Converting NSString to Base64 Data for XML Serialization

Question

I use Apple's NSXMLParser class to load data. I have a separate framework for serializing my data.

I've had enough of the long dash, the Word quotes, etc. showing up in my XML output and causing errors when parsing, which often result in data loss due to the parser's poor handling of those characters.

If I have a user's input as an NSString object, I want to convert it to Base64 (which I have a utility method for), then write those bytes to the XML file, in place of the NSString. I understand this might take up more space, but at least I won't have to deal with the validation issues any more.

My question is, how does one go about converting the NSData bytes (which look like <8d72...> in an NSLog statement) to an NSString, without using encoding to get the original value back. I want those actual bytes, as they appear in the log statement, to be converted to an NSString. And finally (when loading this serialized data), if you have an NSString:

NSString *loadedData = @"8d72...";

How does one go from this form to the human-readable format? The whole encoding process is a little difficult for me to understand, but I think this is a really solid way to ensure that weird user input is properly persisted in my XML files.

Answer 1

Got it. I'm using the encoding/decoding methods from this answer to convert my NSString objects to NSData objects, and vice versa: Any base64 library on iphone-sdk?

And then I wrote these quick methods that make use of the above methods, and allow me to write the Base64 string data to XML. Loading it up works great, has been tested with Chinese characters, Word characters, etc. You can also parse these out if you want, but at least the error isn't in the parser. (Which could easily result in data loss if you don't handle the errors properly.)

+ (NSString *)toBase64String:(NSString *)string {
    NSData *data = [string dataUsingEncoding: NSUnicodeStringEncoding];

    NSString *ret = [NSStringUtil base64StringFromData:data length:[data length]];

    return ret;
}

+ (NSString *)fromBase64String:(NSString *)string {
    NSData  *base64Data = [NSStringUtil base64DataFromString:string];

    NSString* decryptedStr = [[NSString alloc] initWithData:base64Data encoding:NSUnicodeStringEncoding];

    return [decryptedStr autorelease];
}

Edit: As the original link is down, I hunted down my code from a while ago, here are my NSStringUtil methods mentioned above. Note I did not author this code, but it has been working well for years:

+ (NSData *)base64DataFromString: (NSString *)string {
    unsigned long ixtext, lentext;
    unsigned char ch, input[4], output[3];
    short i, ixinput;
    Boolean flignore, flendtext = false;
    const char *temporary;
    NSMutableData *result;

    if (!string) {
        return [NSData data];
    }

    ixtext = 0;

    temporary = [string UTF8String];

    lentext = [string length];

    result = [NSMutableData dataWithCapacity: lentext];

    ixinput = 0;

    while (true) {
        if (ixtext >= lentext) {
            break;
        }

        ch = temporary[ixtext++];

        flignore = false;

        if ((ch >= 'A') && (ch <= 'Z')) {
            ch = ch - 'A';
        } else if ((ch >= 'a') && (ch <= 'z')) {
            ch = ch - 'a' + 26;
        } else if ((ch >= '0') && (ch <= '9')) {
            ch = ch - '0' + 52;
        } else if (ch == '+') {
            ch = 62;
        } else if (ch == '=') {
            flendtext = true;
        } else if (ch == '/') {
            ch = 63;
        } else {
            flignore = true; 
        }

        if (!flignore) {
            short ctcharsinput = 3;
            Boolean flbreak = false;

            if (flendtext) {
                if (ixinput == 0) {
                    break;
                }

                if ((ixinput == 1) || (ixinput == 2)) {
                    ctcharsinput = 1;
                } else {
                    ctcharsinput = 2;
                }

                ixinput = 3;

                flbreak = true;
            }

            input[ixinput++] = ch;

            if (ixinput == 4) {
                ixinput = 0;

                unsigned char0 = input[0];
                unsigned char1 = input[1];
                unsigned char2 = input[2];
                unsigned char3 = input[3];

                output[0] = (char0 << 2) | ((char1 & 0x30) >> 4);
                output[1] = ((char1 & 0x0F) << 4) | ((char2 & 0x3C) >> 2);
                output[2] = ((char2 & 0x03) << 6) | (char3 & 0x3F);

                for (i = 0; i < ctcharsinput; i++) {
                    [result appendBytes: &output[i] length: 1];
                }
            }

            if (flbreak) {
                break;
            }
        }
    }

    return result;
}

+ (NSString *)base64StringFromData: (NSData *)data length: (NSUInteger)length {
    unsigned long ixtext, lentext;
    long ctremaining;
    unsigned char input[3], output[4];
    short i, charsonline = 0, ctcopy;
    const unsigned char *raw;
    NSMutableString *result;

    lentext = [data length];

    if (lentext < 1) {
        return @"";
    }

    result = [NSMutableString stringWithCapacity: lentext];

    raw = [data bytes];

    ixtext = 0;

    while (true) {
        ctremaining = lentext - ixtext;

        if (ctremaining <= 0) {
            break;
        }

        for (i = 0; i < 3; i++) { 
            unsigned long ix = ixtext + i;

            if (ix < lentext) {
                input[i] = raw[ix];
            } else {
                input[i] = 0;
            }
        }

        output[0] = (input[0] & 0xFC) >> 2;
        output[1] = ((input[0] & 0x03) << 4) | ((input[1] & 0xF0) >> 4);
        output[2] = ((input[1] & 0x0F) << 2) | ((input[2] & 0xC0) >> 6);
        output[3] = input[2] & 0x3F;

        ctcopy = 4;

        switch (ctremaining) {
            case 1: 
                ctcopy = 2;
                break;
            case 2: 
                ctcopy = 3;
                break;
        }

        for (i = 0; i < ctcopy; i++) {
            [result appendString: [NSString stringWithFormat: @"%c", base64EncodingTable[output[i]]]];
        }

        for (i = ctcopy; i < 4; i++) {
            [result appendString: @"="];
        }

        ixtext += 3;
        charsonline += 4;

        if ((ixtext % 90) == 0) {
            [result appendString: @"\n"];
        }

        if (length > 0) {
            if (charsonline >= length) {
                charsonline = 0;

                [result appendString: @"\n"];
            }
        }
    }

    return result;
}