Whereas POW deals with the assurance that a client indeed possesses a given file, Provable Data Possession (PDP) and Proof of Retrievability (PoR) deal with the dual problem of ensuring - at the client-side - that a server still stores the files it ought to. PoR and PDP schemes address the requirement of data integrity (ensuring that data has not undergone malicious modifications) and availability (ensuring that data is still available in its entirety and can be downloaded if needed).
Trusted Execution Environments offer a way of securing PoR and PDP protocols. In particular, trusted computing based systems can be used to generate proofs supporting properties on the lower layers of the software stack and the function set of the Trusted Platform Module (TPM). While feasible in theory, such approaches still suffer from the limitations highlighted in the previous sections.
Encryption keys stored in the hard disk are susceptible to tampering, TPM solutions offer a protected storage of keys through hardware and protection of authentication credentials by binding them to the platform, providing a stronger mechanism to prevent unauthorized access to the platform and thus, the integrity of the data stored. Authentication built on top of trusted computing services (based on the use of TPMs) provides higher degrees of assurance, but performance overheads introduced can be significant.