libs & stu_capture

Within libs, the wacom and will folders may be deleted. Optionally, the libs folder itself may also be deleted, provided users retrieve the stu_capture folder inside and place it within sigCaptDialog, so that the final folder structure looks like so: sigCaptDialog/libs/stu_capture.

For the purposes of this guide, it will be assumed that the folder structure is sigCaptDialog/stu_capture.

Within stu_capture, the file stu_capture_encryption.js will require the following adjustments:

On clearKeys

Version 1

    /**
     * Reset all encryption key values
     */
    clearKeys() {
		this.bigint_p    = null;
		this.bigint_g    = null;
		this.sjcl_keyAES = null;
	}

Version 2

    /**
     * Reset all encryption key values
     */
    clearKeys() {
		this.bigint_p = null;
		this.bigint_g = null;
		this.aesCipher = null;
	}

On computeSharedKey and decrypt:

Version 1

    /**
    * Calculate a shared key, given the tablet's public key
    * @param  devicePublicKey   the tablet's public key
    */
    computeSharedKey(devicePublicKey) {
		var B = devicePublicKey;
  
		var bigint_B = BigInt("0x"+arrayToHex(B));

		var bigint_shared = powMod(bigint_B, this.bigint_a, this.bigint_p);

		var str_shared = padLeft(bigint_shared.toString(16), 32, '0');

		this.sjcl_keyAES = new sjcl.cipher.aes( sjcl.codec.hex.toBits(str_shared) );
	}

    /**
     * Decrypts a block of encrypted data
     * @param  data  an array of bytes to decrypt
     * @return decrypted data
     */
    decrypt(data) {
		var arr_cipherText  = data;
		var hex_cipherText  = arrayToHex(arr_cipherText);
		var sjcl_cipherText = sjcl.codec.hex.toBits(hex_cipherText);

		var sjcl_plainText = this.sjcl_keyAES.decrypt(sjcl_cipherText);

		var hex_plainText = sjcl.codec.hex.fromBits(sjcl_plainText);
		var arr_plainText = hexToArray(hex_plainText);
		return arr_plainText;
	}

Version 2

    /**
    * Calculate a shared key, given the tablet's public key
    * @param  devicePublicKey   the tablet's public key
    */
    computeSharedKey(devicePublicKey) {
		const B = devicePublicKey;  
		const bigint_B = BigInt("0x"+arrayToHex(B));
		const bigint_shared = powMod(bigint_B, this.bigint_a, this.bigint_p);
		const str_shared = padLeft(bigint_shared.toString(16), 32, '0');
		const keyAES = AES.Codec.Hex.toBits(str_shared);    
		
        // SubtleCrypto does not supports AES-ECB so we need to use a custom code.
		this.aesCipher = new AES.ECB(keyAES);		
	}

    /**
     * Decrypts a block of encrypted data
     * @param  data  an array of bytes to decrypt
     * @return decrypted data
     */
    decrypt(data) {
		const hexCipherText  = arrayToHex(data);
		const cipherText = AES.Codec.Hex.toBits(hexCipherText);
		const plainText = this.aesCipher.decrypt(cipherText);

		const hexPlainText = AES.Codec.Hex.fromBits(plainText);
		return hexToArray(hexPlainText);
	}

And finally, computeSessionKey and decrypt:

Version 1

	// replace additional left zeros
		const decryptKey = BigInt("0x"+arrayToHex(new Uint8Array(key)));				
		const hexKey = padLeft(decryptKey.toString(16), 64, '0');
		
		// SubtleCrypto only supports AES-CBC with PKCS#7 padding.
        // so we need to use another library as STU devices uses no padding.
        this.keyAES = new sjcl.cipher.aes(sjcl.codec.hex.toBits(hexKey));		
	}

    /**
     * Decrypts a block of encrypted data
     * @param  data  an array of bytes to decrypt
     * @return decrypted data
     */
    decrypt(data) {			
		var hex_cipherText  = arrayToHex(data);
		var sjcl_cipherText = sjcl.codec.hex.toBits(hex_cipherText);

		var sjcl_plainText = this.keyAES.decrypt(sjcl_cipherText);

		var hex_plainText = sjcl.codec.hex.fromBits(sjcl_plainText);
		var arr_plainText = hexToArray(hex_plainText);

		return arr_plainText;
	}

Version 2

		// replace additional left zeros
		const decryptKey = BigInt("0x"+arrayToHex(new Uint8Array(key)));				
		const hexKey = padLeft(decryptKey.toString(16), 64, '0');		
		const keyAES = AES.Codec.Hex.toBits(hexKey);        
		
        // SubtleCrypto does not supports AES-ECB so we need to use a custom code.
		this.aesCipher = new AES.ECB(keyAES);
	}

    /**
     * Decrypts a block of encrypted data
     * @param  data  an array of bytes to decrypt
     * @return decrypted data
     */
    decrypt(data) {			
		const hexCipherText  = arrayToHex(data);
		const cipherText = AES.Codec.Hex.toBits(hexCipherText);
		const plainText = this.aesCipher.decrypt(cipherText);

		const hexPlainText = AES.Codec.Hex.fromBits(plainText);
		return hexToArray(hexPlainText);
	}

Delete the remaining files, stu_capture.js and package.json. From there, add the file sjcl.js, a dependency that's still required for the sample.

This can be downloaded here: https://www.npmjs.com/package/sjcl