How do I create a hash of a file on iOS?

Question

I'm trying to create unique file names by renaming them using their hashed value in iOS. How can I do that?

Answer 1

you could achieve this by extending NSString, Try this in your .h:

@interface NSString(MD5)

- (NSString *)generateMD5Hash

@end

and this in your .m

- (NSString*)generateMD5Hash
{
  const char *string = [self UTF8String];
  unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];
  CC_MD5(string, strlen(string), md5Buffer);
  NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
  for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) 
    [output appendFormat:@"%02x",md5Buffer[i]];
  return output;
}

you can implement this by making a new class called NSString+MD5, and inserting the code above in the corresponding files (.h and .m)

EDIT: Do not forget to import

< CommonCrypto/CommonDigest.h >

EDIT 2:

And for NSData;

@interface NSData(MD5)

- (NSString *)generateMD5Hash;

@end

your .m:

- (NSString *)generateMD5Hash
{
  unsigned char md5Buffer[CC_MD5_DIGEST_LENGTH];
  CC_MD5(self.bytes, (CC_LONG)self.length, md5Buffer);
  NSMutableString *output = [NSMutableString stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
  for(int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) 
    [output appendFormat:@"%02x",md5Buffer[i]];

  return output;
}

Please note that the value returned is autorelease and might need to be retained by the receiver.

Hope this helps.

Answer 2

Why don't you simply generate unique identifiers and use it? like

CFUUIDRef uuidObj = CFUUIDCreate(nil);
NSString *uniqueId = (NSString*)CFUUIDCreateString(nil, uuidObj);
CFRelease(uuidObj);
NSLog(@"%@",uniqueId);
[uniqueId autorelease];

Answer 3

Using NSData is an expensive choice. Better use NSFileHandler extension if you are dealing with big files anytime.