Question

I need to implement salts in my encryption, but to do so, I need to store it in a file format that I need to create so I can later retrieve it to decrypt. I’m a noob when it comes to encryption. The specifications of the file format should be as so:

Ciphertext: length of ciphertext ;
Salt: length of salt ;

Then the ciphertext and salt written out. This is where xcode really confuses me, as in creating a new file, etc.

How can I do this? And then retrieve the salt for decryption?

Thank you, your help is greatly appreciated.

Answer 1

You might consider using NSMutableDictionary and NSKeyedUnarchiver like this:

// Example ciphertext and salt
NSString *ciphertext = @"the ciphertext";
NSString *salt = @"the salt";

// File destination
NSString *path = @"/Users/Anne/Desktop/Archive.dat";

// Create dictionary with ciphertext and salt
NSMutableDictionary *dictionary = [[NSMutableDictionary alloc] init];
[dictionary setObject:ciphertext forKey:@"ciphertext"];
[dictionary setObject:salt forKey:@"salt"];

// Archive dictionary and write to file
NSData *data = [NSKeyedArchiver archivedDataWithRootObject:dictionary];
[data writeToFile:path options:NSDataWritingAtomic error:nil];

// Read file and unarchive
NSMutableDictionary *theDictionary = [NSKeyedUnarchiver unarchiveObjectWithFile:path];

// Get ciphertext and salt
NSString *theCiphertext = [theDictionary objectForKey:@"ciphertext"];
NSString *theSalt = [theDictionary objectForKey:@"salt"];

// Show Result
NSLog(@"Extracted ciphertext: %@",theCiphertext);
NSLog(@"Extracted salt: %@",theSalt);

Output:

Extracted ciphertext: the ciphertext
Extracted salt: the salt

EDIT

Response to comment: Both NSData and NSString feature length.

Quick example:

NSString *theString = @"Example String";
NSData *theData = [theString dataUsingEncoding:NSUTF8StringEncoding];

NSUInteger stringLength = [theString length];
NSUInteger dataLength = [theData length];

NSLog(@"String length: %ld",stringLength);
NSLog(@"Data length: %ld",dataLength);

Output:

String length: 14
Data length: 14