PIN Security Requirements:
21-2.4 Procedures exist to ensure that no custodian ever has access to sufficient key components or shares of a secret or private key to reconstruct a cryptographic key.
For example, in an m-of-n scheme (which must use a recognized secretsharing scheme such as Shamir), where only two of any three shares are required to reconstruct the cryptographic key, a custodian must not have current or prior knowledge of more than one share. If a custodian was previously assigned share A, which was then reassigned, the custodian must not then be assigned share B or C, as this would give them knowledge of two shares, which gives them ability to recreate the key.
In an m-of-n scheme where n=5 and where three shares are required to reconstruct the cryptographic key, a single custodian may be permitted to have access to two of the key shares (for example, share A and share B); and a second custodian (with, in this example, share C) would be required to reconstruct the final key, ensuring that dual control is maintained
Testing Procedures:
21-2.4.a Examine documented procedures for the use of key components/shares to verify that procedures ensure that any custodian never has access to sufficient key components or shares to reconstruct a secret or private cryptographic key.
21-2.4.b Examine key-component/share access controls and access logs to verify that authorized custodians cannot access sufficient key components or shares to reconstruct a secret or private cryptographic key.
