NSData与NSString的转换
NSString* aStr = [[NSString alloc] initWithData:aData encoding:NSASCIIStringEncoding];
NSData* aData = [aStr dataUsingEncoding: NSASCIIStringEncoding];
NSString* aStr = [[NSString alloc] initWithData:aData encoding:NSASCIIStringEncoding];
NSData* aData = [aStr dataUsingEncoding: NSASCIIStringEncoding];