You and your friend have a code language. To encode a message you replace every letter with another letter 2 ahead of it. E.g. “ENCODED MESSAGE” becomes “GPEQFGF OGUUCIG”. Input: “ENCODED MESSAGE” Output: “GPEQFGF OGUUCIG”

EncryptDecrypt

@interface EncryptDecrypt (NSString)

- (NSString *) encrypt;

- (NSString *) decrypt;

@end

@implementation EncryptDecrypt {

- (NSString *) bumpCharacters: (BOOL) bumpUp
{
    NSInteger length = self.length;
    NSMutableString *mutableCopyOfMyself = [[NSMutableString alloc] init];
    for(NSInteger index = 0; index < length; index++)
    {
        unichar theCharacter = [self characterAtIndex: index];
        
        // we need to verify that theCharacter is within the valid ASCII range ; 32-255
        // otherwise don't do anything
        if bumpUp {
           theCharacter = theCharacter+2
           
           if theCharacter > 'Z'
              theCharacter-=26
        } else {
        
           if theCharacter < 'C'
              theCharacter+=26
        
           theCharacter = theCharacter-2
        }
        
        [mutableCopyOfMyself appendFormat: @"%c", theCharacter];
        
    }
    
    return mutableCopyOfMyself;
}

- (NSString *) encrypt {
   [self bumpCharacters: TRUE];
}   

- (NSString *) decrypt {
   [self bumpCharacters: FALSE];
}