Signing with local credentials
Overview
To sign a claim in a C2PA manifest you need an end-entity certificate that complies with the C2PA trust model. Then you can use your private key with the certificate to sign it.
Trust lists connect the end-entity certificate that signed a manifest back to the originating root CA. This is accomplished by supplying the subordinate public X.509 certificates forming the trust chain (the public X.509 certificate chain). If those are not supplied, you can use a private credential store to validate the certificate trust chain. If you do not supply a certificate chain or trust list, validators may reject the manifest. See the C2PA specification for more details.
Signing a manifest
The simplest way to add a C2PA manifest to an asset file and sign it is by using C2PA Tool (c2patool). You can run C2PA Tool manually from the command line (for example, during development) and more generally from any executable program that can call out to the shell.
Similarly, using the Rust SDK, you can add a manifest to an asset file, referencing the certificate and private key file. The Node.js, Python, and C++/C libraries can also add and sign a manifest.
Accessing a private key and certificate directly from the file system is fine during development, but doing so in production is not secure. Instead use a Key Management Service (KMS) or a hardware security module (HSM) to access the certificate and key; For more information, see Using a certificate in production.
Example
Getting a certificate provides a general overview of getting a signing certificate from a certificate authority (CA).
Here is an example of getting signing credentials and then using them with C2PA Tool.
This example uses an inexpensive personal certificate, which is fine for development and testing, but in production, an enterprise certificate is strongly recommended. An enterprise certificate is required for Inspect tool on Adobe Content Authenticity (Beta) to display your organization name when for signed assets.
1. Purchase credentials
Follow the CA's instructions to purchase and download your .pfx file. This file is a PKCS12 container that holds your certificate chain and private signing key. Other certificate providers may have alternate ways of providing your private key and certificate and may include only the end-entity certificate and so you must manually download the rest of the certificate chain.
The rest of this tutorial uses OpenSSL (a set of cryptographic utilities). If OpenSSL is not installed on your system, see OpenSSL for the source distribution or the list of unofficial binary distributions.
2. Extract credentials
To work with the certificate, you need to extract it. When the CAI SDK adds Content Credentials to an asset, it incorporates the certificate (including the associated public key) into the manifest.
Use the commands below to extract the key and certificate chain. If prompted, enter the password that was used to generate the .pfx file.
Make sure you are using a recent version of OpenSSL.
Troubleshooting errors
In this step, OpenSSL may report errors when extracting the key or certificate chain. In many cases, if OpenSSL generates the output file, you can ignore the messages.
For example, the following error message means the .pfx was encrypted with an older standard:
Shrouded Keybag: pbeWithSHA1And3- KeyTripleDES-CBC, Iteration 2000
PKCS7 Encrypted data: pbeWithSHA1And40BitRC2- CBC, Iteration 2000
Error outputting keys and certificates
Extract the key
openssl pkcs12 -in mycertfile.pfx -nocerts -out mykey.pem -nodes
Check to make sure the above command generated a .pem file and it's not an empty file.
Extract certificate chain
For many certificate providers, the .pfx file contains not just your certificate but the complete certificate trust chain, which you can extract with a command like this:
openssl pkcs12 -in mycertfile.pfx -nokeys -out mycerts.pub
When the .pfx file does not contain the certificate chain, you can obtain it from your provider.
3. Determine signature algorithm
To use the credentials extracted above you must know the signature types they support. Typically, you'll already know this information or the certificate provider will provide it.
You can also enter this OpenSSL command to dump information about the public key used with the certificate:
openssl x509 -inform PEM -in mycerts.pub -text
Where mycerts.pub is the file containing the certificate chain from signing certificate to the last certificate before the root CA, concatenated.
This command produces a text summary of the certificate properties, as shown in the example below. Look for a line containing Public Key Algorithm since the public key indicates the signature algorithm used. See the table in Getting a certificate to determine the corresponding signature type.
In this example, Public Key Algorithm: rsassaPss corresponds to the "RSASSA-PSS with SHA-256" or sha256WithRSAEncryption signature algorithm. A example snippet of this output looks something like this:
Validity
Not Before: Jun 10 18:46:28 2022 GMT
Not After : Aug 26 18:46:28 2030 GMT
Subject: C=US, ST=CA, L=Somewhere, O=C2PA Test Signing Cert, OU=FOR TESTING_ONLY, CN=C2PA Signer
Subject Public Key Info:
Public Key Algorithm: rsassaPss
Public-Key: (4096 bit)
Modulus:
...
Exponent: 65537 (0x10001)
PSS parameter restrictions:
Hash Algorithm: SHA2-256
Mask Algorithm: MGF1 with SHA2-256
Minimum Salt Length: 32
Trailer Field: 0x1 (default)
4. Test with C2PA Tool
You now have all the needed information to configure C2PA Tool for manifest signing. Edit your manifest store file to add the following fields that are specific to C2PA Tool:
"alg": "ps256",
"private_key": "mykey.pem",
"sign_cert": "mycerts.pub"
The private_key and sign_cert properties must be full paths to the key and certificate chain files generated above.
You can now use C2PA Tool to add a manifest to an image or other asset file. The command will be something like this:
c2patool -m my_manifest.json -o signed_image.jpg my_image.jpg
The example above uses the information in my_manifest.json to add a new manifest to output signed_image.jpg using source my_image.jpg. The manifest will be signed using the PS256 signature algorithm with private key mykey.pem. The manifest will contain the trust chain specified in mycerts.pub.
This example accesses the private key and certificate directly from the file system, which is handy during development, but is not secure for production use. For more information, see Using a certificate in production.
4. Confirm it worked
Use C2PA Tool to confirm that you successfully signed the asset. Enter a command like this:
c2patool signed_image.jpg
This command displays the manifest attached to signed_image.jpg and should include a section such as this:
...
"signature_info": {
"cert_serial_number": "012345678901234567890123456789",
"time": "2023-11-02T17:18:14+00:00"
},
"label": "urn:uuid:0b9bc2b8-6d66-4258-9fed-694c30abcdef"
...
You can also use the Inspect tool on Adobe Content Authenticity (Beta) to confirm that your image was signed. If your certificate can't be traced back to a certificate on the C2PA trust list, it displays the message "The Content Credential issuer couldn't be recognized...."