I’m trying to create a virtual friend in Unity for iOS, where I use the Vertex AI model Gemini Flash for communication, but I can’t just use an API key for that communication; Vertex AI need OAuth 2.0 authentication. I tried code using the Google.Apis.Auth.OAuth2 library, but it seems to not work when building for iOS. Json is loaded correctly but error:

Failed to initialize Google credential: Json is Empty or null

Is there an alternative option?

private async Task InitializeCredential()
{
    try
    {
        // Cesta ke služebnímu účtu v StreamingAssets
        var resourcePath = Path.Combine(Application.streamingAssetsPath, apiKeyPath + ".json");
        string jsonKey;

        if (Application.platform == RuntimePlatform.IPhonePlayer)
        {
            using (UnityWebRequest www = UnityWebRequest.Get(resourcePath))
            {
                await www.SendWebRequest();
                if (www.result != UnityWebRequest.Result.Success)
                {
                    throw new FileNotFoundException($"Failed to load service account JSON file: {www.error}");
                }
                jsonKey = www.downloadHandler.text;
            }
        }
        else
        {
            jsonKey = File.ReadAllText(resourcePath);
        }

        // Vytvoření Google Credential z JSON klíče
        using (var jsonKeyStream = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(jsonKey)))
        {
            _credential = GoogleCredential.FromStream(jsonKeyStream)
                .CreateScoped(new[] { "https://www.googleapis.com/auth/cloud-platform" });
        }
    }
    catch (Exception ex)
    {
        Debug.LogError($"Failed to initialize Google credentials: {ex.Message}");
    }
}

public async Task<string> GetAccessTokenAsync()
{
    if (_credential == null)
    {
        Debug.LogError("Google Credential is not initialized.");
        return null;
    }

    try
    {
        var tokenResponse = await _credential.UnderlyingCredential.GetAccessTokenForRequestAsync();
        return tokenResponse;
    }
    catch (Exception ex)
    {
        Debug.LogError($"Failed to obtain access token: {ex.Message}");
        throw;
    }
}