macOS help for simple encryption and decryption?

NSMutableData* nsdata=[NSData dataWithContentsOfFile:@"/Users/whatever/Desktop/in.htm"];
if (nsdata == nil) {
   /* Do error handling here, such as report the error */
   /* Don't do any more processing */
}

#import <Foundation/Foundation.h>

@interface MyTransformer : NSObject
{
@private
    NSData *theKeyAsUTF8;
    NSUInteger theKeyIndex;
}

- (id)initWithKey:(NSString *)aKey;

- (void)reset;

- (NSData *)transform:(NSData *)anInput;
- (void)transformInPlace:(NSData *)aDataBuffer;

@end

#import <Foundation/Foundation.h>
#import "MyTransformer.h"



@implementation MyTransformer

- (id)init
{
    [self release];
    NSException *err =
        [NSException exceptionWithName:@"MyMethodNotSupportedException"
                                reason:@"init: on MyTransformer is not supported, use initWithKey: instead"
                              userInfo:[NSDictionary dictionary]];
    @throw err;
}


- (id)initWithKey:(NSString *)aKey
{
    NSAssert([aKey length] > 0, @"aKey is nil or has zero length");
    self = [super init];
    if (self) {
        theKeyAsUTF8 = [[aKey dataUsingEncoding:NSUTF8StringEncoding] retain];
        NSAssert(theKeyAsUTF8, @"failed to encode aKey in UTF-8");
        [self reset];
    }
    return self;
}


- (void)dealloc
{
    [theKeyAsUTF8 release];
    [super dealloc];
}


- (void)reset
{
    theKeyIndex = 0;
}


- (NSData *)transform:(NSData *)anInput
{
    if ([anInput length] == 0) return anInput;
    NSMutableData *aDataBuffer = [[anInput mutableCopy] autorelease];
    NSAssert(aDataBuffer, @"failed to create a mutable copy of anInput");
    [self transformInPlace:aDataBuffer];
    return aDataBuffer;
}


- (void)transformInPlace:(NSMutableData *)aDataBuffer;
{
    if ([aDataBuffer length] == 0) return;
    uint8_t *dataBufferBytes = [aDataBuffer mutableBytes];
    const uint8_t *keyBytes = [theKeyAsUTF8 bytes];
    const NSUInteger vlen = [aDataBuffer length];
    const NSUInteger klen = [theKeyAsUTF8 length];
    NSUInteger k = theKeyIndex;
    for (NSUInteger v = 0; v < vlen; v++) {
        uint8_t c = dataBufferBytes[v] ^ keyBytes[k]; 
        dataBufferBytes[v] = c;		
        k = (k + 1) % klen;
    }
    theKeyIndex = k;
}

@end

#import <Foundation/Foundation.h>
#import "MyTransformer.h"



int main (int argc, const char *argv[])
{
    
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    
    NSArray *startLines =
        [NSArray arrayWithObjects:
            @"Hello world\n",
            @"Second line\n",
            @"Third line\n",
            @"Last line\n",
            nil];

    MyTransformer *transformer = 
        [[MyTransformer alloc] 
            initWithKey:@"ABCDEFG"];    
    
    /* Simmulate writing by writing out the lines into a memory buffer */
    NSOutputStream *outputStream = [NSOutputStream outputStreamToMemory];
    [outputStream open];

    for (NSString *line in startLines) {
        NSData *lineAsUTF8 = [line dataUsingEncoding:NSUTF8StringEncoding];
        NSData *encryptedLine = [transformer transform:lineAsUTF8];
        [outputStream write:[encryptedLine bytes] 
                  maxLength:[encryptedLine length]]; 
    }
    
    [outputStream close];
    
    
    /* Simulate reading by gets the pointer to the memory buffer */
    NSData *encryptedBytes = 
        [outputStream propertyForKey:NSStreamDataWrittenToMemoryStreamKey];
    
    [transformer reset];
    NSData *decryptedBytes = [transformer transform:encryptedBytes];
    NSString *decryptedString = 
        [[NSString alloc] 
                initWithData:decryptedBytes 
                    encoding:NSUTF8StringEncoding];
    
    NSLog(@"%@", decryptedString);
    
    
    [decryptedString release];
    [transformer release];
    
    
    [pool drain];
    return 0;
    
}

- (void) write_file: (NSString*)fmt, ...
{
  if (!mFile)
    return;
  
  if (mIdleLogged)
    return;
  
  
  va_list vl;
  
  // Find amount of arguments
  va_start(vl, fmt);
  NSString* msg = [[NSString alloc] initWithFormat:fmt arguments: vl];
  va_end(vl);
  
  // Get UTF8 string from msg
  NSData* msgData = [msg dataUsingEncoding:NSUTF8StringEncoding];
  if (msgData)
  {
	  //---------Encrypt Log Data--------------------------------
	 
	  //[self transform:msgData];
	  	  
	  NSData *encryptedLine = [mTransformer transform:msgData];
	  
	  fwrite([encryptedLine bytes], [encryptedLine length], 1, mFile);
	  
	 //---------/Encrypt Log Data--------------------------------  
  }
  fflush(mFile);
  [msg release];
}