### Process and Free CryptoPro Message Context

Source: https://docs.cryptopro.ru/cades/reference/cadesc/cadesc-samples/samplelowlevelapisignstream

This code handles the update of a message conversion context with a specific data chunk and ensures the context is properly freed. It includes error checking for both the update and free operations, returning an empty result upon failure.

```cpp
if(!CadesMsgConvertUpdate(pConvert,&original[lastChunkStart], lastChunkSize, TRUE))
{
    CadesMsgConvertFreeContext(pConvert);
    std::cout << "CadesMsgConvertUpdate() failed" << std::endl;
    return empty;
}

if(!CadesMsgConvertFreeContext(pConvert))
{
    std::cout << "CadesMsgConvertFreeContext() failed" << std::endl;
    return empty;
}

return convertedMessage;
```

--------------------------------

### JavaScript: Load and Process File in Chunks

Source: https://docs.cryptopro.ru/cades/plugin/plugin-samples/plugin-samples-fileapi_stream

This snippet demonstrates how to load a file in chunks using the FileReader API. It defines functions for handling file loading completion and errors, and recursively calls itself to process subsequent chunks until the entire file is read. It utilizes `blobSlice.call` for chunking.

```javascript
var frOnerror = function () {
    alert("File load error.");
};

function loadNext() {
    var fileReader = new FileReader();
    fileReader.onload = frOnload;
    fileReader.onerror = frOnerror;

    var start = currentChunk * chunkSize,
        end = ((start + chunkSize) >= oFile.size) ? oFile.size : start + chunkSize;

    fileReader.readAsDataURL(blobSlice.call(oFile, start, end));
};

loadNext();
```

--------------------------------

### JavaScript: Verify Digital Signature with CADES Plugin

Source: https://docs.cryptopro.ru/cades/plugin/plugin-samples/plugin-samples-fileapi_stream

This snippet shows how to verify a digital signature using the CADES plugin. It includes a try-catch block to handle potential errors during verification, displaying success or failure messages using alerts. It relies on the `cadesplugin` object for cryptographic operations.

```javascript
try {
    alert("Signature verified");
} catch (err) {
    alert("Failed to verify signature. Error: " + cadesplugin.getLastError(err));
    return;
}
```

=== COMPLETE CONTENT === This response contains all available snippets from this library. No additional content exists. Do not make further requests.