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YubiHSM 2 + Tendermint KMS

The YubiHSM 2 from Yubico is a relatively low-cost solution for online key storage featuring support for random key generation, encrypted backup and export, and audit logging.

This document describes how to configure a YubiHSM 2 for production use with Tendermint KMS.

Compiling tmkms with YubiHSM support

Please see the toplevel README.md for prerequisites for compiling tmkms from source code. You will need: Rust (stable, 1.35+), a C compiler, pkg-config, and libusb (1.0+) installed.

There are two ways to install tmkms with YubiHSM 2 support, and in either case, you will need to pass the --features=yubihsm parameter to enable YubiHSM 2 support:

Compiling from source code (via git)

tmkms can be compiled directly from the git repository source code using the following method.

$ git clone https://github.com/tendermint/kms.git && cd kms
[...]
$ cargo build --release --features=yubihsm

If successful, this will produce a tmkms executable located at ./target/release/tmkms

Installing with the cargo install command

With Rust (1.31+) installed, you can install tmkms with the following:

cargo install tmkms --features=yubihsm

Or to install a specific version (recommended):

cargo install tmkms --features=yubihsm --version=0.4.0

This command installs tmkms directly from packages hosted on Rust's crates.io service. Package authenticity is verified via the crates.io index (itself a git repository) and by SHA-256 digests of released artifacts.

However, if newer dependencies are available, it may use newer versions besides the ones which are "locked" in the source code repository. We cannot verify those dependencies do not contain malicious code. If you would like to ensure the dependencies in use are identical to the main repository, please build from source code instead.

Verifying YubiHSM support was included in a build

Run the following subcommand of the resulting tmkms executable to ensure that YubiHSM 2 support was compiled-in successfully:

$ tmkms yubihsm help                                                                           127 ↵
tmkms 0.4.0
Tony Arcieri <tony@iqlusion.io>, Ismail Khoffi <Ismail.Khoffi@gmail.com>
Tendermint Key Management System

USAGE:
  tmkms <SUBCOMMAND>

FLAGS:
  -h, --help     Prints help information
  -V, --version  Prints version information

SUBCOMMANDS:
  detect  detect all YubiHSM2 devices connected via USB
  help    show help for the 'yubihsm' subcommand
  keys    key management subcommands
  setup   initial device setup and configuration
  test    perform a signing test

If detect, help, keys, setup etc are listed under SUBCOMMANDS then the build was successfully configured with YubiHSM 2 support.

udev configuration

On Linux, you will need to grant tmkms access to the YubiHSM 2 using rules for the udev subsystem. Otherwise, you'll get an error like this:

$ tmkms yubihsm detect
error: couldn't detect USB devices: USB error: USB(bus=1,addr=4):
       error opening device: Access denied (insufficient permissions)

You'll need to create a POSIX group, e.g. yubihsm which is allowed to access the YubiHSM2, and then add the following rules file under the /etc/udev/rules.d directory, e.g. /etc/udev/rules.d/10-yubihsm.rules:

SUBSYSTEMS=="usb", ATTRS{product}=="YubiHSM", GROUP=="yubihsm"

Note that creating this file does not have an immediate effect: you'll need to reload the udev subsystem, either by rebooting or running the following command:

$ udevadm control --reload-rules && udevadm trigger

For the rules above to apply, make sure you run tmkms as a user which is a member of the yubihsm group!

yubihsm-server feature

The YubiHSM backend includes additional option support for running a local HTTP server which is compatible with Yubico's yubihsm-connector service. This allows administration of YubiHSMs via yubihsm-shell or via tmkms yubihsm while the KMS is running (i.e. communicating with the YubiHSM2 via USB).

To enable support for this feature, enable it when you compile Tendermint KMS, either via:

$ cargo build --release --features=yubihsm-server

or if you're using cargo install:

$ cargo install tmkms --features=yubihsm-server

To enable the HTTP service, you'll need to add connector_server configuration to the [[providers.yubihsm]] section of tmkms.toml:

[[providers.yubihsm]]
adapter = { type = "usb" }
connector_server = { laddr = "tcp://127.0.0.1:12345" }

This will start an HTTP server on tcp://127.0.0.1:12345 which is compatible with Yubico's yubihsm-server. You can now use this with yubihsm-shell, e.g.:

$ yubihsm-shell
Using default connector URL: http://127.0.0.1:12345
yubihsm> connect
Session keepalive set up to run every 15 seconds
yubihsm> session open 2
Enter password:

Using yubihsm-server with tmkms yubihsm CLI commands

You can also configure tmkms yubihsm subcommands to connect via this HTTP server rather than USB, which allows for using them while the KMS is running. To do that, add a cli subsection to connector_server line to the following:

connector_server = { laddr = "tcp://127.0.0.1:12345", cli = { auth_key = 2 } }

The cli = { ... } subsection indicates that the connector_server should be used by all tmkms yubihsm subcommands. It can contain the following additional fields:

auth_key

An alternative authentication key to use (e.g. 2 for the operator role key provisioned by the "Production YubiHSM 2 setup" process documented below).

When used, this will prompt for a password to use as opposed to using the one in the configuration file:

$ tmkms yubihsm keys list
Enter password for YubiHSM2 auth key 0x0002:
Listing keys in YubiHSM #0123456789:
- 0x0001: ...

Production YubiHSM 2 setup

tmkms contains built-in support for fully automated production YubiHSM 2 setup, including deterministically generating authentication keys and backup encryption keys from a BIP39 24-word seed phrase. This allows new YubiHSM 2s to be provisioned with the same initial set of keys from the phrase alone, while also creating multiple "roles" within the HSM.

Alternatively Yubico provides the yubihsm-setup tool, however the setup process internal to tmkms provides a "happy path" for Tendermint validator usage, and also operational processes which should be familiar to cryptocurrency users.

Configuring tmkms for initial setup

In order to perform setup, tmkms needs a minimal configuration file which contains the credentials needed to authenticate to the HSM with an administrator key.

This configuration should be placed in a file called: tmkms.toml. By default tmkms will look for this file in the current working directory, however most subcommands take a -c /path/to/tmkms.toml argument if you would like to place it somewhere else.

Here is an example tmkms.toml file which can be used for initial setup with a YubiHSM 2 which is still configured with its default authentication keys (i.e. authentication key 1, with a default password of password):

[[providers.yubihsm]]
adapter = { type = "usb" }
auth = { key = 1, password = "password" }

If you have changed the default authentication key ID and/or password, you will need to provide the correct credentials.

NOTE: if you have lost or forgotten the admin authentication key, you can factory reset the YubiHSM 2 to a default state (wiping all keys) by pushing down on the top (LED) immediately after inserting it and continuing to push down on it for 10 seconds.

tmkms yubihsm setup: Initial YubiHSM setup

WARNING: THIS PROCESS PERFORMS A FACTORY RESET OF THE YUBIHSM, DELETING ALL EXISTING KEYS AND REPLACING THEM WITH NEW ONES. MAKE SURE YOU HAVE MADE BACKUPS OF IMPORTANT KEYS BEFORE PROCEEDING!!!

After configuring your YubiHSM 2's credentials in tmkms.toml, you can run the following command to perform automatic setup:

$ tmkms yubihsm setup

We recommend this process be performed on an airgapped computer which is not connected to any network. It will generate master secrets which can be used to decrypt encrypted backups of keys within the HSM.

This process will perform the following steps:

  • Generate a random 24-word recovery mnemonic phrase from randomness taken from the host OS as well as the YubiHSM2 itself.
  • Deterministically (ala BIP32) generate authentication keys for the following four roles within the YubiHSM (with yubihsm-shell compatible passwords):
    • admin (authentication key 0x0001): full access to all HSM capabilities
    • operator (authentication key 0x0002): ability to generate new signing keys, export/import encrypted backups of keys, and view the audit log
    • auditor (authentication key 0x0003): ability to view and consume the audit log
    • validator (authentication key 0x0004): ability to generate signatures using signing keys loaded within the device
  • Deterministically generate "wrap key" 0x0001: symmetric encryption key (AES-CCM) used for making encrypted backups of other keys generated within the device. If you have existing keys you would like to transfer out of other YubiHSM 2s, this key can be imported into those HSMs in order to export encrypted backups (see below).

Notably different from cryptocurrency hardware wallets: this process does not actually generate any signing keys, only authentication keys and the wrap key. Generating validator signing keys (and creating backups) occurs in a subsequent step (see below).

The following is example output from running the above command:

$ tmkms yubihsm setup
This process will *ERASE* the configured YubiHSM2 and reinitialize it:

- YubiHSM serial: 9876543210

Authentication keys with the following IDs and passwords will be created:

- key 0x0001: admin:

    double section release consider diet pilot flip shell mother alone what fantasy
    much answer lottery crew nut reopen stereo square popular addict just animal

- authkey 0x0002 [operator]:  kms-operator-password-1k02vtxh4ggxct5tngncc33rk9yy5yjhk
- authkey 0x0003 [auditor]:   kms-auditor-password-1s0ynq69ezavnqgq84p0rkhxvkqm54ks9
- authkey 0x0004 [validator]: kms-validator-password-1x4anf3n8vqkzm0klrwljhcx72sankcw0
- wrapkey 0x0001 [primary]:   21a6ca8cfd5dbe9c26320b5c4935ff1e63b9ab54e2dfe24f66677aba8852be13

*** Are you SURE you want erase and reinitialize this HSM? (y/N):

NOTE: the admin password is displayed on two separate lines. When using it from yubihsm-shell or as the password field in tmkms.toml, it is not split across multiple lines and is separated only by a single space between words.

If you are certain you are ready to initialize your first YubiHSM 2, type y to proceed:

*** Are you SURE you want erase and reinitialize this HSM? (y/N): y
21:08:09 [WARN] factory resetting HSM device! all data will be lost!
21:08:10 [INFO] waiting for device reset to complete
21:08:11 [INFO] installed temporary setup authentication key into slot 65534
21:08:11 [WARN] deleting default authentication key from slot 1
21:08:11 [INFO] installing role: admin:2019-03-05T20:31:07Z
21:08:11 [INFO] installing role: operator:2019-03-05T20:31:08Z
21:08:11 [INFO] installing role: auditor:2019-03-05T20:31:08Z
21:08:11 [INFO] installing role: validator:2019-03-05T20:31:08Z
21:08:11 [INFO] installing wrap key: primary:2019-03-05T20:31:08Z
21:08:11 [INFO] storing provisioning report in opaque object 0xfffe
21:08:11 [WARN] deleting temporary setup authentication key from slot 65534
     Success reinitialized YubiHSM (serial: 9876543210)

Make sure to write down the 24-word recovery phrase and store it in a secure location!

Initializing additional HSMs from an existing 24-word recovery phrase

After initializing your first HSM, you can bootstrap additional YubiHSM 2s as a clone of the initial one using the same 24-word recovery phrase. To do that, pass the -r (or --restore) flag when running the setup command:

$ tmkms yubihsm setup -r
Restoring and reprovisioning YubiHSM from existing 24-word mnemonic phrase.

*** Enter mnemonic (separate words with spaces): double section release consider [...]

Mnemonic phrase decoded/checksummed successfully!

This process will *ERASE* the configured YubiHSM2 and reinitialize it:

- YubiHSM serial: 9876543211

Authentication keys with the following IDs and passwords will be created:

- key 0x0001: admin:

    double section release consider diet pilot flip shell mother alone what fantasy
    much answer lottery crew nut reopen stereo square popular addict just animal

- authkey 0x0002 [operator]:  kms-operator-password-1k02vtxh4ggxct5tngncc33rk9yy5yjhk
- authkey 0x0003 [auditor]:   kms-auditor-password-1s0ynq69ezavnqgq84p0rkhxvkqm54ks9
- authkey 0x0004 [validator]: kms-validator-password-1x4anf3n8vqkzm0klrwljhcx72sankcw0
- wrapkey 0x0001 [primary]:   21a6ca8cfd5dbe9c26320b5c4935ff1e63b9ab54e2dfe24f66677aba8852be13

*** Are you SURE you want erase and reinitialize this HSM? (y/N): y
21:47:18 [WARN] factory resetting HSM device! all data will be lost!
21:47:19 [INFO] waiting for device reset to complete
21:47:21 [INFO] installed temporary setup authentication key into slot 65534
21:47:21 [WARN] deleting default authentication key from slot 1
21:47:21 [INFO] installing role: admin:2019-03-05T21:47:02Z
21:47:21 [INFO] installing role: operator:2019-03-05T21:47:03Z
21:47:21 [INFO] installing role: auditor:2019-03-05T21:47:03Z
21:47:21 [INFO] installing role: validator:2019-03-05T21:47:03Z
21:47:21 [INFO] installing wrap key: primary:2019-03-05T21:47:03Z
21:47:21 [INFO] storing provisioning report in opaque object 0xfffe
21:47:21 [WARN] deleting temporary setup authentication key from slot 65534
     Success reinitialized YubiHSM (serial: 9876543211)

tmkms yubihsm keys generate: signing key generation

The tmkms YubiHSM backend is designed to support signing keys which are randomly generated by the device's internal cryptographically secure random number generator, as opposed to ones which are deterministically generated from the 24-word BIP39 mnemonic phrase.

This means you will need to do the following:

  • Run tmkms yubihsm keys generate to create signing keys (i.e. validator consensus keys)
  • Retain backups of these keys for disaster recovery

This command integrates a feature to export a backup of the keys at the time they are generated, which is compatible with the yubihsm-shell tool.

Below is an example of the command to generate and export an encrypted backup of an Ed25519 signing key:

$ tmkms yubihsm keys generate 1 -p cosmosvalconspub -b steakz4u-validator-key.enc
 Generated key #1: cosmosvalconspub1zcjduepqtvzxa733n7dhrjf247n0jtdwsvvsd4jgqvzexj5tkwerpzy5sugsvmfja3
     Wrote backup of key 1 (encrypted under wrap key 1) to steakz4u-validator-key.enc

This operation must be performed after configuring tmkms.toml with one of the following sets of credentials:

  • The admin key and associated 24-word password (i.e. key ID 0x0001)
  • The operator key (0x0002) and associated kms-operator-password

Parameters

  • tmkms yubihsm keys generate 1 - generates asymmetric key 0x0001, which is by default an Ed25519 signing key.
  • -p (or --prefix): Bech32 prefix to serialize key with (automatically sets label)
  • -l (or --label): an up-to-40-character label describing the key
  • -b (or --backup): path to a file where an encrypted backup of the generated key should be written
  • (not used in the example) -w (or --wrapkey): ID of the "wrap" (i.e encryption) key used to encrypt the backup key. It defaults to wrap key 0x0001 which was automatically generated as part of the tmkms yubihsm setup process.

tmkms yubihsm keys list: list signing keys

The following command lists keys in the HSM:

$ tmkms yubihsm keys list
Listing keys in YubiHSM #9876543211:
- 0x#0001: 1624DE64200FB6DB3175225219D290497E3B78190A3EEDA89AEBBC2E2294547CA98E76F9D5

Exporting and Importing Keys

tmkms contains functionality for exporting and importing keys, including making encrypted backups of keys, and also importing existing priv_validator.json keys.

We recommend you randomly generate keys using the above tmkms yubihsm keys generate procedure to avoid exposing plaintext copies of signing private keys outside of the HSM. However, below are instructions which can hopefully accommodate any situation you happen to be in with regard to exporting and importing existing keys.

tmkms yubihsm keys export: export encrypted backups of signing keys

If you ran tmkms yubihsm keys generate (or equivalent yubihsm-shell) command without creating a backup, the keys export subcommand can also export a backup:

$ tmkms yubihsm keys export --id 1 steakz4u2-validator-key.enc
  Exported key 0x0001 (encrypted under wrap key 0x0001) to steakz4u2-validator-key.enc

The backups generated are compatible with the ones generated by the Yubico yubihsm-shell utility.

Parameters

  • -i (or --id): ID of the asymmetric key to export
  • -w (or --wrapkey): ID of the wrap key under which the exported key will be encrypted.

tmkms yubihsm keys import: import encrypted backups of signing keys

After generating a key on a YubiHSM and exporting a backup, you can import the encrypted copy into another HSM with the following command:

$ tmkms yubihsm keys import steakz4u-validator-key.enc
    Imported key 0x0001: cosmosvalconspub1zcjduepqtvzxa733n7dhrjf247n0jtdwsvvsd4jgqvzexj5tkwerpzy5sugsvmfja3

Exporting keys from previously configured YubiHSM 2s

If you've previously configured a production key within a YubiHSM 2 and wish to securely export it and import it into a YubiHSM 2 provisioned using the tmkms yubihsm setup workflow, here are the steps to securely export it.

Note wrap key during the tmkms yubihsm setup procedure

Among the keys generated during the initial procedure is the wrap key, which is the encryption key used for all backups. It's at the bottom of this list:

- authkey 0x0002 [operator]:  kms-operator-password-1k02vtxh4ggxct5tngncc33rk9yy5yjhk
- authkey 0x0003 [auditor]:   kms-auditor-password-1s0ynq69ezavnqgq84p0rkhxvkqm54ks9
- authkey 0x0004 [validator]: kms-validator-password-1x4anf3n8vqkzm0klrwljhcx72sankcw0
- wrapkey 0x0001 [primary]:   21a6ca8cfd5dbe9c26320b5c4935ff1e63b9ab54e2dfe24f66677aba8852be13

(i.e. wrapkey 0x0001 [primary])

The number 21a6ca8cfd5dbe9c26320b5c4935ff1e63b9ab54e2dfe24f66677aba8852be13 is the hex serialization of an AES-256-CCM encryption key, and also compatible with the syntax used by the yubihsm-shell utility.

If you can authenticate to a YubiHSM 2 which contains an existing key you with to export, you can import the wrap key to export it under into the HSM with the following yubihsm-shell command:

yubihsm> put wrapkey 0 1 wrapkey 1 export-wrapped,import-wrapped exportable-under-wrap,sign-ecdsa,sign-eddsa 21a6ca8cfd5dbe9c26320b5c4935ff1e63b9ab54e2dfe24f66677aba8852be13
Stored Wrap key 0x0001

Parameters

  • put wrapkey 0 1 wrapkey: put the specified wrap key (via session 0) into slot 0x0001
  • 1: put the wrap key into domain 1 (use any of the domains the original key is accessible from)
  • export-wrapped,import-wrapped: grant the capabilities to export and import other keys
  • exportable-under-wrap,sign-ecdsa,sign-eddsa: delegated capabilities which allow imported keys to be exported again, as well as used to generate ECDSA and "EdDSA" (i.e. Ed25519) signatures.
  • [hex string]: raw AES-256-CCM wrap key to import
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