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Content Addressable Storage (CAS) backend

As part of an attestation process, you might want to collect different pieces of evidence such as Software Bill Of Materials (SBOMs), test results, runner logs, etc and then attach them to the final in-toto attestation.

Chainloop helps with this process by providing a Content Addressable Storage API proxy that:

  • Abstracts away the underlying storage backend. Currently, we support OCI registries as storage backends but you can expect blob storage, Artifactory and other storage backends to be supported in the future.
  • Makes sure that the pieces of evidence are stored in a tamper-proof manner. This is achieved by storing the evidences named after their SHA256 content digest, which is calculated by the client, verified by the CAS server.
  • Enables support of large pieces of evidence since the content digest reference is what will be stored in the attestation.

Manage backends

You can setup as many CAS backends as you want, but you can only have one enabled as default at the time. This default backend will be used during the attestation process to store the pieces of evidence.

In Chainloop, CAS backends can be managed with the chainloop cas-backend command.

$ chainloop cas-backend ls
│ LOCATION                        │ PROVIDER │ DESCRIPTION                         │ LIMITS        │ DEFAULT │
│                                 │ INLINE   │ Embed artifacts content in the atte │ MaxSize: 500K │ false   │
│                                 │          │ station (fallback)                  │               │         │
│ │ OCI      │                                     │ MaxSize: 100M │ true    │

Backend providers

New CAS Backends will be added over time. If yours is not implemented yet, please let us know

Inline (fallback)

Chainloop comes pre-configured with what we call an inline backend.

The inline backend embeds the pieces of evidence in the resulting attestations. This is useful to get started quickly but since the metadata is embedded in the attestation, its max size is limited.

We recommend that once you get closer to a production-ready setup, you switch to a more robust backend such as an OCI registry.

OCI registry

Add a new OCI registry backend

  # Using json-based service account

  $ chainloop cas-backend add oci \
    # i.e
    --repo [region][my-project]/[my-repository] \
    --username _json_key \
    --password "$(cat service-account.json)" \

Rotate credentials

chainloop cas-backend update oci --id [BACKEND_ID] --username [NEW_USERNAME] --password [NEW_PASSWORD]

Set as default

chainloop cas-backend update oci --id [BACKEND_ID] --default=true


Chainloop also supports storing artifacts in AWS S3 Blob Storage.


To connect your AWS account to Chainloop you'll need:

  • S3 Bucket Name
  • Bucket Region
  • AccessKeyID
  • SecretAccessKey

Create an S3 bucket

Create an S3 bucket and take note of the bucket name and region

Create an IAM user with access to that bucket

Next we are going to create a policy that has write/read permissions to the bucket.

You can use the snippet below by just replacing [bucketName] with the actual name of the bucket you created in the step before.

	"Version": "2012-10-17",
	"Statement": [
			"Effect": "Allow",
			"Action": [
			"Resource": "arn:aws:s3:::[bucketName]"
			"Effect": "Allow",
			"Action": [
			"Resource": "arn:aws:s3:::[bucketName]/*"

Then create an user, attach the policy to it and click on "create access Key"

Then select third-party service and copy the access key ID and secret access key

We are now ready to connect our AWS account to Chainloop

 $ chainloop cas-backend add aws-s3 \
    --access-key-id [accessKeyID] \
    --secret-access-key [secretAccessKey] \
    --region [region] \
    --bucket [bucketName]

Rotate credentials

chainloop cas-backend update aws-s3 --id [BACKEND_ID] --access-key-id [new-accessKeyID] --secret-access-key [new-secretAccessKey] --region [new-region]

Azure Blob Storage

Chainloop also supports storing artifacts in Azure Blob Storage.


To connect your Azure storage account you'll need the following information

  • Active Directory Tenant ID
  • Service Principal ID
  • Service Principal Secret
  • Storage account name

We'll walk you through the process of how to find this information

Register an application to create the service principal

First, you'll need to register an application in your Azure Active Directory tenant. You can do this using the Azure CLI or from the Azure portal

Once done, in the application overview you should be able to find the tenantID, and Service principal ID

Next, let's create a secret for the service principal

Create a storage account and give permissions to the service principal

Next, we'll create a storage account (or you can use an existing one), take a note on the storage account name.

And once created, we'll give permissions to the service principal, go to IAM assign-roles.

Search for the application we just registered and assign the Storage Blob Data Contributor role

At thi point we have all the information we need to connect our Azure storage account to Chainloop

 $ chainloop cas-backend add azure-blob \
    --client-id [servicePrincipalID] \
    --client-secret [servicePrincipalSecret] \
    --tenant [Active directory tenant] \
    --storage-account [Storage Account name] \
    --container [optional Storage account container]

Rotate credentials

chainloop cas-backend update azure-blob --id [BACKEND_ID] --client-id [new-clientID] --client-secret [new secret] --tenant [updated tenant]

Give it a try

If everything went well, you should be able to upload and download artifact materials, let's give it a try

$ chainloop artifact upload -f myfile
myfile@sha256:c5cc0a2c712497c29f29c3ba11e7fcc0c3cc725ab591720db595e5d6469f3f37 ... done! [1.03KB in 0s; 5.48KB/s]
$ chainloop artifact download -d sha256:c5cc0a2c712497c29f29c3ba11e7fcc0c3cc725ab591720db595e5d6469f3f37
INF downloading file name=myfile to=/tmp/myfile
INF file downloaded! path=/tmp/myfile