Signing and Verifying Containers

Singularity 3.0 introduced the ability to create and manage PGP keys and use them to sign and verify containers. This provides a trusted method for Singularity users to share containers. It ensures a bit-for-bit reproduction of the original container as the author intended it.

Note

Singularity 3.6.0 uses a new signature format. Containers signed by 3.6.0 cannot be verifed by older versions of Singularity.

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

Verifying containers from the Container Library

The verify command will allow you to verify that a container has been signed using a PGP key. To use this feature with images that you pull from the container library, you must first generate an access token to the Sylabs Cloud. If you don’t already have a valid access token, follow these steps:

  1. Go to: https://cloud.sylabs.io/

  2. Click “Sign in to Sylabs” and follow the sign in steps.

  3. Click on your login id (same and updated button as the Sign in one).

  4. Select “Access Tokens” from the drop down menu.

  5. Enter a name for your new access token, such as “test token”

  6. Click the “Create a New Access Token” button.

  7. Click “Copy token to Clipboard” from the “New API Token” page.

  8. Run singularity remote login and paste the access token at the prompt.

Now you can verify containers that you pull from the library, ensuring they are bit-for-bit reproductions of the original image.

$ singularity verify alpine_latest.sif

Container is signed by 1 key(s):

Verifying partition: FS:
8883491F4268F173C6E5DC49EDECE4F3F38D871E
[REMOTE]  Sylabs Admin <support@sylabs.io>
[OK]      Data integrity verified

INFO:    Container verified: alpine_latest.sif

In this example you can see that Sylabs Admin has signed the container.

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, Singularity 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:.

$ singularity key newpair

Enter your name (e.g., John Doe) : David Trudgian
Enter your email address (e.g., john.doe@example.com) : david.trudgian@sylabs.io
Enter optional comment (e.g., development keys) : demo
Enter a passphrase :
Retype your passphrase :
Would you like to push it to the keystore? [Y,n] Y
Generating Entity and OpenPGP Key Pair... done
Key successfully pushed to: https://keys.sylabs.io

Note that I chose Y when asked if I wanted to push my key to the keystore. This will push my public key to whichever keystore has been configured by the singularity remote command, so that it can be retrieved by other users running singularity verify. If you do not wish to push your public key, say n during the newpair process.

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

$ singularity key list

Public key listing (/home/dave/.singularity/sypgp/pgp-public):

0) U: David Trudgian (demo) <david.trudgian@sylabs.io>
   C: 2019-11-15 09:54:54 -0600 CST
   F: E5F780B2C22F59DF748524B435C3844412EE233B
   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 chose not to push your key to the keystore during the newpair process, but later wish to, you can push it to a keystore configured using singularity remote like so:

$ singularity key push E5F780B2C22F59DF748524B435C3844412EE233B

public key `E5F780B2C22F59DF748524B435C3844412EE233B` pushed to server successfully

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

$ singularity key search Trudgian

Showing 1 results

KEY ID    BITS  NAME/EMAIL
12EE233B  4096  David Trudgian (demo) <david.trudgian@sylabs.io>

$ singularity key pull 12EE233B

1 key(s) added to keyring of trust /home/dave/.singularity/sypgp/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

Singularity 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:
$ singularity key search 0x8883491F4268F173C6E5DC49EDECE4F3F38D871E

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

# search for user:
$ singularity key search Godlove

# search for email:
$ singularity 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:

$ singularity 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 Keystore.

$ singularity verify my_container.sif
Verifying image: my_container.sif
[LOCAL]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[LOCAL]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |FS
Container verified: my_container.sif

If you’ve pushed your key to the Keystore 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.

$ singularity key remove E5F780B2C22F59DF748524B435C3844412EE233B

$ singularity verify my_container.sif
Verifying image: my_container.sif
[REMOTE]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[REMOTE]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |FS
Container verified: my_container.sif

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

$ singularity key pull E5F780B2C22F59DF748524B435C3844412EE233B

1 key(s) added to keyring of trust /home/dave/.singularity/sypgp/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.

$ singularity sif list my_container.sif

Container id: e455d2ae-7f0b-4c79-b3ef-315a4913d76a
Created on:   2019-11-15 10:11:58 -0600 CST
Modified on:  2019-11-15 10:11:58 -0600 CST
----------------------------------------------------
Descriptor list:
ID   |GROUP   |LINK    |SIF POSITION (start-end)  |TYPE
------------------------------------------------------------------------------
1    |1       |NONE    |32768-32800               |Def.FILE
2    |1       |NONE    |36864-36961               |JSON.Generic
3    |1       |NONE    |40960-25890816            |FS (Squashfs/*System/amd64)

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

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

$ singularity verify --sif-id 1 my_container.sif
Verifying image: my_container.sif
[LOCAL]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[LOCAL]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448
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.

$ singularity verify my_container.sif
Verifying image: my_container.sif
[LOCAL]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[LOCAL]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448

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.

$ singularity sign --groupid 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.

$ singularity verify --group-id 1 my_container.sif
Verifying image: my_container.sif
[LOCAL]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[LOCAL]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |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:

$ singularity verify my_container.sif
Verifying image: my_container.sif
[LOCAL]   Signing entity: David Trudgian (Demo keys) <david.trudgian@sylabs.io>
[LOCAL]   Fingerprint: 65833F473098C6215E750B3BDFD69E5CEE85D448
Objects verified:
ID  |GROUP   |LINK    |TYPE
------------------------------------------------
1   |1       |NONE    |Def.FILE
2   |1       |NONE    |JSON.Generic
3   |1       |NONE    |FS
Container verified: my_container.sif