Here I mention a connection between public key crypto and capability discipline. The idea is that knowledge of a public key conveys the authority to request a service of the holder of the private key. Knowledge of the private key conveys the ability to interpret and act on such requests. This way of using public key crypto illustrates both the use of restricting the distribution of public keys and limited distribution of private keys. We want to examine these issues more closely here and further to compare these protocols with those described by Needham and Schroeder and now used in Kerberos.

I have not yet read the literature cited by Schneier who ascribes the basic Kerberos protocol to Needham and Schroeder. Here is my understanding of the protocol:

If the U.S. state department decided, in 1960, before the public invention of public keys crypto, that it would be expedient for the U.S. embassy in Moscow to be able to communicate directly and securely with the U.S. embasy in Madrid then it might send the following enciphered message to Moscow:
Please communicate directly with Madrid with the following secret key: 0x434B4968d45e6401. Include the following message in the clear that will convey to them same key. Madrid can decipher it: 0x234 .... 9b.
The enclosed enciphered message is in the key shared by the State Department and the U.S. embassy in Madrid. The Department could have sent the enclosed message directly to Madrid but perhaps communication links are not always available, or perhaps that would betray traffic patterns better hidden, or perhaps there is reason not to inform Madrid until the need to communicate has come.
Note that Washington knows the secret key used between Moscow and Madrid and can intercept messages. Note that Moscow cannot share its authority with another embassy without further exposing the key it uses with Madrid.

I don’t know if such schemes were used in practice but they do solve some of the problems often cited as the achievement of public key crypto. I see no extension of these ideas to digital signature.

The above message to Moscow is a bit like sending Madrid’s public key to Moscow in an enciphered message.