The Sigsum transparency system enables detection of unexpected or malicious use of a signing key. The owner of a signing key submits each signature made to a Sigsum log, and gets back a corresponding Sigsum proof of logging. The parties that rely on these signatures (e.g., for installing signed software updates) are expected to reject any signature that does not come with a valid proof of logging. The key owner can then monitor the log and discover all signatures made that are going to be accepted.
Discoverability of signatures is a desirable property, but what if you are making signatures in a context where those relying on your signatures are unable to process a Sigsum proof? E.g., the signature may have to follow some standard that’s not easily extended with a Sigsum proof, or the signatures have to be verified on a slow embedded system where extended verification is impractical for performance reasons.
The Tillitis TKey is an open source USB hardware security device which allows running small arbitrary applications in a more secure environment, using measured boot to give each application its own secret. Security keys and HSMs, including the TKey, are often used as signing oracles. But we can do better, by having the device also enforce Sigsum logging of the data before signing.
The sign-if-logged tool consists of a host program, and a TKey device app. The host program configures the device app with a Sigsum policy and a list of authorized submitter keys. The host program can then ask the device to sign data, but the device requires that the host also provides a Sigsum proof, valid according to its configuration.
This way, the signing party gets discoverability of signatures, and the ability to detect unexpected or malicious key usage, without the parties relying on those signatures having to know about it.
The talk will give a brief introduction to Sigsum and the TKey hardware. It will explain how the sign-if-logged tool works and how to use it. If time permits, it will also introduce the Sigsum C library, which is designed with constrained embedded systems in mind.
A version 1.0.0 of the sign-if-logged tool was released this summer.
Source code and documentation: https://git.glasklar.is/sigsum/apps/sign-if-logged
Speaker
Working with the Sigsum project at Glasklar Teknik since 2022. Main author and maintainer of the GNU Nettle crypto library. PhD from the Automatic Control group at KTH.
