YubiHSM 2: Backup and Restore

The YubiHSM 2 supports encrypted export and import of objects using a symmetric AES-CCM based scheme.

The examples below assume the default authentication key (0x0001). If you use some other authentication key make sure that it has the capability put-wrap-key and has the correct delegated capabilities, otherwise you will get a wrong permissions for operation error.

You can perform these operations using:

YubiHSM Shell for backing up and restoring
YubiHSM Setup for backing up and restoring
YubiHSM Key Storage Provider for backing up and restoring certificate as well as private key.

In all three cases, the process is done by taking the following steps:

Create a wrap key, call it wrapkey.
Import wrapkey into the primary YubiHSM2.
Export other objects in the primary YubiHSM2 using wrapkey.
Import wrapkey into the backup YubiHSM2.
Import the objects exported in step 3 into the backup YubiHSM2.

In order for a full backup to be successful, the following conditions need to be fulfilled (any object that does not fulfill these conditions is not exported):

wrapkey is accessible in all the domains the other objects are available in.
wrapkey has delegated capabilities that include all the capabilities any other object has.
wrapkey has the capabilities export-wrapped and import-wrapped.
All other objects have the capability exportable-under-wrap.

Backup and Restore Using YubiHSM Shell

Backup

For the purpose of this guide, we will start by generating an asymmetric key that we will then make a backup of.
```
$ yubihsm-shell -a generate-asymmetric-key -A rsa2048
 --capabilities exportable-under-wrap, sign-pkcs,
 decrypt-pkcs
...
Generated Asymmetric key 0x6e77
...
```
OBS: This will generate an asymmetric key accessible in all domains.
Start by getting a pseudo random number from the YubiHSM2 and store it in a file. This will be the wrap key.
```
$ yubihsm-shell -a get-pseudo-random --count=32
--out=wrap.key
```
Important

The file wrap.key here contains the Wrap Key loaded into your YubiHSM in clear text. It should therefore be considered sensitive.

Import wrap.key into the primary YubiHSM2.

...
yubihsm-shell -a put-wrap-key --capabilities
export-wrapped,import-wrapped --delegated=sign-pkcs,
decrypt-pkcs,exportable-under-wrap --in=wrap.key
...
Stored Wrap key 0xd581

OBS: This will import a wrap key accessible in all domains.

Make an encrypted backup of the Asymmetric Key 0x6e77 in the file key_6e77.yhw.

yubihsm-shell -a get-wrapped --wrap-id=0xd581
--object-id=0x6e77 -t asymmetric-key
--out=key_6e77.yhw

Restore

This assumes a fresh device where you want to restore the previously backed up key 0x6e77.

Import the wrap key into the backup YubiHSM2.

$ yubihsm-shell -a put-wrap-key -A aes256-ccm-wrap -c export-wrapped,
import-wrapped --delegated=sign-pkcs,decrypt-pkcs,exportable-under-wrap
--in=wrap.key -i 0xd581
...
Stored Wrap key 0xd581

Import the Asymmetric key 0x6e77 into the backup YubiHEM2.

yubihsm-shell -a put-wrapped --wrap-id=0xd581 --in=key_6e77.yhw
...
Object imported as 0x6e77 of type asymmetric-key

Backup and Restore Using YubiHSM Setup

The YubiHSM 2 Setup Tool can be used to backup and restore all exportable objects simultaneously.

Backup

OBS: This assumes that a wrap key fulfilling all the conditions mentioned above already exists in the primary YubiHSM2. For the following command line examples, we will assume that such a key has ObjectID 0xd581.

$ yubihsm-setup dump
Enter the wrapping key ID to use for exporting objects: 0xd581
...
Successfully exported object Asymmetric with ID 0x6e77 to ./0x6e77.yhw
All done

Note

When creating a wrap key using yubihsm-setup with the subcommand ksp or ejbca, an option is presented to split the wrap key into shares to be held by different custodians. It would also be possible to set the minimum number of custodians required to reconstruct the wrap key.

Important

Split and reconstruction of the wrap key is done in the software (yubihsm-setup). The YubiHSM2 itself is not aware of such split or any shares.

Restore

Running the store command will import all \*.yhw files in the current directory. If some of those files are not encrypted/wrapped with a wrap key that exists in the backup YubiHSM2, they will not be imported.

$ yubihsm-setup restore

Note

If the wrap key was split, the shares to reconstruct it will need to be provided in this step.

Backup and Restore Using YubiHSM KSP (Windows Only)

YubiHSM Key Storage Provider (KSP) enables backing up and restoring the keys managed using this tool.

Note

Microsoft Active Directory Certificate Services (ADCS) does not set the NCRYPT_ALLOW_EXPORT_FLAG when generating a key, either through the setup UI or the Install-ADCSCertificationAuthority PowerShell module.

When creating an ADCS root CA key using the YubiHSM 2, we add the exportable-under-wrap Capability by default. Backup and restore functionality is therefore available using the following manual processes.

Identify Your Private Key Container Name
Backup the Target Certificate
Backup the Target Private Key
Restore the Target Private Key
Restore the Target Certificate

Identify Your Private Key Container Name

To view the currently installed certificates in the Local Machine “My” store, open an elevated command prompt/shell by using the certutil command.

PS1> certutil -store My

Find the target certificate in the list and then find its Key Container property. The Provider property should be the same as YubiHSM Key Storage Provider.
To identify the certificate, record the Cert Hash property.

Backup the Target Certificate

Using any available means (certmgr.msc, PowerShell, certutil), export the target certificate, but without the private key in DER format.

Note

The YubiHSM does not provide a mechanism for returning the raw private key to Windows, so generating a PKCS#12 container is not currently possible.

For example, to export the certificate in .crt ``format to a file named ``<Cert Hash>.crt, use the command.

PS1> certutil -split -store My <Cert Hash>.

Backup the Target Private Key

Export the target private key with the label property equal to the Key Container property.

Use an Authentication Key with the export-wrapped capability set.
Use the instructions for exporting a private key under wrap via yubihsm-shell (see Backup and Restore Using YubiHSM Shell).

Restore the Target Private Key

Import the target private key file to your backup YubiHSM.

Use an Authentication Key with the import-wrapped capability set.
Use the instructions for importing a private key under wrap via yubihsm-shell (see Backup and Restore Using YubiHSM Shell).

The imported key object should have the same Label property as the original object.

Restore the Target Certificate

Before the certificate is imported to the local machine, it does not have an associated private key.

Move the target certificate file generated as per Backup and Restore Using YubiHSM Shell to the target machine by importing the certificate to the LocalMachine “My” store. Use your preferred method.
Re-associate the certificate to the private key by using the -repairstore functionality of certutil.
Verify that the target private key is visible via the YubiHSM KSP: list all private keys (and their corresponding container names - which are equal to the Label property in the YubiHSM visible to the current Authentication Key).

PS1> certutil -key -csp "YubiHSM Key Storage Provider"

Open an elevated prompt and execute the command:

PS1> certutil -repairstore MY <Cert Hash>

To verify that the certificate has been associated with the YubiHSM Key Storage Provider and has the correct Key Container property value, repeat the steps under Identify Your Private Key Container Name.