x01, 0x00,    //   INTEGER 0
    0x30, 0x05,          //   SEQUENCE(1 elem) (5 bytes)
    0x06, 0x03,          //     OBJECT IDENTIFIER (3 bytes)
    0x2b, 0x65, 0x6e,    //       OBJ_X25519 -> 1.3.101.110 -> 1*40+3, 101, 110
    0x04, 0x22,          //   OCTET STRING(1 elem) (nested)
    0x04, 0x20};         //     OCTET STRING(32 bytes)

  std::string private_asn1;
  private_asn1.assign(private_prefix, sizeof(private_prefix));
  private_asn1.append(private_key.data(), private_key.size());
  const unsigned char *ptr = reinterpret_cast<const unsigned char*> (private_asn1.data());
  const unsigned char *end = ptr + private_asn1.size();

  PKCS8_PRIV_KEY_INFO* p8(d2i_PKCS8_PRIV_KEY_INFO(nullptr, &ptr, private_asn1.size()));
  if (!p8 || ptr != end)
  {
    return nullptr;
  }

  EVP_PKEY* key(EVP_PKCS82PKEY(p8));
  if (!key)
  {
    return nullptr;
  }

  X509_PUBKEY *ppubkey = nullptr;
  if (1 != X509_PUBKEY_set(&ppubkey, key))
  {
    return nullptr;
  }

  const unsigned char *p = nullptr;
  int plen;

  if (1 != X509_PUBKEY_get0_param(nullptr, &p, &plen, nullptr, ppubkey) || !p || plen != k25519PublicKeyBytes)
  {
  } 
  else 
  {
  }

  X509_PUBKEY_free(ppubkey);
  EVP_PKEY_free(key);
  PKCS8_PRIV_KEY_INFO_free(p8);
  return nullptr;
}

int main()
{
  for(int i = 0 ; i < 256; i++)
  {
    private_key[0] = i;
    for(int j = 0 ; j < 256; j++)
    {
      private_key[1] = j;
      foo({private_key, k25519PrivateKeyBytes});
    }
  }
  return 0;
}