Basic

On-demand image creation by secure URL parameters

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

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

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()

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.

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>.

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).