Question

I am trying to implement the new VerificationController.m class that Apple released to fix the in-app purchase fraud problem.

As everything released by Apple, this is one more vague, incomplete and bad explained document with a lot of voids and unknowns that cannot be circumvented/understood by everyone.

I am trying to implement that, but at the end of the code we see these four methods:

- (NSString *)encodeBase64:(const uint8_t *)input length:(NSInteger)length
{
#warning Replace this method.
    return nil;
}

- (NSString *)decodeBase64:(NSString *)input length:(NSInteger *)length
{
#warning Replace this method.
    return nil;
}

#warning Implement this function.
char* base64_encode(const void* buf, size_t size)
{ return NULL; }

#warning Implement this function.
void * base64_decode(const char* s, size_t * data_len)
{ return NULL; }

You can see that Apple was lazy to implement the C functions at the end of the code. As my C/C++ abilities stink, I see I need to implement these two functions in C/C++ and that they must return char and void (???). Other people have posted routines to do that on SO, but they are either in Objective-C or not returning chars and void (??).

NOTE: this is another problem I have: how can a method return void if it is used by Apple in this form?

uint8_t *purchase_info_bytes = base64_decode([purchase_info_string cStringUsingEncoding:NSASCIIStringEncoding],                                                 &purchase_info_length);

shouldn’t it be returning uint8_t?

NOTE2: another problem I have is that apple says base64_encode is required but it is not being used on the code provided by them. I think they are smoking bad stuff or my C/C++ knowledge really stink.

So, returning to my first question. Can someone post/point a method that can do the job that follows the requirements of the declared methods base64_encode and base64_decode? Please refrain from posting objective-c methods that are not compatible with these requirements imposed by Apple.

Thanks.

Answer 1

This solution should be pretty straight forward, which includes all the methods to populate the missing information. Tested and functional within the sandbox.

//  single base64 character conversion
static int POS(char c)
{
    if (c>='A' && c<='Z') return c - 'A';
    if (c>='a' && c<='z') return c - 'a' + 26;
    if (c>='0' && c<='9') return c - '0' + 52;
    if (c == '+') return 62;
    if (c == '/') return 63;
    if (c == '=') return -1;

    [NSException raise:@"invalid BASE64 encoding" format:@"Invalid BASE64 encoding"];
    return 0;
}

- (NSString *)encodeBase64:(const uint8_t *)input length:(NSInteger)length
{
    return [NSString stringWithUTF8String:base64_encode(input, (size_t)length)];
}

- (NSString *)decodeBase64:(NSString *)input length:(NSInteger *)length
{
    size_t retLen;
    uint8_t *retStr = base64_decode([input UTF8String], &retLen);
    if (length)
        *length = (NSInteger)retLen;
    NSString *st = [[[NSString alloc] initWithBytes:retStr
                                             length:retLen
                                           encoding:NSUTF8StringEncoding] autorelease];
    free(retStr);    // If base64_decode returns dynamically allocated memory
    return st;
}

char* base64_encode(const void* buf, size_t size)
{
    static const char base64[] =  "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    char* str = (char*) malloc((size+3)*4/3 + 1);

    char* p = str;
    unsigned char* q = (unsigned char*) buf;
    size_t i = 0;

    while(i < size) {
        int c = q[i++];
        c *= 256;
        if (i < size) c += q[i];
        i++;

        c *= 256;
        if (i < size) c += q[i];
        i++;

        *p++ = base64[(c & 0x00fc0000) >> 18];
        *p++ = base64[(c & 0x0003f000) >> 12];

        if (i > size + 1)
            *p++ = '=';
        else
            *p++ = base64[(c & 0x00000fc0) >> 6];

        if (i > size)
            *p++ = '=';
        else
            *p++ = base64[c & 0x0000003f];
    }

    *p = 0;

    return str;
}

void* base64_decode(const char* s, size_t* data_len_ptr)
{
    size_t len = strlen(s);

    if (len % 4)
        [NSException raise:@"Invalid input in base64_decode" format:@"%d is an invalid length for an input string for BASE64 decoding", len];

    unsigned char* data = (unsigned char*) malloc(len/4*3);

    int n[4];
    unsigned char* q = (unsigned char*) data;

    for(const char*p=s; *p; )
    {
        n[0] = POS(*p++);
        n[1] = POS(*p++);
        n[2] = POS(*p++);
        n[3] = POS(*p++);

        if (n[0]==-1 || n[1]==-1)
            [NSException raise:@"Invalid input in base64_decode" format:@"Invalid BASE64 encoding"];

        if (n[2]==-1 && n[3]!=-1)
            [NSException raise:@"Invalid input in base64_decode" format:@"Invalid BASE64 encoding"];

        q[0] = (n[0] << 2) + (n[1] >> 4);
        if (n[2] != -1) q[1] = ((n[1] & 15) << 4) + (n[2] >> 2);
        if (n[3] != -1) q[2] = ((n[2] & 3) << 6) + n[3];
        q += 3;
    }

    // make sure that data_len_ptr is not null
    if (!data_len_ptr)
        [NSException raise:@"Invalid input in base64_decode" format:@"Invalid destination for output string length"];

    *data_len_ptr = q-data - (n[2]==-1) - (n[3]==-1);

    return data;
}