function StreamToMD5(x: TStream): ArDigest;
const
  bufsize = 65536;
var
  context: MD5_CTX;
  buffer: array[1..bufsize] of byte;
  length: integer;
begin
  MD5Init(context);
  x.Position := 0;
  repeat
    length := x.Read(buffer, bufsize);
    MD5Update(context, @buffer[1], length);
  until length <= 0;
  MD5Final(context);
  result := context.digest;
end;

function MD5_File(infile_name: WideString): ArDigest;
const
  bufsize = 65536;
var
  infile: THandle;
  inbuffer: pointer;
  amount_read: DWORD;
  cbHashDataLen: DWORD;
  Digest: ArDigest;
  hProv: TCryptProv;
  hHash: TCryptHash;
begin
  infile := CreateFileW(PWideChar(infile_name), GENERIC_READ, FILE_SHARE_READ, nil,
         OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL or FILE_FLAG_SEQUENTIAL_SCAN, 0);

  if infile = INVALID_HANDLE_VALUE then
  begin
    ShowMessage('1: ' + SysErrorMessage(GetLastError));
    exit;
  end;

  try
    GetMem(inbuffer, bufsize);
    hHash := nil;
    hProv := 0;
    if not CryptAcquireContext(hProv, nil, nil, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT) then ShowMessage('1: ' + SysErrorMessage(GetLastError));
    if not CryptCreateHash(hProv, CALG_MD5, nil, 0, hHash) then ShowMessage('2: ' + SysErrorMessage(GetLastError));
    try
      ReadFile(infile, inbuffer^, bufsize, amount_read, nil);
      repeat
        if not CryptHashData(hHash, inbuffer, amount_read, 0) then ShowMessage('3: ' + SysErrorMessage(GetLastError));
        ReadFile(infile, inbuffer^, bufsize, amount_read, nil);
      until amount_read = 0;
      cbHashDataLen := 16;
      if not CryptGetHashParam(hHash, HP_HASHVAL, @Digest[0], cbHashDataLen, 0) then ShowMessage('4: ' + SysErrorMessage(GetLastError));
    finally
      CryptDestroyHash(hhash);
      CryptReleaseContext(hprov, 0);
    end;
    result := Digest;
  finally
    CloseHandle(infile);
    FreeMem(inbuffer);
  end;
end;