From NSDLWiki

PublicKey

The PublicKey datastream contains an Agent's public key, which is used to authenticate signed requests to the repository.

Namespace 

http://ns.nsdl.org/api/relationships#

Prefix 

nsdl

Cardinality 

zero or one.

Found in 

Agent

Values 

XML encoded public RSA key

Usage and Semantics

• Present only in Agents that have reason to log in to the repository (typically application agents)