public async Task PerformDecodeAsync(string sourcepath, string targetPath)
{
    using (var streamWriter = File.AppendText(targetPath))
    {
        foreach(var tempLine in File.ReadLines(sourcePath))
        {
            short decodeCounter = 0;
            StringBuilder line = new StringBuilder();
            foreach (var singleChar in tempLine)
            {
                var positionInDecodeKey = decodingKeysList[decodeCounter].IndexOf(singleChar);

                if (positionInDecodeKey > 0)
                    line.Append(model.Substring(positionInDecodeKey, 1));
                else
                    line.Append(singleChar);

                if (decodeCounter > 18)
                    decodeCounter = 0;
                else ++decodeCounter;
            }

            await streamWriter.WriteLineAsync(line);
        }
    }
}