Implementing a file format to be used with Encryption - Cocoa - cocoa

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