  The Internet of Things (IoT) is an emerging paradigm that allows to set large
networks of small and independent devices. To ensure their integrity,
practitioners employ so-called Remote Attestation (RA) schemes. Classic RA
schemes require a central and powerful entity, called Verifier, that has mainly
two duties: (i) it manages the entire process of attestation, and (ii) it
contains all the proofs for validating the devices' integrity. However, having
a central Verifier makes the network dependent upon an external entity and
introduces a single point of failure for security.
  In this work, we propose SAFE^d: the first RA schema that allows a pair of
IoT devices to validate their integrity without relying on an external
Verifier. Our approach overcomes previous limitations by spreading the proofs
among multiple IoT devices and using novel cryptographic mechanisms to ensure
secure communications. Moreover, the entire IoT network can collaboratively
isolate tampered devices and recover missing proofs in case of anomalies. We
evaluate our schema through an implementation for Raspberry Pi platform and a
network simulation. The results show that SAFE^d can detect infected devices
and recover up to 99.9% of proofs in case of faults or attacks. Moreover, we
managed to protect up to 10K devices with a logarithmic overhead on the network
and on the devices' memory.
