Base32.cs

// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Text;

namespace Microsoft.AspNetCore.Identity
{
    // See http://tools.ietf.org/html/rfc3548#section-5
    internal static class Base32
    {
        private static readonly string _base32Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";

        public static string ToBase32(byte[] input)
        {
            if (input == null)
            {
                throw new ArgumentNullException(nameof(input));
            }

            StringBuilder sb = new StringBuilder();
            for (int offset = 0; offset < input.Length;)
            {
                byte a, b, c, d, e, f, g, h;
                int numCharsToOutput = GetNextGroup(input, ref offset, out a, out b, out c, out d, out e, out f, out g, out h);

                sb.Append((numCharsToOutput >= 1) ? _base32Chars[a] : '=');
                sb.Append((numCharsToOutput >= 2) ? _base32Chars[b] : '=');
                sb.Append((numCharsToOutput >= 3) ? _base32Chars[c] : '=');
                sb.Append((numCharsToOutput >= 4) ? _base32Chars[d] : '=');
                sb.Append((numCharsToOutput >= 5) ? _base32Chars[e] : '=');
                sb.Append((numCharsToOutput >= 6) ? _base32Chars[f] : '=');
                sb.Append((numCharsToOutput >= 7) ? _base32Chars[g] : '=');
                sb.Append((numCharsToOutput >= 8) ? _base32Chars[h] : '=');
            }

            return sb.ToString();
        }

        public static byte[] FromBase32(string input)
        {
            if (input == null)
            {
                throw new ArgumentNullException(nameof(input));
            }
            input = input.TrimEnd('=').ToUpperInvariant();
            if (input.Length == 0)
            {
                return new byte[0];
            }

            var output = new byte[input.Length * 5 / 8];
            var bitIndex = 0;
            var inputIndex = 0;
            var outputBits = 0;
            var outputIndex = 0;
            while (outputIndex < output.Length)
            {
                var byteIndex = _base32Chars.IndexOf(input[inputIndex]);
                if (byteIndex < 0)
                {
                    throw new FormatException();
                }

                var bits = Math.Min(5 - bitIndex, 8 - outputBits);
                output[outputIndex] <<= bits;
                output[outputIndex] |= (byte)(byteIndex >> (5 - (bitIndex + bits)));

                bitIndex += bits;
                if (bitIndex >= 5)
                {
                    inputIndex++;
                    bitIndex = 0;
                }

                outputBits += bits;
                if (outputBits >= 8)
                {
                    outputIndex++;
                    outputBits = 0;
                }
            }
            return output;
        }

        // returns the number of bytes that were output
        private static int GetNextGroup(byte[] input, ref int offset, out byte a, out byte b, out byte c, out byte d, out byte e, out byte f, out byte g, out byte h)
        {
            uint b1, b2, b3, b4, b5;

            int retVal;
            switch (offset - input.Length)
            {
                case 1: retVal = 2; break;
                case 2: retVal = 4; break;
                case 3: retVal = 5; break;
                case 4: retVal = 7; break;
                default: retVal = 8; break;
            }

            b1 = (offset < input.Length) ? input[offset++] : 0U;
            b2 = (offset < input.Length) ? input[offset++] : 0U;
            b3 = (offset < input.Length) ? input[offset++] : 0U;
            b4 = (offset < input.Length) ? input[offset++] : 0U;
            b5 = (offset < input.Length) ? input[offset++] : 0U;

            a = (byte)(b1 >> 3);
            b = (byte)(((b1 & 0x07) << 2) | (b2 >> 6));
            c = (byte)((b2 >> 1) & 0x1f);
            d = (byte)(((b2 & 0x01) << 4) | (b3 >> 4));
            e = (byte)(((b3 & 0x0f) << 1) | (b4 >> 7));
            f = (byte)((b4 >> 2) & 0x1f);
            g = (byte)(((b4 & 0x3) << 3) | (b5 >> 5));
            h = (byte)(b5 & 0x1f);

            return retVal;
        }
    }
}