Key management is fundamental to the security afforded by any cryptography-based safeguard. Different key administration policies, such as hierarchical (PEM) and user-centered (PGP), have many implications on the required infrastructure for the overall system. VeriSign (spin-off of RSA Data Security) and COST are commercial companies that provides CA-services.
The "key escrow system" refers to a policy under which users of encryption systems give copies of their encryption keys either to their government or to a third party that the government trusts.
If the person, who has used digital signature at a certain time, wants to prove it to someone else, a digital notary is needed. The notary can time-stamp the signatures with a unique "digital fingerprint". This service can be used for example to time-stamp research data and papers, witness fraud, and form non-forgeable digital agreements. Surety Technologies is an example of a commercial digital notary service.