Recovery

If the user has lost access to their credentials or their credentials have been compromised, effective recovery mechanisms are needed. However, in setups where users are responsible for managing their own keys, this is a significant challenge. In the context of a PoP protocol, there are multiple mechanisms that can be used:

• Restoring a User-Managed Backup: The simplest method for credential recovery involves storing encrypted user-managed backups of their credentials. This allows users to restore their credentials, such as on a new device when their previous one is lost.

• Social Recovery: If no user-managed backup exists, but the user has set up social recovery, the credentials can be recovered through the help of friends and family.

• Recover Keys: If neither backups nor social recovery are available, the user needs to return to the issuer to regain access to their original credential. The user needs to prove to the issuer that they are the legitimate owner of a certain credential. Upon successful authentication, the issuer grants access to the credential again. This process is similar to obtaining a new government ID after losing the previous one. The user can get a new ID with the same information on it. This process may not be viable for some credentials: for example, if a private key was generated by the user and only the public key is recorded by the issuer (e.g. ChainX ID).

• Re-Issuance: In situations where regaining access to the original credential through the issuer is not possible or undesirable (e.g. due to identity theft). In that case, re-issuance provides a way to invalidate the previous credential and issue a new credential. This can be compared to freezing a credit card and ordering a new one. Importantly, the availability of a re-issuance mechanism to rotate keys makes the illegitimate acquisition of other individuals’ PoP credentials financially unviable from a game-theoretic perspective. The true holder of the credential can always recover their credentials and invalidate the bought/stolen credential. However, this does not protect against all cases of identity transfer, especially those that involve collusion or coercion.

Two other properties add to the integrity of a PoP mechanism:

Revocation

While the hope is that all participants act with integrity, this cannot be assumed. In instances where an issuer is found to be compromised or malicious, the impact can be mitigated by issuers or developers removing affected PoP credentials from their list of accepted credentials. If the issuance of a credential is decentralized across multiple issuing locations and only a subset is affected, the respective subset could be revoked by the issuing authority itself. An example in terms of today's credentials could be a university granting a diploma to a person who hasn't met all the criteria. If the fraud is identified, the diploma is revoked.

Expiry

The efficacy of security mechanisms degrades over time and new mechanisms are continuously being developed. As a result, many identity systems incorporate a predefined expiry date to credentials at the point of issuance. An example are passports. Although expiry is not required for a PoP mechanism to work, its inclusion can increase the PoP’s integrity.