Objective-C Implicit conversion loses integer prec

问题:

I have a function that's generating a sha256 encryption of a string,

Here's the function:

    -(NSString*)sha256HashFor:(NSString*)input
{
    const char* str = [input UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(str, strlen(str), result);

    NSMutableString *ret = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH*2];
    for(int i = 0; i<CC_SHA256_DIGEST_LENGTH; i++)
    {
        [ret appendFormat:@"%02x",result[i]];
    }
    return ret;
}

Now this line right here CC_SHA256(str, strlen(str), result); is what's producing this warning (the warning is for the strlen(str) variable).

Implicit conversion loses integer precision: 'size_t' (aka 'unsigned long') to 'CC_LONG' (aka 'unsigned int')

I'm guessing I just need to convert the strlen(str) to a CC_Long, but I have no idea how to do that.

回答1:

1. Presumably that's not an error but a warning.

2. "I just need to convert the strlen(str) to a CC_Long, but I have no idea how to do that." - explicit type conversion (type casting): (CC_LONG)strlen(str), but I don't think you really need this.

回答2:

This code will not show any warning and works perfectly.

- (NSString*) sha256 {
    const char * pointer = [self UTF8String];
    unsigned char result[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(pointer, (CC_LONG)strlen(pointer), result);

    NSMutableString *ret = [NSMutableString stringWithCapacity:CC_SHA256_DIGEST_LENGTH*2];
    for(int i = 0; i<CC_SHA256_DIGEST_LENGTH; i++)
    {
        [ret appendFormat:@"%02x",result[i]];
    }
    return ret;
}

回答3:

Speaking from the iOS perspective, the fact that CC_LONG (a.k.a. uint32_t) and size_t (unsigned long) are incompatible sizes can be a security/stability concern in some applications, especially when dealing with reusable library functions.

An MD5 hash is a 128 bit hash of a potentially unlimited length message, so there is a good reason for this warning. If you truncate a length greater than 2^32, you will come up with an incorrect hash.

Your code should logically decide on how large of a string it can support. In this case using CC_MD5, it would have to be 2^32 bytes.

Maximum length for MD5 input/output