Anchoring self-sovereign identity to a hardware TPM 2.0

Self-sovereign identity (SSI) is a decentralized identity model. When applied in the Internet of Things domain, this model enables digital trust independently of the protocol agreed among peers for data exchange. Each peer has full control on its own identity and it must publish a decentralized identifier on a trusted public utility. Such identifiers are the basic building block to provide authentication in a decentralized fashion. However, SSI framework requires secure management of the cryptographic material required for building the digital identity. Each device is the true owner of its own identity, therefore a proper key management system needs to be integrated for autonomously generating identity keys and produce digital signatures. Today, trusted platform modules (TPMs) are used in IoT. TPMs have key management capabilities: they can generate asymmetric key pair in its isolated environment from the operating system and securely produce digital signatures. This thesis describes the usage of an hardware TPM 2.0 as a key management system for the IOTA Identity framework, an SSI library that relies on the IOTA Tangle, a largely used Distributed Ledger technology (DLT). Both projects are created by IOTA Foundation. The \emph{de facto} standard key management system implementation for IOTA Identity is Stronghold. This solution is a software component that can execute the required cryptographic procedures and implements a secure storage of the cryptographic material, both at rest and in memory. The IOTA Identity framework contains generic interfaces to ease the implementation of custom key management systems to support SSI operations. The aforementioned interfaces have been implemented to offload key management operations to the hardware TPM 2.0 device. Specifically, the TPM 2.0 creates keypair with signing capability and exposes the public part when a new decentralized identifier needs to be publish. On the other hand, the sensitive part of the key pair is never exposed outside of the TPM 2.0 context. Similarly, if a verifier requires to verify the credential, the TPM 2.0 loads a previously generated signing key and produces a digital signature. In addition, the usage of hardware TPM 2.0 can be propagated to the higher level of the SSI framework. A credential issuer that is also a Privacy Certification Authority can verify the TPM 2.0 device identity and the policy of the key used for decentralized identity. The issuer releases credentials that enforce a strong policy about the usage of identity keys. A peer that requires a stronger authentication may require for such credentials and trust only digital signatures performed by keys loaded in the hardware TPM 2.0 that can neither be exported or generated externally.