DPAPI fails with CryptographicException when trying to decrypt Chrome cookies

Question

i am trying to get session from my Chrome browser. i can see 2 cookie files in Developer Tools. but this is inconvenient for the user to get cookie values from browser, i wo

Answer 1

For people who are looking for the code, I'm expanding on Cerberus answer. Starting Chrome 80 version, cookies are encrypted using the AES256-GCM algorithm, and the AES encryption key is encrypted with the DPAPI encryption system, and the encrypted key is stored inside the ‘Local State’ file.

byte[] encryptedData=<data stored in cookie file>
string encKey = File.ReadAllText(localAppDataPath + @"\Google\Chrome\User Data\Local State");
encKey = JObject.Parse(encKey)["os_crypt"]["encrypted_key"].ToString();
var decodedKey = System.Security.Cryptography.ProtectedData.Unprotect(Convert.FromBase64String(encKey).Skip(5).ToArray(), null, System.Security.Cryptography.DataProtectionScope.LocalMachine);
_cookie = _decryptWithKey(encryptedData, decodedKey, 3);

Key size is 256 bits. Encypted message format is, pay load('v12')+nonce (12 bytes)+cipherText

private string _decryptWithKey(byte[] message, byte[] key, int nonSecretPayloadLength)
{
    const int KEY_BIT_SIZE = 256;
    const int MAC_BIT_SIZE = 128;
    const int NONCE_BIT_SIZE = 96;

    if (key == null || key.Length != KEY_BIT_SIZE / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KEY_BIT_SIZE), "key");
    if (message == null || message.Length == 0)
        throw new ArgumentException("Message required!", "message");

    using (var cipherStream = new MemoryStream(message))
    using (var cipherReader = new BinaryReader(cipherStream))
    {
        var nonSecretPayload = cipherReader.ReadBytes(nonSecretPayloadLength);
        var nonce = cipherReader.ReadBytes(NONCE_BIT_SIZE / 8);
        var cipher = new GcmBlockCipher(new AesEngine());
        var parameters = new AeadParameters(new KeyParameter(key), MAC_BIT_SIZE, nonce);
        cipher.Init(false, parameters);
        var cipherText = cipherReader.ReadBytes(message.Length);
        var plainText = new byte[cipher.GetOutputSize(cipherText.Length)];
        try
        {
            var len = cipher.ProcessBytes(cipherText, 0, cipherText.Length, plainText, 0);
            cipher.DoFinal(plainText, len);
        }
        catch (InvalidCipherTextException)
        {
            return null;
        }
        return Encoding.Default.GetString(plainText);
    }
}

Needed packages

1) Newtonsoft JSON .net

2) Bouncy Castle Crypto package

Answer 2

I finally figured it out. according to Chromium sources, two methods are used to decrypt the cookie value.

1. if the cookie value starts with v10 chars, we use AES_256_GCM
2. otherwise, DPAPI is used

for the first method we need key and nonce. key is located in Google Chrome files and nonce is located in encrypted cookie value.

it remains unclear for me - what determines which method is used