使用 C# 实现 URL 安全的 Base62 转码

Base62 编码与 Base64 编码类似，都用于数据内容编码。Base64 和 Base62 的目的在都在于将需要传输的内容进行编码，尤其是一些特殊字符（如不可见字符、传输时与协议头冲突的字符）。

与 Base64 不同的是，Base62 编码是由10个数字、26个大写英文字母和26个小写英文字母组成，不包含 Base64 中的符号：+，/和=。这让经过 Base62 编码的数据很适合在 URL 中进行传输。

微信的扫码登录规定了 state 只能使用 a-zA-Z0-9 的参数值，这种场景下使用 Base62 就很有效。

public static class Base62
{
	private const string DefaultCharacterSet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
	private const string InvertedCharacterSet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

	/// <summary>
	/// Encode a byte array with Base62
	/// </summary>
	/// <param name="original">Byte array</param>
	/// <param name="inverted">Use inverted character set</param>
	/// <returns>Base62 string</returns>
	public static string ToBase62(byte[] original, bool inverted = false)
	{
		var characterSet = inverted ? InvertedCharacterSet : DefaultCharacterSet;
		var arr = Array.ConvertAll(original, t => (int)t);

		var converted = BaseConvert(arr, 256, 62);
		var builder = new StringBuilder();
		foreach (var t in converted)
		{
			builder.Append(characterSet[t]);
		}
		return builder.ToString();
	}

	/// <summary>
	/// Decode a base62-encoded string
	/// </summary>
	/// <param name="base62">Base62 string</param>
	/// <param name="inverted">Use inverted character set</param>
	/// <returns>Byte array</returns>
	public static byte[] FromBase62(string base62, bool inverted = false)
	{
		if (string.IsNullOrWhiteSpace(base62))
		{
			throw new ArgumentNullException(nameof(base62));
		}

		var characterSet = inverted ? InvertedCharacterSet : DefaultCharacterSet;
		var arr = Array.ConvertAll(base62.ToCharArray(), characterSet.IndexOf);

		var converted = BaseConvert(arr, 62, 256);
		return Array.ConvertAll(converted, Convert.ToByte);
	}

	private static int[] BaseConvert(int[] source, int sourceBase, int targetBase)
	{
		var result = new List<int>();
		var leadingZeroCount = Math.Min(source.TakeWhile(x => x == 0).Count(), source.Length - 1);
		int count;
		while ((count = source.Length) > 0)
		{
			var quotient = new List<int>();
			var remainder = 0;
			for (var i = 0; i != count; i++)
			{
				var accumulator = source[i] + remainder * sourceBase;
				var digit = accumulator / targetBase;
				remainder = accumulator % targetBase;
				if (quotient.Count > 0 || digit > 0)
				{
					quotient.Add(digit);
				}
			}

			result.Insert(0, remainder);
			source = quotient.ToArray();
		}
		result.InsertRange(0, Enumerable.Repeat(0, leadingZeroCount));
		return result.ToArray();
	}
}

将字节数组转换为 Base62 字符串

var guid = Guid.Parse("40943a11e5434d2f869d4e566e48c4f2");
Console.WriteLine(Base62.ToBase62(guid.ToByteArray()));

输出：

WVdhFoDbSqBBYSKzptnko

将 Base62 字符串转换为字节数组

var str = "WVdhFoDbSqBBYSKzptnko";
var bytes = Base62.FromBase62(str);
var guid = new Guid(bytes);
Console.WriteLine(guid.ToString("N"));

输出：

40943a11e5434d2f869d4e566e48c4f2