AWS News Blog – New – Code Signing, a Trust and Integrity Control for AWS Lambda

Code signing is an industry standard technique used to confirm that the code is unaltered and from a trusted publisher. Code running inside AWS Lambda functions is executed on highly hardened systems and runs in a secure manner. However, function code is susceptible to alteration as it moves through deployment pipelines that run outside AWS.

Today, we are launching Code Signing for AWS Lambda. It is a trust and integrity control that helps administrators enforce that only signed code packages from trusted publishers run in their Lambda functions and that the code has not been altered since signing.

Code Signing for Lambda provides a first-class mechanism to enforce that only trusted code is deployed in Lambda. This frees up organizations from the burden of building gatekeeper components in their deployment pipelines. Code Signing for AWS Lambda leverages AWS Signer, a fully managed code signing service from AWS. Administrators create Signing Profile, a resource in AWS Signer that is used for creating signatures and grant developers access to the signing profile using AWS Identity and Access Management (IAM). Within Lambda, administrators specify the allowed signing profiles using a new resource called Code Signing Configuration (CSC). CSC enables organizations to implement a separation of duties between administrators and developers. Administrators can use CSC to set code signing policies on the functions, and developers can deploy code to the functions.

How to Create a Signing Profile
You can use AWS Signer console to create a new Signing profile. A signing profile can represent a group of trusted publishers and is analogous to the use of a digital signing certificate.

By clicking Create Signing Profile, you can create a Signing Profile that can be used to create signed code packages.

You can assign Signature validity period for the signatures generated by a Signing Profile between 1 day and 135 months.

How to create a Code Signing Configuration (CSC)
You can configure your functions to use Code Signing through the AWS Lambda console, Command-line Interface (CLI), or APIs by creating and attaching a new resource called Code Signing Configuration to the function. You can find Code signing configurations under Additional resources menu.

You can click Create configuration to define signing profiles that are allowed to sign code artifacts for this configuration, and set signature validation policy. To add an allowed signing profile, you can either select from the dropdown, which shows all signing profiles in your AWS account, or add a signing profile from a different account by specifying the version ARN.

Also, you can set the signature validation policy to either ‘Warn’ or ‘Enforce’. With ‘Warn’, Lambda logs a Cloudwatch metric if there is a signature check failure but accepts the deployment. With ‘Enforce’, Lambda rejects the deployment if there is a signature check failure. Signature check fails if the signature signing profile does not match one of the allowed signing profiles in the CSC, the signature is expired, or the signature is revoked. If the code package is tampered or altered since signing, the deployment is always rejected, irrespective of the signature validation policy.

You can use new Lambda API CreateCodeSigningConfig to create a CSC too. You can see the JSON request syntax below.

{
     "CodeSigningConfigId": string,
     "CodeSigningConfigArn": string,
     "Description": string,
     "AllowedPublishers": {
           "SigningProfileVersionArns": [string]
      },
     "CodeSigningPolicies": {
     "UntrustedArtifactOnDeployment": string,   // WARN OR ENFORCE
    },
     "LastModified”: string
}

Let’s Enable Code Signing for Your Lambda Functions
To enable Code Signing feature for your Lambda functions, you can select a function and click Edit in Code signing configuration section.

Select one of the available CSCs and click the Save button.

Once your function is configured to use code signing, you need to upload signed .zip file or Amazon S3 URL of a signed .zip made by a signing job in AWS Signer.

How to Create a Signed Code Package
Choose one of the allowed signing profiles and specify the S3 location of the code package ZIP file to be signed. Also, specify a destination path where the signed code package should be uploaded.

A signing job is an asynchronous process that generates a signature for your code package and puts the signed code package in the specified destination path.

Once signing job is succeeded, you can find signed ZIP packages in your assigned S3 bucket.

Back to Lambda console, you can now publish the signed code package to the Lambda function. Lambda will perform signature checks to verify that the code has not been altered since signing and that the code is signed by one of the allowed signing profile.

You can also enable code signing for a function using CreateFunction or PutFunctionCodeSigningConfig APIs by attaching a CSC to the function.

Developers can also use SAM CLI to sign code packages. They do this by specifying the signing profiles at package or deploy stage. SAM CLI automatically starts the signing workflow before deploying the code to Lambda.

Code Signing is also supported by Infrastructure as code tools like AWS CloudFormation and Terraform. Terraform also allows developers to sign code, in addition to declaring and creating code signing resources.

Now Available
Code Signing for AWS Lambda is available in all commercial regions except AWS China Regions, AWS GovCloud (US) Regions, and Asia Pacific (Osaka) Region. There is no additional charge for using code signing, and customers pay the standard price for Lambda functions.