Objective C: AES 256 Encryption with NSData Key - objective-c

Question

Previously I did Triple DES encryption to encrypt some data using a string key file, by defining a method
+ (NSData *)tripleDESEncryptWithKey:(NSString *)key dataToEncrypt:(NSData*)convertedData {}.
Now, similarly I am doing AES 256 encryption. But this time, I can not use a string as the key. I need to get NSData from a file in resources using
NSData *keyData = [NSData dataWithContentsOfFile:keyPath];
As I need to pass this key as parameter, I tried to convert it into string with
NSString *key = [[NSString alloc] initWithData:keyData encoding:NSUTF8StringEncoding];
but it returned NULL. So, How to use the Data key for AES 256 encryption ?
EDIT: I got that I should not use UTF8 encoding as the data file was made by Base64 encoding of a string. So Now the question is, how to get the string key using Base64 from keyData ?

Answer 1

If key variable returned NULL. Try to use like this.
NSString* key = [NSString stringWithUTF8String:[keyData bytes]];
Refer to KennyTM's Answer
UPDATE 1: You can also read the string directly from your file without convert in NSData
By NSFileHandle:
NSString * path = #"your key file path";
NSFileHandle * fileHandle = [NSFileHandle fileHandleForReadingAtPath:path];
NSData * buffer = nil;
while ((buffer = [fileHandle readDataOfLength:1024])) {
//do something with buffer
}
or use NSString
NSString * key = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:nil];
Refer to Dave DeLong's Answer
Note: Ensure your keyPath variable , Is it correct in the format of full or relative path?
UPDATE 2: You can also deal with Base64 Encoding with Matt Gallagher NSData+Base64 Class&Header (See also his blog there)
For encode base64
NSString * key = [[path dataUsingEncoding:NSUTF8StringEncoding] base64EncodedString];
Hope it helps you!

Answer 2

Read the file:
NSData* myData = [NSData dataWithContentsOfFile:myFileWithPath];
Then use one of the implementations in this post:
Base64 encoding options on the Mac and iPhone
to do your Base64 conversion.
Note: simply sticking your decryption key into a file is a bad idea. Optimally, the key would be stored securely by being encrypted by the user's own key.