base64加解密

平常工作中偶尔会用到base64进行简单的数据加解密，或者将结构体转换成可打印的字符串进行传递。

原理

1、将三个字节转换成四个字节：三个字节24位，分成四组，再在每个6位前面添加两个0，所得的字节值在0-63之间。而0-63之前有许多不可打印的字符，则进行如下转换

0～25: A~Z

26~51: a~z

52~61: 0~9

62: +

63: -

如代码中的base64_table数组所示。

2、如不足三个字节的

只有2个字节，即16位。进行分组后，（6位、6位、4位、0位），第三个分组除了前面要添加两个0，后面还要添加两个0。前面三个字节仍旧按数组对应进行转换，最后一个指节用‘＝’字符进行替换。只有一个字节的时候也做相似处理。

以下代码直接来至wpa_supplicant\src\utils\base64.c

static const unsigned char base64_table[65] =
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

/**
* base64_encode - Base64 encode
* @src: Data to be encoded
* @len: Length of the data to be encoded
* @out_len: Pointer to output length variable, or %NULL if not used
* Returns: Allocated buffer of out_len bytes of encoded data,
* or %NULL on failure
*
* Caller is responsible for freeing the returned buffer. Returned buffer is
* nul terminated to make it easier to use as a C string. The nul terminator is
* not included in out_len.
*/
unsigned char * base64_encode(const unsigned char *src, size_t len,
size_t *out_len)
{
unsigned char *out, *pos;
const unsigned char *end, *in;
size_t olen;
int line_len;

olen = len * 4 / 3 + 4; /* 3-byte blocks to 4-byte */
olen += olen / 72; /* line feeds */
olen++; /* nul termination */
if (olen < len)
return NULL; /* integer overflow */
out = os_malloc(olen);
if (out == NULL)
return NULL;

end = src + len;
in = src;
pos = out;
line_len = 0;
while (end - in >= 3) {
*pos++ = base64_table[in[0] >> 2];
*pos++ = base64_table[((in[0] & 0x03) << 4) | (in[1] >> 4)];
*pos++ = base64_table[((in[1] & 0x0f) << 2) | (in[2] >> 6)];
*pos++ = base64_table[in[2] & 0x3f];
in += 3;
line_len += 4;
if (line_len >= 72) {
*pos++ = '\n';
line_len = 0;
}
}

if (end - in) {
*pos++ = base64_table[in[0] >> 2];
if (end - in == 1) {
*pos++ = base64_table[(in[0] & 0x03) << 4];
*pos++ = '=';
} else {
*pos++ = base64_table[((in[0] & 0x03) << 4) |
(in[1] >> 4)];
*pos++ = base64_table[(in[1] & 0x0f) << 2];
}
*pos++ = '=';
line_len += 4;
}

if (line_len)
*pos++ = '\n';

*pos = '\0';
if (out_len)
*out_len = pos - out;
return out;
}

/**
* base64_decode - Base64 decode
* @src: Data to be decoded
* @len: Length of the data to be decoded
* @out_len: Pointer to output length variable
* Returns: Allocated buffer of out_len bytes of decoded data,
* or %NULL on failure
*
* Caller is responsible for freeing the returned buffer.
*/
unsigned char * base64_decode(const unsigned char *src, size_t len,
size_t *out_len)
{
unsigned char dtable[256], *out, *pos, in[4], block[4], tmp;
size_t i, count, olen;
memset(in, 0, sizeof(in));
memset(block, 0, sizeof(block));

os_memset(dtable, 0x80, 256);
for (i = 0; i < sizeof(base64_table) - 1; i++)
dtable[base64_table[i]] = (unsigned char) i;
dtable['='] = 0;

count = 0;
for (i = 0; i < len; i++) {
if (dtable[src[i]] != 0x80)
count++;
}

if (count == 0 || count % 4)
return NULL;

olen = count / 4 * 3;
pos = out = os_malloc(olen);
if (out == NULL)
return NULL;

count = 0;
for (i = 0; i < len; i++) {
tmp = dtable[src[i]];
if (tmp == 0x80)
continue;

in[count] = src[i];
block[count] = tmp;
count++;
if (count == 4) {
*pos++ = (block[0] << 2) | (block[1] >> 4);
*pos++ = (block[1] << 4) | (block[2] >> 2);
*pos++ = (block[2] << 6) | block[3];
count = 0;
}
}

if (pos > out) {
if (in[2] == '=')
pos -= 2;
else if (in[3] == '=')
pos--;
}

*out_len = pos - out;
return out;
}

参考链接
http://www.ruanyifeng.com/blog/2008/06/base64.html