Code signing

Nextcloud supports code signing for the core releases, and for Nextcloud applications. Code signing gives our users an additional layer of security by ensuring that nobody other than authorized persons can push updates.

It also ensures that all upgrades have been executed properly, so that no files are left behind, and all old files are properly replaced. In the past, invalid updates were a significant source of errors when updating Nextcloud.

FAQ

Why did Nextcloud add code signing?

By supporting Code Signing we add another layer of security by ensuring that nobody other than authorized persons can push updates for applications, and ensuring proper upgrades.

Do we lock down Nextcloud?

The Nextcloud project is open source and always will be. We do not want to make it more difficult for our users to run Nextcloud. Any code signing errors on upgrades will not prevent Nextcloud from running, but will display a warning on the Admin page. For applications that are not tagged “Featured” the code signing process is optional.

Not open source anymore?

The Nextcloud project is open source and always will be. The code signing process is optional, though highly recommended. The code check for the core parts of Nextcloud is enabled when the Nextcloud release version branch has been set to stable.

For custom distributions of Nextcloud it is recommended to change the release version branch in version.php to something else than “stable”.

Is code signing mandatory for apps?

Code signing is required for all applications on apps.nextcloud.com.

Technical details

Nextcloud uses a X.509 based approach to handle authentication of code. Each Nextcloud release contains the certificate of a shipped Nextcloud Code Signing Root Authority. The private key of this certificate is only accessible to the project leader, who may grant trusted project members with a copy of this private key.

This Root Authority is only used for signing certificate signing requests (CSRs) for additional certificates. Certificates issued by the Root Authority must always be limited to a specific scope, usually the application identifier. This enforcement is done using the CN attribute of the certificate.

Code signing is then done by creating a signature.json file with the following content:

{
    "hashes": {
        "/filename.php":
        "2401fed2eea6f2c1027c482a633e8e25cd46701f811e2d2c10dc213fd95fa60e350b
        ccbbebdccc73a042b1a2799f673fbabadc783284cc288e4f1a1eacb74e3d",
        "/lib/base.php":
        "55548cc16b457cd74241990cc9d3b72b6335f2e5f45eee95171da024087d114fcbc2
        effc3d5818a6d5d55f2ae960ab39fd0414d0c542b72a3b9e08eb21206dd9"
    },
    "certificate": "-----BEGIN CERTIFICATE-----
    MIIBvTCCASagAwIBAgIUPvawyqJwCwYazcv7iz16TWxfeUMwDQYJKoZIhvcNAQEF\
    nBQAwIzEhMB8GA1UECgwYb3duQ2xvdWQgQ29kZSBTaWduaW5nIENBMB4XDTE1MTAx\
    nNDEzMTcxMFoXDTE2MTAxNDEzMTcxMFowEzERMA8GA1UEAwwIY29udGFjdHMwgZ8w\
    nDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBANoQesGdCW0L2L+a2xITYipixkScrIpB\
    nkX5Snu3fs45MscDb61xByjBSlFgR4QI6McoCipPw4SUr28EaExVvgPSvqUjYLGps\
    nfiv0Cvgquzbx/X3mUcdk9LcFo1uWGtrTfkuXSKX41PnJGTr6RQWGIBd1V52q1qbC\
    nJKkfzyeMeuQfAgMBAAEwDQYJKoZIhvcNAQEFBQADgYEAvF/KIhRMQ3tYTmgHWsiM\
    nwDMgIDb7iaHF0fS+/Nvo4PzoTO/trev6tMyjLbJ7hgdCpz/1sNzE11Cibf6V6dsz\
    njCE9invP368Xv0bTRObRqeSNsGogGl5ceAvR0c9BG+NRIKHcly3At3gLkS2791bC\
    niG+UxI/MNcWV0uJg9S63LF8=\n
    -----END CERTIFICATE-----",
    "signature": "U29tZVNpZ25lZERhdGFFeGFtcGxl"
}

hashes is an array of all files in the folder with their corresponding SHA-512 hashes. certificate is the certificate used for signing. It has to be issued by the Nextcloud Root Authority, and its CN needs to be permitted to perform the required action. The signature is then a signature of the hashes which can be verified using the certificate.

Having the certificate bundled within the signature.json file has the advantage that even if a developer loses their certificate, future updates can still be ensured by having a new certificate issued.

How code signing affects apps in the app store

Apps which have a Featured tag MUST be code signed. Unsigned Featured apps won’t be installable anymore.
Apps which have been signed in a previous release MUST be code-signed in all future releases as well, otherwise the update will be refused.

How to get your app signed

The following commands require that you have OpenSSL installed on your machine. Ensure that you keep all generated files to sign your application. The following examples will assume that you are trying to sign an application named “contacts”.

Generate a private key and CSR: openssl req -nodes -newkey rsa:4096 -keyout contacts.key -out contacts.csr -subj "/CN=contacts". Replace “contacts” with your application identifier.
Post the CSR at https://github.com/nextcloud/app-certificate-requests, in a new pull request with the link of a public repository with the code of your app. Make sure to keep the private key file (contacts.key) secret and not disclose it to any third parties.
Nextcloud will provide you with the signed certificate.
Run ./occ integrity:sign-app to sign your application, and specify your private and public keys as well as the path to the application. A valid example looks like: ./occ integrity:sign-app --privateKey=/Users/lukasreschke/contacts.key --certificate=/Users/lukasreschke/CA/contacts.crt --path=/Users/lukasreschke/Programming/contacts

The occ tool will store a signature.json file within the appinfo folder of your application. Then compress the application folder and upload it to apps.nextcloud.com. Be aware that doing any changes to the application after it has been signed requires another signing. So if you do not want to have some files shipped remove them before running the signing command.

In case you lose your certificate please submit a new CSR as described above and mention that you have lost the previous one. Nextcloud will revoke the old certificate.

If you maintain an app together with multiple people it is recommended to designate a release manager responsible for the signing process as well as the uploading to apps.nextcloud.com. If there are cases where this is not feasible and multiple certificates are required Nextcloud can create them on a case by case basis. We do not recommend developers to share their private key.

Errors

The following errors can be encountered when trying to verify a code signature. For information about how to get access to those results please refer to the Issues section of the Nextcloud Server Administration manual.

INVALID_HASH
- The file has a different hash than specified within signature.json. This usually happens when the file has been modified after writing the signature data.
MISSING_FILE
- The file cannot be found but has been specified within signature.json. Either a required file has been left out, or signature.json needs to be edited.
EXTRA_FILE
- The file does not exist in signature.json. This usually happens when a file has been removed and signature.json has not been updated.
EXCEPTION
- Another exception has prevented the code verification. There are currently these following exceptions:
  - Signature data not found.
    - The app has mandatory code signing enforced but no signature.json file has been found in its appinfo folder.
  - Certificate is not valid.
    - The certificate has not been issued by the official Nextcloud Code Signing Root Authority.
  - Certificate is not valid for required scope. (Requested: %s, current: %s)
    - The certificate is not valid for the defined application. Certificates are only valid for the defined app identifier and cannot be used for others.
  - Signature could not get verified.
    - There was a problem with verifying the signature of signature.json.