var
  BIOPrivateKey: PBIO;
  PrivateKey: PEVP_PKEY;
  Ctx: PEVP_MD_CTX;
  SHA: PEVP_MD;
  Size: Cardinal;
begin
	BIOPrivateKey := Libeay32.BIO_new_mem_buf(@AKey[0], Length(AKey));
  PrivateKey := Libeay32.PEM_read_bio_PrivateKey(BIOPrivateKey, nil, nil, nil);
  Ctx := Libeay32.EVP_MD_CTX_create();
  try
    case Algorithm of
      RS256: SHA := Libeay32.EVP_sha256();
      RS384: SHA := Libeay32.EVP_sha384();
      RS512: SHA := Libeay32.EVP_sha512();
    else
      SHA := nil;
    end;
    if (Libeay32.EVP_DigestSignInit(Ctx, nil, SHA, nil, PrivateKey) > 0) and
      (Libeay32.EVP_DigestUpdate(Ctx, @ABytesToSign[0], Length(ABytesToSign)) > 0) and
      (Libeay32.EVP_DigestSignFinal(Ctx, nil, Size) > 0) then
    begin
      SetLength(Result, Size);
      Libeay32.EVP_DigestSignFinal(Ctx, @Result[0], Size);
    end else
      SetLength(Result, 0);
  finally
    Libeay32.EVP_MD_CTX_destroy(Ctx);
  end;

  key := TNetEncoding.Base64.DecodeStringToBytes(csSecretKey);

  if not CryptAcquireContext(@rsa, nil, nil, PROV_RSA_FULL, 0) then
    Exit;
  try
    if not CryptImportKey(rsa, @Key[0], Length(Key), 0, 0, @PublicKey) then
    begin
      ShowMessage(SysErrorMessage(GetLastError)); // ->  плохая версия постовщика
      Exit;
    end;

"-----BEGIN CERTIFICATE-----\nMIIDJjCCAg6gAwIBAgIITBMpwgJ5WykwDQYJKoZIhvcNAQEFBQAwNjE0MDIGA1UE
...
/NicQ3cLG9FvD24KbqhQGRblr+2CZQM4xPO\n-----END CERTIFICATE-----\n"