Signing and Verifying Containers

Apptainer has the ability to create and manage PGP keys and use them to sign and verify containers. This provides a trusted method for Apptainer users to share containers. It ensures a bit-for-bit reproduction of the original container as the author intended it.

Note

To verify containers signed with Singularity versions older than 3.6.0 the --legacy-insecure flag must be provided to the apptainer verify command.

Verifying containers from the remote sources

The verify command will allow you to verify that a container has been signed using a PGP key. This ensures that the container image on your disk is a bit-for-bit reproduction of the original image.

$ apptainer verify alpine_latest.sif

[REMOTE]  Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[REMOTE]  Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |JSON.Generic
4   |1       |NONE    |FS
Container verified: my_container.sif

In this example you can see that Ian Kaneshiro has signed the container.

This feature is available with SIF images like those you can pull from container libraries or OCI registries via oras://.

Signing your own containers

Generating and managing PGP keys

To sign your own containers you first need to generate one or more keys.

If you attempt to sign a container before you have generated any keys, Apptainer will guide you through the interactive process of creating a new key. Or you can use the newpair subcommand in the key command group like so:

$ apptainer key newpair

Enter your name (e.g., John Doe) : Ian Kaneshiro
Enter your email address (e.g., john.doe@example.com) : ikaneshiro@apptainer.org
Enter optional comment (e.g., development keys) : example key
Enter a passphrase :
Retype your passphrase :
Generating Entity and OpenPGP Key Pair... done

The list subcommand will show you all of the keys you have created or saved locally.`

$ apptainer key list

Public key listing (/home/ian/.apptainer/keys/pgp-public):

0) U: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
   C: 2022-02-23 15:12:19 -0800 PST
   F: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
   L: 4096
   --------

In the output above the index of my key is 0 and the letters stand for the following:

  • U: User

  • C: Creation date and time

  • F: Fingerprint

  • L: Key length

If you would like others in the community to easily be able to fetch your public key for image verification, you can push your key to a public keyserver.

First we can check which key server we have configured using:

$ apptainer remote list

Cloud Services Endpoints
========================

NAME           URI                  ACTIVE  GLOBAL  EXCLUSIVE
DefaultRemote  cloud.apptainer.org  YES     YES     NO

Keyservers
==========

URI                       GLOBAL  INSECURE  ORDER
https://keys.openpgp.org  YES     NO        1*

* Active cloud services keyserver

Authenticated Logins
=================================

URI             INSECURE
oras://ghcr.io  NO

Here we can see that we will be pushing to https://keys.openpgp.org. Now we can use the following command to push our key:

$ apptainer key push 8232570480B868E1473AEEB03DBCBA1EE9D661E5

public key `8232570480B868E1473AEEB03DBCBA1EE9D661E5' pushed to server successfully

If you delete your local public PGP key, you can always locate and download it again like so.

$ apptainer key search --long-list ikaneshiro@apptainer.org

Showing 1 results

FINGERPRINT                               ALGORITHM  BITS  CREATION DATE                  EXPIRATION DATE  STATUS     NAME/EMAIL
8232570480B868E1473AEEB03DBCBA1EE9D661E5  RSA        4096  2022-02-23 15:12:19 -0800 PST                   [enabled]  Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>

$ apptainer key pull 8232570480B868E1473AEEB03DBCBA1EE9D661E5
1 key(s) added to keyring of trust /home/ian/.apptainer/keys/pgp-public

But note that this only restores the public key (used for verifying) to your local machine and does not restore the private key (used for signing).

Searching for keys

Apptainer allows you to search the keystore for public keys. You can search for names, emails, and fingerprints (key IDs). When searching for a fingerprint, you need to use 0x before the fingerprint, check the example:

# search for key ID:
$ apptainer key search 0x8883491F4268F173C6E5DC49EDECE4F3F38D871E

# search for the sort ID:
$ apptainer key search 0xF38D871E

# search for user:
$ apptainer key search Godlove

# search for email:
$ apptainer key search @gmail.com

Signing and validating your own containers

Now that you have a key generated, you can use it to sign images like so:

$ apptainer sign my_container.sif

Signing image: my_container.sif
Enter key passphrase :
Signature created and applied to my_container.sif

Because your public PGP key is saved locally you can verify the image without needing to contact the key server.

$ apptainer verify my_container.sif

Verifying image: my_container.sif
[LOCAL]   Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[LOCAL]   Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |JSON.Generic
4   |1       |NONE    |FS
Container verified: my_container.sif

If you’ve pushed your key to a key server you can also verify this image in the absence of a local public key. To demonstrate this, first remove your local public key, and then try to use the verify command again.

$ apptainer key remove 8232570480B868E1473AEEB03DBCBA1EE9D661E5

$ apptainer verify my_container.sif

Verifying image: my_container.sif
[REMOTE]  Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[REMOTE]  Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |JSON.Generic
4   |1       |NONE    |FS
Container verified: my_container.sif

Note that the [REMOTE] message shows the key used for verification was obtained from a key server, and is not present on your local computer. You can retrieve it, so that you can verify even if you are offline with apptainer key pull

$ apptainer key pull 8232570480B868E1473AEEB03DBCBA1EE9D661E5

1 key(s) added to keyring of trust /home/ian/.apptainer/keys/pgp-public

Advanced Signing - SIF IDs and Groups

As well as the default behaviour, which signs all objects, fine-grained control of signing is possible.

If you sif list a SIF file you will see it is comprised of a number of objects. Each object has an ID, and belongs to a GROUP.

$ apptainer sif list my_container.sif

------------------------------------------------------------------------------
ID   |GROUP   |LINK    |SIF POSITION (start-end)  |TYPE
------------------------------------------------------------------------------
1    |1       |NONE    |32768-32800               |Def.FILE
2    |1       |NONE    |36864-39751               |JSON.Generic
3    |1       |NONE    |40960-41055               |JSON.Generic
4    |1       |NONE    |45056-2781184             |FS (Squashfs/*System/amd64)
5    |NONE    |1   (G) |2781184-2782981           |Signature (SHA-256)

I can choose to sign and verify a specific object with the --sif-id option to sign and verify.

$ apptainer sign --sif-id 1 my_container.sif

Signing image: my_container.sif
Enter key passphrase :
Signature created and applied to my_container.sif

$ apptainer verify --sif-id 1 my_container.sif

Verifying image: my_container.sif
[LOCAL]   Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[LOCAL]   Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
Container verified: my_container.sif

Note that running the verify command without specifying the specific sif-id gives a fatal error. The container is not considered verified as whole because other objects could have been changed without my knowledge.

$ apptainer verify my_container.sif

Verifying image: my_container.sif
[LOCAL]   Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[LOCAL]   Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5

Error encountered during signature verification: object 2: object not signed
FATAL:   Failed to verify container: integrity: object 2: object not signed

I can sign a group of objects with the --group-id option to sign.

$ apptainer sign --group-id 1 my_container.sif
Signing image: my_container.sif
Enter key passphrase :
Signature created and applied to my_container.sif

This creates one signature over all objects in the group. I can verify that nothing in the group has been modified by running verify with the same --group-id option.

$ apptainer verify --group-id 1 my_container.sif

Verifying image: my_container.sif
[LOCAL]   Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[LOCAL]   Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |JSON.Generic
4   |1       |NONE    |FS
Container verified: my_container.sif

Because every object in the SIF file is within the signed group 1 the entire container is signed, and the default verify behavior without specifying --group-id can also verify the container:

$ apptainer verify my_container.sif

Verifying image: my_container.sif
[LOCAL]   Signing entity: Ian Kaneshiro (example key) <ikaneshiro@apptainer.org>
[LOCAL]   Fingerprint: 8232570480B868E1473AEEB03DBCBA1EE9D661E5
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |JSON.Generic
4   |1       |NONE    |FS
Container verified: my_container.sif