Base64編碼解碼C語言實現
阿新 • • 發佈：2019-02-05
static const char *codes = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"; static const unsigned char map[256] = { 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 253, 255, 255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 253, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 62, 255, 255, 255, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 255, 255, 255, 254, 255, 255, 255, 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, 255, 255, 255, 255, 255, 255, 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, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255 }; int base64_encode(const unsigned char *in, unsigned long len, unsigned char *out) { unsigned long i, len2, leven; unsigned char *p; /* valid output size ? */ len2 = 4 * ((len + 2) / 3); p = out; leven = 3*(len / 3); for (i = 0; i < leven; i += 3) { *p++ = codes[in[0] >> 2]; *p++ = codes[((in[0] & 3) << 4) + (in[1] >> 4)]; *p++ = codes[((in[1] & 0xf) << 2) + (in[2] >> 6)]; *p++ = codes[in[2] & 0x3f]; in += 3; } /* Pad it if necessary... */ if (i < len) { unsigned a = in[0]; unsigned b = (i+1 < len) ? in[1] : 0; unsigned c = 0; *p++ = codes[a >> 2]; *p++ = codes[((a & 3) << 4) + (b >> 4)]; *p++ = (i+1 < len) ? codes[((b & 0xf) << 2) + (c >> 6)] : '='; *p++ = '='; } /* append a NULL byte */ *p = '\0'; return p - out; } int base64_decode(const unsigned char *in, unsigned char *out) { unsigned long t, x, y, z; unsigned char c; int g = 3; for (x = y = z = t = 0; in[x]!=0;) { c = map[in[x++]]; if (c == 255) return -1; if (c == 253) continue; if (c == 254) { c = 0; g--; } t = (t<<6)|c; if (++y == 4) { // if (z + g > *outlen) { return CRYPT_BUFFER_OVERFLOW; } out[z++] = (unsigned char)((t>>16)&255); if (g > 1) out[z++] = (unsigned char)((t>>8)&255); if (g > 2) out[z++] = (unsigned char)(t&255); y = t = 0; } } // if (y != 0) { // return -1; // } return z; }
Base64編碼基本上理解為：將二進位制資料轉變為可見的ASCII碼字串（反之亦然）。這個很有用，比如向資料庫裡面寫東西，如果是二進位制（如圖片）的話，轉變成字串就會很方便的存入。
該編碼的好處是：速度快，編碼為ASCII碼後體積增加三分之一左右。