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Re: Feeding NSUnarchiver bad data can cause crash

Hello...

Another possibility would be to append some sort of signature or checksum to the end of the data produced by the archiver, before you encrypt it.

Then after you decrypt the data, you could check the last bytes of the data for the signature or verify the checksum before passing the rest of the data to the unarchiver.

/* typed in email, etc... */

In other words, something like the following when you archive:

- (void)saveEncryptedArchiveFromObject:(id)plaintext
{
NSData *archivedData = [NSArchiver archivedDataWithRootObject:plaintext];

// create a checksum NSData object, however you prefer (but it should be a fixed length)
NSData *checksumData = [self checksumDataCalculatedForData:archivedData];

NSMutableData *combinedData = [NSMutableData dataWithData:archivedData];
[combinedData appendData:checksumData];

// encrypt your data
NSData *encryptedData = [self encryptedDataUsingData:combinedData];

// write the data out
[encryptedData writeToFile:@"/some/Path" atomically:TRUE];

Then, when decrypting:

- (id)archivedObjectFromEncryptedData:(NSData *)fileData
{
id plaintext = nil;

// unencrypt the data from the file
NSData *unencryptedData = [self unencryptedDataFromData:fileData];

// break the data back into parts
NSData *archivedData = nil;
NSData *checksumData = nil;
unsigned rawLength = [unecryptedData length];
if (rawLength > CHECKSUM_LENGTH)
{
archivedData = [unecryptedData subDataWithRange:NSMakeRange(0, rawLength - CHECKSUM_LENGTH)];
checksumData = [unecryptedData subDataWithRange:NSMakeRange(rawLength - CHECKSUM_LENGTH , CHECKSUM_LENGTH)];

// verify the checksum
if ([self isValidChecksum:checksumData forData:archivedData])
{
// unarchive the data since checksum was valid
plaintext = [NSUnarchiver unarchiveObjectWithData:archivedData];
}
}

return plaintext;
}

The checksumDataCalculatedForData:, isValidChecksum:forData:, encryptedDataUsingData:, and unencryptedDataFromData: methods used above would all be part of your own implementation.

Hope that helps,

Louis


The setup I'm using is this. An attributed string is archived, and the archived data is encrypted using a Blowfish symmetric cipher, producing a second data object which is saved to disk. Later, the encrypted data is decrypted. If the key used for decryption is the wrong one, the data will decrypt, but will be "bad" data. If this is passed to an NSUnarchiver for unarchiving as an attributed string, the unarchiver forces it to unarchive, resulting an error:

*** +[NSUnarchiver unarchiveObjectWithData:]: extra data discarded

and, sometimes, a SIGNAL 11 crash:

*** malloc_zone_malloc[414]: argument too large: 4294967286

If exactly the same encrypted data is decrypted with the correct key, producing "good" data, it unarchives just fine, never crashes, and all is cool.

The magic numbers you mention would be extremely useful if I knew how to access them. Perhaps it's possible to parse the decrypted data to see if the magic number is there? Or is it only present when the archive is written to file? In my case, it's the encrypted archive, not the archive itself, which is saved to disk.

Regards,
-Jeremy
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References: 
 >Re: Feeding NSUnarchiver bad data can cause crash (From: Jeremy Dronfield <email@hidden>)



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