Weak subjectivity

In some proof-of-stake systems, former validators’ keys can keep signing after their deposits cease to be exposed to penalties. A new node may encounter alternative histories that appear to follow the rules. Weak subjectivity describes the need for recent external information to select the accepted history.

A trusted checkpoint identifies a recent block or state. The node checks subsequent blocks under normal rules and rejects branches that do not continue that checkpoint. How long a checkpoint remains sufficiently recent depends on the protocol and its parameters.

The checkpoint fixes that node’s starting point; it does not make rewriting the past physically impossible. Security combines cryptographic checks with selecting a trusted source for the initial checkpoint.