Provides three access levels of storage key protection, comprising a supervisory level (key 0), an intermediate level of non-public and non-supervisory keys (keys 1-8, 10-15), and a unique public level (key 9). The program routines operating with a supervisory-level access key can access both the public level and the intermediate level of storage blocks. Although a program routine operating with an access key in the intermediate access level cannot access any supervisory level storage block, it can access any block assigned a public level storage key, as well as any storage block assigned the respective intermediate level key. One or more third-level public storage keys (PSKs) may be provided. A program access key using one of the PSK values can only access blocks having the same PSK value, and it cannot access blocks having any other key value.