C# Converting base64 string to 16-bit words stored in little-endian byte order

Question

I am trying to upload a base64 of a signature but I need it to be a base64 encoding of an array of 16-bit words stored in little-endian byte order. Can anyone help me convert the base64 to 16-bit array in little-endian byte and then convert it again to base64?

Answer 1

To do this you can create arrays of the correct type (byte[] and short[]) and use Buffer.BlockCopy() to copy the bytes between them, thus converting the data.

This does not account for little-endian/big-endian differences, but since you state that this only needs to run on little-endian systems, we don't need to worry about it.

Here's a sample console app that demonstrates how to do the conversion. It does the following:

1. Create an array of shorts 0..99 inclusive.
2. Convert array of shorts to array of bytes (preserving endianness).
3. Convert array of bytes to base 64 string.
4. Convert base 64 string back into array of bytes.
5. Convert array of bytes back into array of shorts (preserving endianness).
6. Compare converted array of shorts with original array to prove correctness.

Here's the code:

using System;
using System.Linq;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create demo array of shorts 0..99 inclusive.

            short[] sourceShorts = Enumerable.Range(0, 100).Select(i => (short)i).ToArray();

            // Convert array of shorts to array of bytes. (Will be little-endian on Intel.)

            int byteCount = sizeof(short) * sourceShorts.Length;
            byte[] dataAsByteArray = new byte[byteCount];
            Buffer.BlockCopy(sourceShorts, 0, dataAsByteArray, 0, byteCount);

            // Convert array of bytes to base 64 string.

            var asBase64 = Convert.ToBase64String(dataAsByteArray);
            Console.WriteLine(asBase64);

            // Convert base 64 string back to array of bytes.

            byte[] fromBase64 = Convert.FromBase64String(asBase64);

            // Convert array of bytes back to array of shorts.

            if (fromBase64.Length % sizeof(short) != 0)
                throw new InvalidOperationException("Byte array size must be multiple of sizeof(short) to be convertable to shorts");

            short[] destShorts = new short[fromBase64.Length/sizeof(short)];
            Buffer.BlockCopy(fromBase64, 0, destShorts, 0, fromBase64.Length);

            // Prove that the unconverted shorts match the source shorts.

            if (destShorts.SequenceEqual(sourceShorts))
                Console.WriteLine("Converted and unconverted successfully");
            else
                Console.WriteLine("Error: conversion was unsuccessful");
        }
    }
}