fix: One more example

Author: tmathern

examples/sign.py

@@ -11,15 +11,18 @@
 # each license.
 
 # This example shows how to sign an image with a C2PA manifest
-# and read the metadata added to the image.
+# using a callback signer and read the metadata added to the image.
 
 import os
 import c2pa
+from cryptography.hazmat.primitives import hashes, serialization
+from cryptography.hazmat.primitives.asymmetric import ec
+from cryptography.hazmat.backends import default_backend
 
 fixtures_dir = os.path.join(os.path.dirname(__file__), "../tests/fixtures/")
 output_dir = os.path.join(os.path.dirname(__file__), "../output/")
 
-# ensure the output directory exists
+# Ensure the output directory exists
 if not os.path.exists(output_dir):
     os.makedirs(output_dir)
 
@@ -33,26 +36,43 @@
     reader = c2pa.Reader("image/jpeg", file)
     print(reader.json())
 
-# Create a signer from certificate and key files
+# Load certificates and private key (here from the test fixtures)
+# This is OK for development, but in production you should use a
+# secure way to load the certificates and private key.
 certs = open(fixtures_dir + "es256_certs.pem", "rb").read()
 key = open(fixtures_dir + "es256_private.key", "rb").read()
 
-signer_info = c2pa.C2paSignerInfo(
-    alg=b"es256",  # Use bytes instead of encoded string
-    sign_cert=certs,
-    private_key=key,
-    ta_url=b"http://timestamp.digicert.com"  # Use bytes and add timestamp URL
-)
+# Define a callback signer function
+def callback_signer_es256(data: bytes) -> bytes:
+    """Callback function that signs data using ES256 algorithm."""
+    private_key = serialization.load_pem_private_key(
+        key,
+        password=None,
+        backend=default_backend()
+    )
+    signature = private_key.sign(
+        data,
+        ec.ECDSA(hashes.SHA256())
+    )
+    return signature
 
-signer = c2pa.Signer.from_info(signer_info)
+# Create a signer using the callback function we defined
+signer = c2pa.Signer.from_callback(
+    callback=callback_signer_es256,
+    alg=c2pa.C2paSigningAlg.ES256,
+    certs=certs.decode('utf-8'),
+    tsa_url="http://timestamp.digicert.com"
+)
 
 # Create a manifest definition as a dictionary
+# This manifest follows the V2 manifest format
 manifest_definition = {
     "claim_generator": "python_example",
     "claim_generator_info": [{
         "name": "python_example",
         "version": "0.0.1",
     }],
+    "claim_version": 2,
     "format": "image/jpeg",
     "title": "Python Example Image",
     "ingredients": [],
@@ -73,13 +93,20 @@
     ]
 }
 
+# Create the builder with the manifest definition
 builder = c2pa.Builder(manifest_definition)
 
-# Sign the image
-print("\nSigning the image...")
-with open(fixtures_dir + "C.jpg", "rb") as source:
-    with open(output_dir + "C_signed.jpg", "wb") as dest:
-        result = builder.sign(signer, "image/jpeg", source, dest)
+# Sign the image with the signer created above,
+# which will use the callback signer
+print("\nSigning the image file...")
+builder.sign_file(
+    source_path=fixtures_dir + "C.jpg",
+    dest_path=output_dir + "C_signed.jpg",
+    signer=signer
+)
+
+# Clean up the signer
+signer.close()
 
 # Read the signed image to verify
 print("\nReading signed image metadata:")

examples/sign_using_signer_info.py

@@ -0,0 +1,92 @@
+# Copyright 2025 Adobe. All rights reserved.
+# This file is licensed to you under the Apache License,
+# Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
+# or the MIT license (http://opensource.org/licenses/MIT),
+# at your option.
+# Unless required by applicable law or agreed to in writing,
+# this software is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR REPRESENTATIONS OF ANY KIND, either express or
+# implied. See the LICENSE-MIT and LICENSE-APACHE files for the
+# specific language governing permissions and limitations under
+# each license.
+
+# This example shows how to sign an image with a C2PA manifest
+# and read the metadata added to the image.
+
+import os
+import c2pa
+
+fixtures_dir = os.path.join(os.path.dirname(__file__), "../tests/fixtures/")
+output_dir = os.path.join(os.path.dirname(__file__), "../output/")
+
+# Ensure the output directory exists
+if not os.path.exists(output_dir):
+    os.makedirs(output_dir)
+
+print("c2pa version:")
+version = c2pa.sdk_version()
+print(version)
+
+# Read existing C2PA metadata from the file
+print("\nReading existing C2PA metadata:")
+with open(fixtures_dir + "C.jpg", "rb") as file:
+    reader = c2pa.Reader("image/jpeg", file)
+    print(reader.json())
+
+# Create a signer from certificate and key files
+certs = open(fixtures_dir + "es256_certs.pem", "rb").read()
+key = open(fixtures_dir + "es256_private.key", "rb").read()
+
+signer_info = c2pa.C2paSignerInfo(
+    alg=b"es256",  # Use bytes instead of encoded string
+    sign_cert=certs,
+    private_key=key,
+    ta_url=b"http://timestamp.digicert.com"  # Use bytes and add timestamp URL
+)
+
+signer = c2pa.Signer.from_info(signer_info)
+
+# Create a manifest definition as a dictionary
+# This examples signs using a V1 manifest
+manifest_definition = {
+    "claim_generator": "python_example",
+    "claim_generator_info": [{
+        "name": "python_example",
+        "version": "0.0.1",
+    }],
+    "format": "image/jpeg",
+    "title": "Python Example Image",
+    "ingredients": [],
+    "assertions": [
+        {
+            "label": "c2pa.actions",
+            "data": {
+                "actions": [
+                    {
+                        "action": "c2pa.created",
+                        "parameters": {
+                            # could hold additional information about this step
+                        }
+                    }
+                ]
+            }
+        }
+    ]
+}
+
+builder = c2pa.Builder(manifest_definition)
+
+# Sign the image
+print("\nSigning the image...")
+with open(fixtures_dir + "C.jpg", "rb") as source:
+    with open(output_dir + "C_signed.jpg", "wb") as dest:
+        result = builder.sign(signer, "image/jpeg", source, dest)
+
+# Read the signed image to verify
+print("\nReading signed image metadata:")
+with open(output_dir + "C_signed.jpg", "rb") as file:
+    reader = c2pa.Reader("image/jpeg", file)
+    print(reader.json())
+
+print("\nExample completed successfully!")
+