# Basic

Your image modifications are signed with your project's API token, thus your image can only be modified by you.

{% tabs %}
{% tab title="Elixir" %}

```elixir
def create_signed_image do
  user_id = ""
  secret_key = ""
  base_id = ""
  
  modifications = 
    Jason.encode!([
      %{"name" => "text_1", "text" => "secure image"},
      %{"name" => "rating_2", "rating" => 5}
    ])
    |> Base.url_encoded64(padding: false)
    
  parameters = "#{user_id}+#{base_id}+#{modifications}"
  
  signature =
    :crypto.hmac(:sha256, secret_key, parameters)
    |> Base.encode16()
    |> String.downcase()
    
  "https://images.usestencil.com/signed-images/#{user_id}/#{base_id}.png?modifications=#{modifications}&s=#{signature}"
end
```

{% endtab %}

{% tab title="Python" %}

```python
import json
import hmac
import hashlib
import base64

def base64_encode(string):
    """
    Removes any `=` used as padding from the encoded string.
    """
    encoded = base64.b64encode(string.encode())
    encoded = encoded.rstrip(b'=')
    return encoded

def generate_url():
  user_id = ""
  secret_key = ""
  base_id = ""

  modifications = [
    { "name": "text_1", "text": "secure image" },
    { "name": "rating_2", "rating": 5 },
  ]

  # ensure no spaces in json output
  encoded = json.dumps(modifications, separators=(',', ':'))
  encoded = base64_encode(encoded).decode()

  parameters = "{user_id}+{base_id}+{modifications}".format(user_id=user_id, base_id=base_id, modifications=encoded)

  signature = hmac.new(secret_key.encode(), parameters.encode(), hashlib.sha256).hexdigest()

  url = "https://images.usestencil.com/signed-images/{user_id}/{base_id}.png?modifications={modifications}&s={signature}".format(
      base_id=base_id,
      user_id=user_id,
      modifications=encoded,
      signature=signature
  )
  print(url)

if __name__ == "__main__":
    generate_url()

```

{% endtab %}
{% endtabs %}

### Changing image format on the fly

We support both JPEG and PNG. To use PNG, change the image in the url to `.png` and to use JPEG, change the image in the url to `.jpg`

### Cache

Image is only generated once (synchronously on the first request) and subsequent images are pulled from cache. Images pulled from cache do not count towards your quota.

{% hint style="warning" %}
**Order matters.** 

In order for cache to work properly, please specify the signature parameter last in the query string i.e. `modifications=<your-modifications>&s=<your-signature>`.
{% endhint %}

#### Cache invalidation

Sometimes it is useful to invalidate cache, you can add `ts=<random number>` to invalidate the cache and a new image will be generated (this will count towards your quota).


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.usestencil.com/integrations/secure-signed-image/signed-image.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.