A prefix that is composed of a basic Base-64 (URL safe) derivation code pre-pended to Base-64 encoding of a basic public digital signing key.
Including the derivation code in the prefix binds the derivation process along with the public key to the resultant identifier.
An example of the prefix with a one character derivation code and a 32 byte public key encoded into a 44 character Based-64 string follows:
BDKrJxkcR9m5u1xs33F5pxRJP6T7hJEbhpHrUtlDdhh0