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Vault Master Key Encryption Feature


The EdgeX secret store threat model calls out a particular aspect of the Vault-based secret store architecture upon which the whole EdgeX secret store depends: the Vault master key. Because plaintext storage of the Vault master key at rest would be a known security weakness, the high level design calls for the Vault master key to be encrypted on storage.

One way of doing this would be to simply encrypt the whole drive upon which the Vault master key is stored. This is a good solution: it would encrypt not only the Vault master key, but also other part of the system to harden them against offline tampering and information disclosure risks. This solution also has drawbacks as well: whole volume encryption may slow down boot times and have a runtime performance impact on constrained devices without hardware-accelerated crypto.

The Vault Master Key Encryption feature of EdgeX enables a system designer to specifically target encryption of the Vault master key, and enables a variety of flexible use cases that are not tied to volume encryption such as key escrow (where a key is stored on another machine on the network), smart cards or USB HSMs (where a key us stored in a dongle or chip card), or TPM (security hardware found on many PC-class motherboards).

Internal design

As stated in the high level design, an RFC-5869 key derivation function (KDF) is used to produce a set of wrapping keys that are used by the vault-worker process to encrypt the Vault master key.

An RFC-5869 KDF requires three inputs. A change to any input results in a different output key:

  • Input keying material (IKM). It need not be (but should be) cryptographically strong, and is the "secret" part of the KDF.

  • A salt. A non-secret random number that adds to the strength of the KDF.

  • An "info" argument. The info argument allows multiple keys to be generated from the same IKM and salt. This allows the same KDF to generate multiple keys each used for a different purpose. For instance, the same KDF can be used to generate an encryption key to protect the PKI at-rest.

The Vault Master Key Encryption feature consumes the IKM from a Unix-style pipe. The IKM is provided by a vendor-defined mechanism, and is intended to be tied into security hardware on the device, be device-unique, and explicitly not stored in the file system.

To further strengthen the solution, an implementation could choose to engineer a solution whereby the IKM is only released a configurable number of times per boot, so that malware that runs on the system post-boot cannot retrieve it.


The Vault Master Key Encryption feature is embedded into the EdgeX security-secretsetore-setup utility. It is enabled by setting an environment variable, IKM_HOOK, containing the path to an executable that implements the IKM interface, described below, when the security-secretstore-setup executable is run in early boot to initialize or unseal the EdgeX secret store.

When this feature is enabled, the Vault master key is encrypted at rest, and cannot be recovered unless the same IKM is provided as when the secretstore was initialized.

IKM interface


ikm - Return input key material for a hash-based KDF.




ikm outputs initial keying material to stdout as a lowercase hex string to be used for the default EdgeX software implementation of an RFC-5869 KDF.

The ikm can output any number of octets. Typically, the KDF will pad the ikm if it is shorter than hashlen, and hash the ikm if it is longer than hashlen. Thus, if ikm returns variable-length output it is advantageous to ensure that the output is always greater than hashlen, where hashlen depends on the hash function used by the KDF.



Sample implementations

This section lists example implementations of the EdgeX Hardware Security Hook.

Tutorial: Configuring EdgeX Hardware Security Hooks to use a TPM on Intel® Developer Zone

There is a tutorial published on Intel® Developer Zone that uses TPM hardware through a device driver interface to encrypt the Vault master key shares. The sample uses TPM-based local attestation to attest the system state prior to releasing the IKM. The sample is based on the tpm2-software project in GitHub and is specifically designed to run as a statically-linked executable that could be injected into a Docker container. Although not a complete solution, it is an illustrative sample that demonstrates in concrete terms how to use the TSS C API to access TPM functionality.