A category on NSString that exploits a set of CoreFoundation methods to create Base64 encoded strings. This is a simple solution that removes the need for external Base64 libraries in projects that targets apps that support iOS 2 - iOS 6. iOS 7 has a more concrete solution, but due to NDA, I cannot post it yet.

Native Base64 (Pre iOS7)

+ (NSString *)base64EncodeString:(NSString *)stringToEncode
{
    CFHTTPMessageRef messageRef = CFHTTPMessageCreateEmpty(kCFAllocatorDefault, FALSE);
    CFHTTPMessageAddAuthentication(messageRef, NULL, CFSTR("AS"), (__bridge CFStringRef)stringToEncode, kCFHTTPAuthenticationSchemeBasic, FALSE);

    CFStringRef authStringRef = CFHTTPMessageCopyHeaderFieldValue(messageRef, CFSTR("Authorization"));
    NSString *encodedString = [(__bridge NSString *)authStringRef substringFromIndex:10];

    CFRelease(messageRef);
    CFRelease(authStringRef);

    return encodedString;
}