iOS MD5加密

为NSString做一个分类，返回一个MD5过后的字符串

@interface NSString ((gzp_DownLoader))

- (NSString *)md5Str;

@end

@implementation NSString (gzp_DownLoader)

- (NSString *)md5Str {
    
    const char *data = self.UTF8String;
    
    unsigned char digest[CC_MD5_DIGEST_LENGTH];
    
    CC_MD5(data, (CC_LONG)strlen(data), digest);
    
    NSMutableString *result = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
    
    for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i ++) {
        [result appendFormat:@"%02x", digest[i]];
    }
    
    return result;
    
}