C# 转换为C++；C的方法# 我有一个解密Base64的方法，我需要把它转换成C，我尝试使用C的转换。从FasBase64字符串和我这里看到的一些其他技巧，但是最后的字符串与C++的结果不匹配。p> 有人能帮助理解C++代码的和平吗？

输入示例：

lWXtYpNpwBBXoLmcuktDqg==

预期输出：

AC0105E92ED496ACD373D7496B76C1C8

使用Convert.FromBase64进行输出：

95-65-ED-62-93-69-C0-10-57-A0-B9-9C-BA-4B-43-AA

static BYTE BackFrom64[]={
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0x3e,0xff,0xff,0xff,0x3f,
0x34,0x35,0x36,0x37,0x38,0x39,0x3a,0x3b,
0x3c,0x3d,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0x00,0x01,0x02,0x03,0x04,0x05,0x06,
0x07,0x08,0x09,0x0a,0x0b,0x0c,0x0d,0x0e,
0x0f,0x10,0x11,0x12,0x13,0x14,0x15,0x16,
0x17,0x18,0x19,0xff,0xff,0xff,0xff,0xff,
0xff,0x1a,0x1b,0x1c,0x1d,0x1e,0x1f,0x20,
0x21,0x22,0x23,0x24,0x25,0x26,0x27,0x28,
0x29,0x2a,0x2b,0x2c,0x2d,0x2e,0x2f,0x30,
0x31,0x32,0x33,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff,
0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff

})

static BYTE*TabBase64=（BYTE*）“abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzo123456789+/”；
int FromBase64（字节*输出、字节*输入、int BufSize、int MaxSize）{
int-xut，txut；
inti，t，k，shift；
字节c；
德沃德博士，dr1；
xut=txut=t=0；
dr=0L；
对于（i=0；i=MaxSize）{
返回（-1）；
}
}
dr=0；
}
}
如果（t>0）{
对于（k=0；k<（t-1）；++k）{
dr1=dr；
dr1>>=（32-（8*（k+1））；
dr1&=0xff；
*输出+++=（字节）dr1；
如果（++txut>=MaxSize）{
返回（-1）；
}
}
}
返回（txut）；
}

接下来是一个非常简单的转换，只需很少的工作

• 类型已更改为C类型
• 指针增量已更改为数组中的索引

唯一的其他更改是将未应用的

BackFrom64

数组计算为

TabBase64

数组的逆数组，并为提供的输入和输出添加单元测试。如果它做了标准算法以外的事情，那么我会整理计数器，其中一些是冗余的（可能

I

和

iInput

都可以用

foreach（输入中的var c）

生成输出

95 65 ed 62 93 69 c0 10 57 a0 b9 9c ba 4b 43 aa

---

这又说明它与标准算法相同。

如果用简单的索引数组访问替换指针算法，那么代码应该按原样编译。如果……C++实现输出不同，那么我会认真考虑它是BGGY（即使不检查代码，我可能错了）。.BTW您不是解密而是解码。什么是

BackFrom64

？为什么不使用标准库方法：Convert.FromBase64String Convert。ToBase64String@jdweng.因为它产生不同的输出。OP说：“最后的字符串与c++上的结果不匹配。”我没有从代码中得到预期的输出，我得到的是与C#相同的结果：这是假设Backrom64是以明显的方式从TabBase64构建的。如果不是，即字符没有以标准方式映射，那么您必须向我们显示Backrom64。

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace SO
{
    [TestClass]
    public class UnitTest1
    {
        private const string TabBase64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
        private static readonly byte[] BackFrom64 = CreateBackFrom64();

        private static byte[] CreateBackFrom64()
        {
            var backFrom64 = new byte[128];

            for (var i = 0; i < backFrom64.Length; ++i)
                backFrom64[i] = 0xff;

            for (byte i = 0; i < TabBase64.Length; i++)
            {
                var c = TabBase64[i];
                backFrom64[c] = i;
            }

            return backFrom64;
        }

        int FromBase64(byte[] Output, string Input, int BufSize, int MaxSize)
        {
            int xut, txut;
            int i, t, k, shift;
            byte c;
            int dr, dr1;

            xut = txut = t = 0;
            dr = 0;

            var iInput = 0;
            var iOutput = 0;

            for (i = 0; i < BufSize; ++i)
            {
                c = (byte)Input[iInput++];
                if (c == '=')
                {
                    break;
                }
                if ((c = BackFrom64[c]) == 0xff)
                {
                    continue;
                }
                dr1 = c;
                shift = (32 - (6 * (t + 1)));
                dr1 <<= shift;
                dr |= dr1;
                t++;
                if (t == 4)
                {
                    t = 0;
                    for (k = 0; k < 3; ++k)
                    {
                        dr1 = dr;
                        dr1 >>= (32 - (8 * (k + 1)));
                        dr1 &= 0xff;
                        Output[iOutput++] = (byte)dr1;
                        if (++txut >= MaxSize)
                        {
                            return (-1);
                        }
                    }
                    dr = 0;
                }
            }
            if (t > 0)
            {
                for (k = 0; k < (t - 1); ++k)
                {
                    dr1 = dr;
                    dr1 >>= (32 - (8 * (k + 1)));
                    dr1 &= 0xff;
                    Output[iOutput++] = (byte)dr1;
                    if (++txut >= MaxSize)
                    {
                        return (-1);
                    }
                }
            }
            return (txut);
        }
        [TestMethod]
        public void TestMethod1()
        {
            var input = @"lWXtYpNpwBBXoLmcuktDqg==";
            var expectedOutput = "AC0105E92ED496ACD373D7496B76C1C8";

            var converted = new byte[(input.Length * 2) / 3];

            var result = FromBase64(converted, input, input.Length, int.MaxValue);

            Assert.AreNotEqual(-1, result, "result is valid");

            Assert.AreEqual(
                BitConverter.ToString(converted).Replace("-",""),
                BitConverter.ToString(Convert.FromBase64String(input)).Replace("-",""),
                "Supplied algorithm matches standard algorithm" );

            Assert.AreEqual(
                expectedOutput,
                BitConverter.ToString(converted).Replace("-",""),
                "Supplied algorithm matches expected output" );
        }
    }
}

int main()
{
    const char* input = "lWXtYpNpwBBXoLmcuktDqg==";

    BYTE Input[100];
    BYTE Output[100];

    int length = strlen(input);

    memcpy(Input, input, length);

    int result = FromBase64(Output, Input, 100, length);

    for (int i = 0; i < result; ++i)
        std::cout << std::hex << (int)Output[i] << " ";

    std::cout << "\n";
}

95 65 ed 62 93 69 c0 10 57 a0 b9 9c ba 4b 43 aa