Can UTF8-byte-chunks be decoded to a string in a safe manner?

Is it safe to decode a UTF8-string that has been split into arbitrary byte-chunks into a string (chunk by chunk)?

Also, what about an arbitrary encoding?

The scenario involves the following method:

async getFileAsync(fileName: string, encoding: string):string
{
    const textDecoder = new TextDecoder(encoding);
    const response = await fetch(fileName);
    
    console.log(response.ok);
    console.log(response.status);
    console.log(response.statusText);
    
    const reader = response.body.getReader();
    let result:ReadableStreamReadResult<Uint8Array>;
    let chunks:Uint8Array[] = [];
    
    do
    {
        result = await reader.read();
        chunks.push(result.value);

        let partN = textDecoder.decode(result.value);

        console.log("result: ", result.value, partN);
    } while(!result.done)

    let chunkLength:number = chunks.reduce(
        function(a, b)
        {
            return a + (b||[]).length;
        }
        , 0
    );
    
    let mergedArray = new Uint8Array(chunkLength);
    let currentPosition = 0;
    for(let i = 0; i < chunks.length; ++i)
    {
        mergedArray.set(chunks[i],currentPosition);
        currentPosition += (chunks[i]||[]).length;
    }

    let file:string = textDecoder.decode(mergedArray);
    
    return file;
} // End Function getFileAsync

Now, my question is, when dealing with arbitrary encoding, is it safe to decode the chunks like this:

result = await reader.read();
// would this be safe ? 
chunks.push(textDecoder.decode(result.value));

By "safe," I mean will it correctly decode the overall string?

I suspect it might not, but I would appreciate confirmation.

I thought that since I have to wait until the end to merge the array of chunks, I could just use:

let responseBuffer:ArrayBuffer = await response.arrayBuffer();
let text:string = textDecoder.decode(responseBuffer);

instead.

Answer №1

Defining what constitutes safety depends on the context.

Knowing the original string size provides a maximum decode size, which can help mitigate certain modern DoS attacks.

The algorithms involved are relatively straightforward, but how data is utilized presents significant security considerations. For example, UTF-8 encoding may include unnecessary long sequences that should be discarded by a good decoder. Ensuring proper handling of characters like U+0000 is crucial to prevent buffer overflows.

UCS allows for encoding more bits than UTF-8, potentially leading to larger byte consumption. While some UTF-8 decoders support this, it is generally considered an error due to compatibility issues with manipulation functions beyond Unicode limits.

Normalization plays a critical role in removing extraneous code points, as excessive encoding can cause problems with certain characters. Additionally, issues such as sorting codepoints and character impersonation present separate security challenges.

An effective decoder must identify invalid bytes, overlong UTF-8 sequences, and non-Unicode codepoints. Handling different levels of permissiveness among decoders is essential for robust program functionality. Invalid sequences may pose decoding challenges, with various decoders responding differently to errors.

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