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C++ single header base64 decode/encoder.
#ifndef _MACARON_BASE64_H_
#define _MACARON_BASE64_H_
/**
* The MIT License (MIT)
* Copyright (c) 2016 tomykaira
*
* Permission is hereby granted, free of charge, to any person obtaining
* a copy of this software and associated documentation files (the
* "Software"), to deal in the Software without restriction, including
* without limitation the rights to use, copy, modify, merge, publish,
* distribute, sublicense, and/or sell copies of the Software, and to
* permit persons to whom the Software is furnished to do so, subject to
* the following conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
* LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
* OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
* WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
#include <string>
namespace macaron {
class Base64 {
public:
static std::string Encode(const std::string data) {
static constexpr char sEncodingTable[] = {
'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
'w', 'x', 'y', 'z', '0', '1', '2', '3',
'4', '5', '6', '7', '8', '9', '+', '/'
};
size_t in_len = data.size();
size_t out_len = 4 * ((in_len + 2) / 3);
std::string ret(out_len, '\0');
size_t i;
char *p = const_cast<char*>(ret.c_str());
for (i = 0; i < in_len - 2; i += 3) {
*p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
*p++ = sEncodingTable[((data[i] & 0x3) << 4) | ((int) (data[i + 1] & 0xF0) >> 4)];
*p++ = sEncodingTable[((data[i + 1] & 0xF) << 2) | ((int) (data[i + 2] & 0xC0) >> 6)];
*p++ = sEncodingTable[data[i + 2] & 0x3F];
}
if (i < in_len) {
*p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
if (i == (in_len - 1)) {
*p++ = sEncodingTable[((data[i] & 0x3) << 4)];
*p++ = '=';
}
else {
*p++ = sEncodingTable[((data[i] & 0x3) << 4) | ((int) (data[i + 1] & 0xF0) >> 4)];
*p++ = sEncodingTable[((data[i + 1] & 0xF) << 2)];
}
*p++ = '=';
}
return ret;
}
static std::string Decode(const std::string& input, std::string& out) {
static constexpr unsigned char kDecodingTable[] = {
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63,
52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64,
64, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 64, 64, 64, 64, 64,
64, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64
};
size_t in_len = input.size();
if (in_len % 4 != 0) return "Input data size is not a multiple of 4";
size_t out_len = in_len / 4 * 3;
if (input[in_len - 1] == '=') out_len--;
if (input[in_len - 2] == '=') out_len--;
out.resize(out_len);
for (size_t i = 0, j = 0; i < in_len;) {
uint32_t a = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
uint32_t b = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
uint32_t c = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
uint32_t d = input[i] == '=' ? 0 & i++ : kDecodingTable[static_cast<int>(input[i++])];
uint32_t triple = (a << 3 * 6) + (b << 2 * 6) + (c << 1 * 6) + (d << 0 * 6);
if (j < out_len) out[j++] = (triple >> 2 * 8) & 0xFF;
if (j < out_len) out[j++] = (triple >> 1 * 8) & 0xFF;
if (j < out_len) out[j++] = (triple >> 0 * 8) & 0xFF;
}
return "";
}
};
}
#endif /* _MACARON_BASE64_H_ */
@noloader
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noloader commented Oct 25, 2021

@Paril,

Please put an end to this once and for all. Please cite the portion of the standard that says you get a writeable pointer from data or string prior to C++17.

@Paril
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Paril commented Oct 25, 2021

@noloader Sure. See section 21.4.7 of the C++11 standard (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3690.pdf) - it states that the return value of data() and c_str() are equivalent to &operator[](i), which is the exact same access that you're saying you have to do to get a "non-const pointer". Thus, you are technically allowed to write to this pointer since they are equivalent. There is a guarantee that they are the same exact thing. Just because the return value of data() or c_str() are const doesn't mean you can't reinterpret-cast them.

This wasn't the case in the C++98 standard, which was a bit vague on exactly what pointer c_str() and data() would return. Both say you can't alter the return value, but, if it's guaranteed to be the same as &[](i) then it's a moot clause now.

EDIT: I'll concede that it's not legal to modify the pointer given to you by data() and c_str() in C++11, but given that c_str() + i is guaranteed to be equivalent to &::operator[](i), I can safely say that it really doesn't matter. If you wanted portable code even for C++98 then yes, you're absolutely right. My main point was just that it's not UB, it's well-defined that you'll be modifying the same data since C++11.

@noloader
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noloader commented Oct 25, 2021

@Paril,

And from the document you just cited under 21.4.7.1 basic_string accessors for data and c_str (p. 659):

Requires: The program shall not alter any of the values stored in the character array

@matheusgomes28
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matheusgomes28 commented Nov 28, 2021

Just a note to people who are thinking of copying and pasting this into their projects: don't! There are several issues with the example here, and I've unfortunately seen people copying this into their productions systems before.

If you are looking for a safer modern Cpp Base64 alternative, I've made a post on how to write a good C++ Base64 encoder. At the very least you will learn how base64 encoding works.

@skullchap
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skullchap commented Dec 23, 2021

@ravstrix
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ravstrix commented Apr 28, 2022

static std::string Encode(const std::string data)

Did you forget a '&'?

@theicfire
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theicfire commented Jun 20, 2022

+1 @ravstrix , otherwise this seems like a copy

@zdj1414
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zdj1414 commented Aug 12, 2022

when the input data length is 1, it will be crashed.

static std::string Encode(const std::string data) {
    static constexpr char sEncodingTable[] = {
      'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
      'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
      'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
      'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
      'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
      'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
      'w', 'x', 'y', 'z', '0', '1', '2', '3',
      '4', '5', '6', '7', '8', '9', '+', '/'
    };

    size_t in_len = data.size();
    size_t out_len = 4 * ((in_len + 2) / 3);
    std::string ret(out_len, '\0');
    size_t i = 0;
    char *p = const_cast<char*>(ret.c_str());

    if (in_len >= 2) {
      for (i = 0; i < in_len - 2; i += 3) {
        *p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
        *p++ = sEncodingTable[((data[i] & 0x3) << 4) | ((int) (data[i + 1] & 0xF0) >> 4)];
        *p++ = sEncodingTable[((data[i + 1] & 0xF) << 2) | ((int) (data[i + 2] & 0xC0) >> 6)];
        *p++ = sEncodingTable[data[i + 2] & 0x3F];
      }
    }

    if (i < in_len) {
      *p++ = sEncodingTable[(data[i] >> 2) & 0x3F];
      if (i == (in_len - 1)) {
        *p++ = sEncodingTable[((data[i] & 0x3) << 4)];
        *p++ = '=';
      }
      else {
        *p++ = sEncodingTable[((data[i] & 0x3) << 4) | ((int) (data[i + 1] & 0xF0) >> 4)];
        *p++ = sEncodingTable[((data[i + 1] & 0xF) << 2)];
      }
      *p++ = '=';
    }

    return ret;
  }

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