EncryptionTransformer class with initialization vector

EncryptionTransformer.m

- (id)transformedValue:(NSData*)data
{
   ...

   // Use another NSData category method (left as an exercise 
   // to the reader) to randomly generate the IV data
   NSData* iv = [NSData randomDataOfLength:32];

   data = [data dataAES256EncryptedWithKey:[self key] Iv:iv];

   // Return a data object that includes the IV with the 
   // encrypted data appended
   NSMutableData* mutableData = [NSMutableData dataWithData:iv];
   [mutableData appendData:data];
   return mutableData;
}

- (id)reverseTransformedValue:(NSData*)data
{
   ...

   // The IV was stored in the first 32 bytes of the data
   NSData* iv = [data subdataWithRange:NSMakeRange(0, 32)];

   // Remove the IV from the encrypted data and decrypt it
   NSMutableData* mutableData = [NSMutableData dataWithData:data];
   [mutableData replaceBytesInRange:NSMakeRange(0, 32) withBytes:NULL];
   return [mutableData dataAES256DecryptedWithKey:[self key] Iv:iv];
}