Class was just taught public key cryptography and symmetric cryptography.

Describe how the following access control policies can be enforced using encryption and digital signatures:

1. Alice can read any document stored in a shared folder named "Blackboard"

2. Bob can read and write documents in folder "Blackboard"

3. Carl can create new documents stored in folder "Blackboard", but cannot read any document from that folder

Assume that each user has a copy of every other users' public key.

Compare and contrast the approach you proposed with traditional access control implemented using file system access control lists. (For example: can the system administrator override read or write permissions on files within the "Blackboard" folder using the two techniques?)