Question

I get an error every time I try to store data into the keychain

The error comes from the dictionaryToSecItemFormat method at these lines

NSString *passwordString = [dictionaryToConvert objectForKey:(id)kSecValueData];
[returnDictionary setObject:[passwordString dataUsingEncoding:NSUTF8StringEncoding] forKey:(id)kSecValueData]'

The error is

I’m calling the KeychainItemWrapper methods like this

KeychainItemWrapper *keychain = [[KeychainItemWrapper alloc] initWithIdentifier:@"credentials" accessGroup:nil];
[keychain setObject:username forKey:kSecAttrAccount];
[keychain release];

The error is

 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 
  '-[NSConcreteData dataUsingEncoding:]: unrecognized selector sent to instance"

Answer 1

dataUsingEncoding is a method from the NSString class, make username a NSString.