/* CPL_SHA256* functions derived from http://code.google.com/p/ulib/source/browse/trunk/src/base/sha256sum.c?r=39 */

/* The MIT License

   Copyright (C) 2011 Zilong Tan (tzlloch@gmail.com)
   Copyright (C) 2015 Even Rouault <even.rouault at spatialys.com>

   Permission is hereby granted, free of charge, to any person obtaining
   a copy of this software and associated documentation files (the
   "Software"), to deal in the Software without restriction, including
   without limitation the rights to use, copy, modify, merge, publish,
   distribute, sublicense, and/or sell copies of the Software, and to
   permit persons to whom the Software is furnished to do so, subject to
   the following conditions:

   The above copyright notice and this permission notice shall be
   included in all copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
   MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
   BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
   ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
   CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
   SOFTWARE.
*/

/*
 *  Original code (for SHA256 computation only) is derived from the author:
 *  Allan Saddi
 */

#ifdef HAVE_CRYPTOPP
#define DO_NOT_USE_DEBUG_BOOL
#endif

#include <string.h>
#include "cpl_conv.h"
#include "cpl_error.h"
#include "cpl_sha256.h"
#include "cpl_string.h"

CPL_CVSID("$Id: cpl_sha256.cpp 8ca42e1b9c2e54b75d35e49885df9789a2643aa4 2020-05-17 21:43:40 +0200 Even Rouault $")

#define ROTL(x, n) (((x) << (n)) | ((x) >> (32 - (n))))
#define ROTR(x, n) (((x) >> (n)) | ((x) << (32 - (n))))

#define Ch(x, y, z) ((z) ^ ((x) & ((y) ^ (z))))
#define Maj(x, y, z) (((x) & ((y) | (z))) | ((y) & (z)))
#define SIGMA0(x) (ROTR((x), 2) ^ ROTR((x), 13) ^ ROTR((x), 22))
#define SIGMA1(x) (ROTR((x), 6) ^ ROTR((x), 11) ^ ROTR((x), 25))
#define sigma0(x) (ROTR((x), 7) ^ ROTR((x), 18) ^ ((x) >> 3))
#define sigma1(x) (ROTR((x), 17) ^ ROTR((x), 19) ^ ((x) >> 10))

#define DO_ROUND() {                                                    \
                GUInt32 t1 = h + SIGMA1(e) + Ch(e, f, g) + *(Kp++) + *(W++);    \
                GUInt32 t2 = SIGMA0(a) + Maj(a, b, c);                          \
                h = g;                                                  \
                g = f;                                                  \
                f = e;                                                  \
                e = d + t1;                                             \
                d = c;                                                  \
                c = b;                                                  \
                b = a;                                                  \
                a = t1 + t2;                                            \
        }

constexpr GUInt32 K[64] = {
    0x428a2f98U, 0x71374491U, 0xb5c0fbcfU, 0xe9b5dba5U,
    0x3956c25bU, 0x59f111f1U, 0x923f82a4U, 0xab1c5ed5U,
    0xd807aa98U, 0x12835b01U, 0x243185beU, 0x550c7dc3U,
    0x72be5d74U, 0x80deb1feU, 0x9bdc06a7U, 0xc19bf174U,
    0xe49b69c1U, 0xefbe4786U, 0x0fc19dc6U, 0x240ca1ccU,
    0x2de92c6fU, 0x4a7484aaU, 0x5cb0a9dcU, 0x76f988daU,
    0x983e5152U, 0xa831c66dU, 0xb00327c8U, 0xbf597fc7U,
    0xc6e00bf3U, 0xd5a79147U, 0x06ca6351U, 0x14292967U,
    0x27b70a85U, 0x2e1b2138U, 0x4d2c6dfcU, 0x53380d13U,
    0x650a7354U, 0x766a0abbU, 0x81c2c92eU, 0x92722c85U,
    0xa2bfe8a1U, 0xa81a664bU, 0xc24b8b70U, 0xc76c51a3U,
    0xd192e819U, 0xd6990624U, 0xf40e3585U, 0x106aa070U,
    0x19a4c116U, 0x1e376c08U, 0x2748774cU, 0x34b0bcb5U,
    0x391c0cb3U, 0x4ed8aa4aU, 0x5b9cca4fU, 0x682e6ff3U,
    0x748f82eeU, 0x78a5636fU, 0x84c87814U, 0x8cc70208U,
    0x90befffaU, 0xa4506cebU, 0xbef9a3f7U, 0xc67178f2U
};

#ifdef WORDS_BIGENDIAN

#define BYTESWAP(x) (x)
#define BYTESWAP64(x) (x)

#else  // WORDS_BIGENDIAN

#define BYTESWAP(x) ((ROTR((x), 8) & 0xff00ff00U) | \
                     (ROTL((x), 8) & 0x00ff00ffU))
#define BYTESWAP64(x) _byteswap64(x)

static inline GUInt64 _byteswap64(GUInt64 x)
{
    GUInt32 a = static_cast<GUInt32>(x >> 32);
    GUInt32 b = static_cast<GUInt32>(x);
    return
        (static_cast<GUInt64>(BYTESWAP(b)) << 32) |
        static_cast<GUInt64>(BYTESWAP(a));
}

#endif /* !(WORDS_BIGENDIAN) */

constexpr GByte padding[64] = {
        0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
};

void CPL_SHA256Init(CPL_SHA256Context * sc)
{
        sc->totalLength = 0;
        sc->hash[0] = 0x6a09e667U;
        sc->hash[1] = 0xbb67ae85U;
        sc->hash[2] = 0x3c6ef372U;
        sc->hash[3] = 0xa54ff53aU;
        sc->hash[4] = 0x510e527fU;
        sc->hash[5] = 0x9b05688cU;
        sc->hash[6] = 0x1f83d9abU;
        sc->hash[7] = 0x5be0cd19U;
        sc->bufferLength = 0U;
}

static GUInt32 burnStack( int size )
{
    GByte buf[128];
    GUInt32 ret = 0;

    memset(buf, static_cast<GByte>(size & 0xff), sizeof(buf));
    for( size_t i = 0; i < sizeof(buf); i++ )
        ret += ret * buf[i];
    size -= static_cast<int>(sizeof(buf));
    if( size > 0 )
        ret += burnStack(size);
    return ret;
}

CPL_NOSANITIZE_UNSIGNED_INT_OVERFLOW
static void CPL_SHA256Guts(CPL_SHA256Context * sc, const GUInt32 * cbuf)
{
        GUInt32 buf[64] = {};

        GUInt32 *W = buf;

        for( int i = 15; i >= 0; i-- )
        {
                *(W++) = BYTESWAP(*cbuf);
                cbuf++;
        }

        GUInt32 *W16 = &buf[0];
        GUInt32 *W15 = &buf[1];
        GUInt32 *W7 = &buf[9];
        GUInt32 *W2 = &buf[14];

        for( int i = 47; i >= 0; i-- )
        {
                *(W++) = sigma1(*W2) + *(W7++) + sigma0(*W15) + *(W16++);
                W2++;
                W15++;
        }

        GUInt32 a = sc->hash[0];
        GUInt32 b = sc->hash[1];
        GUInt32 c = sc->hash[2];
        GUInt32 d = sc->hash[3];
        GUInt32 e = sc->hash[4];
        GUInt32 f = sc->hash[5];
        GUInt32 g = sc->hash[6];
        GUInt32 h = sc->hash[7];

        const GUInt32 *Kp = K;
        W = buf;

#ifndef CPL_SHA256_UNROLL
#define CPL_SHA256_UNROLL 1
#endif                          /* !CPL_SHA256_UNROLL */

#if CPL_SHA256_UNROLL == 1
        for( int i = 63; i >= 0; i-- )
                DO_ROUND();
#elif CPL_SHA256_UNROLL == 2
        for( int i = 31; i >= 0; i-- )
        {
                DO_ROUND();
                DO_ROUND();
        }
#elif CPL_SHA256_UNROLL == 4
        for( int i = 15; i >= 0; i-- )
        {
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
        }
#elif CPL_SHA256_UNROLL == 8
        for( int i = 7; i >= 0; i-- )
        {
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
        }
#elif CPL_SHA256_UNROLL == 16
        for( int i = 3; i >= 0; i-- )
        {
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
        }
#elif CPL_SHA256_UNROLL == 32
        for( int i = 1; i >= 0; i-- )
        {
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
                DO_ROUND();
        }
#elif CPL_SHA256_UNROLL == 64
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
        DO_ROUND();
#else
#error "CPL_SHA256_UNROLL must be 1, 2, 4, 8, 16, 32, or 64!"
#endif

        sc->hash[0] += a;
        sc->hash[1] += b;
        sc->hash[2] += c;
        sc->hash[3] += d;
        sc->hash[4] += e;
        sc->hash[5] += f;
        sc->hash[6] += g;
        sc->hash[7] += h;
}

void CPL_SHA256Update( CPL_SHA256Context * sc, const void *data, size_t len )
{
    int needBurn = 0;

    if( sc->bufferLength )
    {
        const GUInt32 bufferBytesLeft = 64U - sc->bufferLength;

        GUInt32 bytesToCopy = bufferBytesLeft;
        if( bytesToCopy > len )
            bytesToCopy = static_cast<GUInt32>(len);

        memcpy(&sc->buffer.bytes[sc->bufferLength], data, bytesToCopy);

        sc->totalLength += bytesToCopy * 8U;

        sc->bufferLength += bytesToCopy;
        data = static_cast<const GByte *>(data) + bytesToCopy;
        len -= bytesToCopy;

        if( sc->bufferLength == 64U )
        {
            CPL_SHA256Guts(sc, sc->buffer.words);
            needBurn = 1;
            sc->bufferLength = 0U;
        }
    }

    while( len > 63U )
    {
        sc->totalLength += 512U;

        CPL_SHA256Guts(sc, static_cast<const GUInt32 *>(data));
        needBurn = 1;

        data = static_cast<const GByte *>(data) + 64U;
        len -= 64U;
    }

    if( len )
    {
        memcpy(&sc->buffer.bytes[sc->bufferLength], data, len);

        sc->totalLength += static_cast<GUInt32>(len) * 8U;

        sc->bufferLength += static_cast<GUInt32>(len);
    }

    if( needBurn )
    {
        // Clean stack state of CPL_SHA256Guts()

        // We add dummy side effects to avoid burnStack() to be
        // optimized away (#6157).
        static GUInt32 accumulator = 0;
        accumulator += burnStack(
            static_cast<int>(sizeof(GUInt32[74]) + sizeof(GUInt32 *[6]) +
                             sizeof(int) + ((len%2) ? sizeof(int) : 0)) );
        if( accumulator == 0xDEADBEEF )
            fprintf(stderr, "%s", ""); /*ok*/
    }
}

void CPL_SHA256Final( CPL_SHA256Context * sc, GByte hash[CPL_SHA256_HASH_SIZE] )
{
    GUInt32 bytesToPad = 120U - sc->bufferLength;
    if( bytesToPad > 64U )
        bytesToPad -= 64U;

    const GUInt64 lengthPad = BYTESWAP64(sc->totalLength);

    CPL_SHA256Update(sc, padding, bytesToPad);
    CPL_SHA256Update(sc, &lengthPad, 8U);

    if( hash )
    {
        for( int i = 0; i < CPL_SHA256_HASH_WORDS; i++ )
        {
            *reinterpret_cast<GUInt32 *>(hash) = BYTESWAP(sc->hash[i]);
            hash += 4;
        }
    }
}

void CPL_SHA256( const void *data, size_t len,
                 GByte hash[CPL_SHA256_HASH_SIZE] )
{
    CPL_SHA256Context sSHA256Ctxt;
    CPL_SHA256Init(&sSHA256Ctxt);
    CPL_SHA256Update(&sSHA256Ctxt, data, len);
    CPL_SHA256Final(&sSHA256Ctxt, hash);
    memset(&sSHA256Ctxt, 0, sizeof(sSHA256Ctxt));
}

#define CPL_HMAC_SHA256_BLOCKSIZE 64U

// See https://en.wikipedia.org/wiki/Hash-based_message_authentication_code#Implementation
void CPL_HMAC_SHA256( const void *pKey, size_t nKeyLen,
                      const void *pabyMessage, size_t nMessageLen,
                      GByte abyDigest[CPL_SHA256_HASH_SIZE] )
{
    GByte abyPad[CPL_HMAC_SHA256_BLOCKSIZE] = {};
    if( nKeyLen > CPL_HMAC_SHA256_BLOCKSIZE )
    {
        CPL_SHA256(pKey, nKeyLen, abyPad);
    }
    else
    {
        memcpy(abyPad, pKey, nKeyLen);
    }

    // Compute ipad.
    for( size_t i = 0; i < CPL_HMAC_SHA256_BLOCKSIZE; i++ )
        abyPad[i] = 0x36 ^ abyPad[i];

    CPL_SHA256Context sSHA256Ctxt;
    CPL_SHA256Init(&sSHA256Ctxt);
    CPL_SHA256Update(&sSHA256Ctxt, abyPad, CPL_HMAC_SHA256_BLOCKSIZE);
    CPL_SHA256Update(&sSHA256Ctxt, pabyMessage, nMessageLen);
    CPL_SHA256Final(&sSHA256Ctxt, abyDigest);

    // Compute opad.
    for( size_t i = 0; i < CPL_HMAC_SHA256_BLOCKSIZE; i++ )
        abyPad[i] = (0x36 ^ 0x5C) ^ abyPad[i];

    CPL_SHA256Init(&sSHA256Ctxt);
    CPL_SHA256Update(&sSHA256Ctxt, abyPad, CPL_HMAC_SHA256_BLOCKSIZE);
    CPL_SHA256Update(&sSHA256Ctxt, abyDigest, CPL_SHA256_HASH_SIZE);
    CPL_SHA256Final(&sSHA256Ctxt, abyDigest);

    memset(&sSHA256Ctxt, 0, sizeof(sSHA256Ctxt));
    memset(abyPad, 0, CPL_HMAC_SHA256_BLOCKSIZE);
}


#ifdef HAVE_CRYPTOPP

/* Begin of crypto++ headers */
#ifdef _MSC_VER
#pragma warning( push )
#pragma warning( disable : 4189 )
#pragma warning( disable : 4512 )
#pragma warning( disable : 4244 )
#endif

#ifdef USE_ONLY_CRYPTODLL_ALG
#include "cryptopp/dll.h"
#else
#include "cryptopp/rsa.h"
#include "cryptopp/queue.h"
#endif

#include "cryptopp/base64.h"
#include "cryptopp/osrng.h"

// Fix compatibility with Crypto++
#if CRYPTOPP_VERSION >= 600
typedef CryptoPP::byte cryptopp_byte;
#else
typedef byte cryptopp_byte;
#endif

#ifdef _MSC_VER
#pragma warning( pop )
#endif

#endif // HAVE_CRYPTOPP

#ifdef HAVE_OPENSSL_CRYPTO
#include <openssl/bio.h>
#include <openssl/evp.h>
#include <openssl/pem.h>
#endif


/************************************************************************/
/*                  CPLOpenSSLNullPassphraseCallback()                  */
/************************************************************************/

#if defined(HAVE_OPENSSL_CRYPTO)

static int CPLOpenSSLNullPassphraseCallback(char * /*buf*/,
                                            int /*size*/,
                                            int /*rwflag*/, void * /*u*/)
{
    CPLError(CE_Failure, CPLE_NotSupported,
             "A passphrase was required for this private key, "
             "but this is not supported");
    return 0;
}

#endif

/************************************************************************/
/*                         CPL_RSA_SHA256_Sign()                        */
/************************************************************************/

GByte* CPL_RSA_SHA256_Sign(const char* pszPrivateKey,
                                  const void* pabyData,
                                  unsigned int nDataLen,
                                  unsigned int* pnSignatureLen)
{
    *pnSignatureLen = 0;

#ifdef HAVE_CRYPTOPP
  if( EQUAL(CPLGetConfigOption("CPL_RSA_SHA256_Sign", "CRYPTOPP"), "CRYPTOPP") )
  {
    // See https://www.cryptopp.com/wiki/RSA_Cryptography
    // https://www.cryptopp.com/wiki/RSA_Signature_Schemes#RSA_Signature_Scheme_.28PKCS_v1.5.29
    // https://www.cryptopp.com/wiki/Keys_and_Formats#PEM_Encoded_Keys

    CPLString osRSAPrivKey(pszPrivateKey);
    static std::string HEADER = "-----BEGIN PRIVATE KEY-----";
    static std::string HEADER_RSA = "-----BEGIN RSA PRIVATE KEY-----";
    static std::string HEADER_ENCRYPTED = "-----BEGIN ENCRYPTED PRIVATE KEY-----";
    static std::string FOOTER = "-----END PRIVATE KEY-----";

    size_t pos1, pos2;
    pos1 = osRSAPrivKey.find(HEADER);
    if(pos1 == std::string::npos)
    {
        if( osRSAPrivKey.find(HEADER_RSA) != std::string::npos )
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                     "'Traditional' PEM header found, whereas PKCS#8 is "
                     "expected. You can use for example "
                     "'openssl pkcs8 -topk8 -inform pem -in file.key "
                     "-outform pem -nocrypt -out file.pem' to generate "
                     "a compatible PEM file");
        }
        else if( osRSAPrivKey.find(HEADER_ENCRYPTED) != std::string::npos )
        {
            CPLError(CE_Failure, CPLE_AppDefined,
                     "Encrypted PEM header found. Only PKCS#8 unencrypted "
                     "private keys are supported");
        }
        else
        {
            CPLError(CE_Failure, CPLE_AppDefined, "PEM header not found");
        }
        return nullptr;
    }

    pos2 = osRSAPrivKey.find(FOOTER, pos1+1);
    if(pos2 == std::string::npos)
    {
        CPLError(CE_Failure, CPLE_AppDefined, "PEM footer not found");
        return nullptr;
    }

    // Strip header and footer to get the base64-only portion
    pos1 = pos1 + HEADER.size();
    std::string osKeyB64 = osRSAPrivKey.substr(pos1, pos2 - pos1);

    // Base64 decode, place in a ByteQueue
    CryptoPP::ByteQueue queue;
    CryptoPP::Base64Decoder decoder;

    decoder.Attach(new CryptoPP::Redirector(queue));
    decoder.Put(reinterpret_cast<const cryptopp_byte*>(osKeyB64.data()), osKeyB64.length());
    decoder.MessageEnd();

    CryptoPP::RSA::PrivateKey rsaPrivate;
    try
    {
        rsaPrivate.BERDecode(queue);
    }
    catch( const std::exception& e )
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Exception while decoding private key: %s", e.what());
        return nullptr;
    }

    // Check that we have consumed all bytes.
    if( !queue.IsEmpty() )
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Invalid private key: extraneous trailing bytes");
        return nullptr;
    }

    CryptoPP::AutoSeededRandomPool prng;
    bool valid = rsaPrivate.Validate(prng, 3);
    if(!valid)
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Invalid private key: validation failed");
        return nullptr;
    }

    std::string signature;
    try
    {
        typedef CryptoPP::RSASS<CryptoPP::PKCS1v15,
                                CryptoPP::SHA256>::Signer
                                            RSASSA_PKCS1v15_SHA256_Signer;
        RSASSA_PKCS1v15_SHA256_Signer signer(rsaPrivate);

        std::string message;
        message.assign(static_cast<const char*>(pabyData), nDataLen);

        CryptoPP::StringSource stringSource(message, true,
            new CryptoPP::SignerFilter(prng, signer,
                new CryptoPP::StringSink(signature)));
    }
    catch( const std::exception& e )
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "Exception while signing: %s", e.what());
        return nullptr;
    }

    *pnSignatureLen = static_cast<unsigned int>(signature.size());
    GByte* pabySignature = static_cast<GByte*>(CPLMalloc(signature.size()));
    memcpy(pabySignature, signature.c_str(), signature.size());
    return pabySignature;
  }
#endif

#if defined(HAVE_OPENSSL_CRYPTO)
  if( EQUAL(CPLGetConfigOption("CPL_RSA_SHA256_Sign", "OPENSSL"), "OPENSSL") )
  {
    const EVP_MD* digest = EVP_sha256();
    if( digest == nullptr )
    {
        CPLError(CE_Failure, CPLE_AppDefined, "EVP_sha256() failed");
        return nullptr;
    }

    // Old versions expect a void*, newer a const void*
    BIO* bio = BIO_new_mem_buf(const_cast<void*>(static_cast<const void*>(pszPrivateKey)),
                               static_cast<int>(strlen(pszPrivateKey)));
    if( bio == nullptr )
    {
        CPLError(CE_Failure, CPLE_AppDefined, "BIO_new_mem_buf() failed");
        return nullptr;
    }
    EVP_PKEY* pkey = PEM_read_bio_PrivateKey(bio, nullptr,
                                             CPLOpenSSLNullPassphraseCallback,
                                             nullptr);
    BIO_free(bio);
    if( pkey == nullptr )
    {
        CPLError(CE_Failure, CPLE_AppDefined,
                 "PEM_read_bio_PrivateKey() failed");
        return nullptr;
    }
    EVP_MD_CTX* md_ctx = EVP_MD_CTX_create();
    CPLAssert(md_ctx != nullptr);
    int ret = EVP_SignInit(md_ctx, digest);
    CPLAssert(ret == 1);
    ret = EVP_SignUpdate(md_ctx, pabyData, nDataLen);
    CPLAssert(ret == 1);
    const int nPKeyLength = EVP_PKEY_size(pkey);
    CPLAssert(nPKeyLength > 0);
    GByte* abyBuffer = static_cast<GByte*>(CPLMalloc(nPKeyLength));
    ret = EVP_SignFinal(md_ctx, abyBuffer, pnSignatureLen, pkey);
    if( ret != 1 )
    {
        CPLError(CE_Failure, CPLE_AppDefined, "EVP_SignFinal() failed");
        EVP_MD_CTX_destroy(md_ctx);
        EVP_PKEY_free(pkey);
        CPLFree(abyBuffer);
        return nullptr;
    }

    EVP_MD_CTX_destroy(md_ctx);
    EVP_PKEY_free(pkey);
    return abyBuffer;
  }
#endif

    CPL_IGNORE_RET_VAL(pszPrivateKey);
    CPL_IGNORE_RET_VAL(pabyData);
    CPL_IGNORE_RET_VAL(nDataLen);

    CPLError(CE_Failure, CPLE_NotSupported,
             "CPLRSASHA256Sign() not implemented: "
             "GDAL must be built against libcrypto++ or libcrypto (openssl)");
    return nullptr;
}
