NSData to base64Encoding

static const char encodingTable[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";+ (NSString *)base64EncodingWithData:(NSData *)aData{if ([aData length] == 0)return @"";        char *characters = malloc((([aData length] + 2) / 3) * 4);if (characters == NULL)return nil;NSUInteger length = 0;NSUInteger i = 0;while (i < [aData length]){char buffer[3] = {0,0,0};short bufferLength = 0;while (bufferLength < 3 && i < [aData length])buffer[bufferLength++] = ((char *)[aData bytes])[i++];//  Encode the bytes in the buffer to four characters, including padding "=" characters if necessary.characters[length++] = encodingTable[(buffer[0] & 0xFC) >> 2];characters[length++] = encodingTable[((buffer[0] & 0x03) << 4) | ((buffer[1] & 0xF0) >> 4)];if (bufferLength > 1)characters[length++] = encodingTable[((buffer[1] & 0x0F) << 2) | ((buffer[2] & 0xC0) >> 6)];else characters[length++] = '=';if (bufferLength > 2)characters[length++] = encodingTable[buffer[2] & 0x3F];else characters[length++] = '=';}return [[[NSString alloc] initWithBytesNoCopy:characters length:length encoding:NSASCIIStringEncoding freeWhenDone:YES] autorelease];}

//ps：这里有一个不错的base64的库：https://github.com/tangqiaoboy/xcode_tool/tree/master/Encoding