problem with MD5 in simulator and device(ipad)

i used a MD5 algorithm to create unique string/name for a same image..so i tested with simulator vs ipad.

i downloaded one image in Simulator using safari and created MD5 string for that.it working fine in simulator.in simulator its always creating a same string for same image.

then i downloaded the same image in to ipad from the same site/url through safari. and created a MD5 for this. there is a pr开发者_如何转开发oblem.the two MD5 are different.can any one tell me the exact problem.

is simulator and ipad saving same images in a different size when it downloading.

i used the code:

MyExtensions.h

@interface NSData (MyExtensions)
    - (NSString*)md5;
@end

MyExtensions.m

#import "MyExtensions.h"
#import <CommonCrypto/CommonDigest.h> // Need to import for CC_MD5 access

@implementation NSData (MyExtensions)
- (NSString*)md5
{
    unsigned char result[16];
    CC_MD5( self.bytes, self.length, result ); // This is the md5 call
    return [NSString stringWithFormat:
        @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
        result[0], result[1], result[2], result[3], 
        result[4], result[5], result[6], result[7],
        result[8], result[9], result[10], result[11],
        result[12], result[13], result[14], result[15]
        ];  
}
@end


-(NSString *) md5Image:(UIImage *)img {
    return [UIImagePNGRepresentation(img) md5];
}

Check the answer to this post for a possible explanation.

As you can see, the CC_MD5 implementation can contain some data type which takes different number of bytes on 32-bit or 64-bit OS. And this could explain the difference you see in hashing on the simulator (Mac OSX) or the device (iOS).