Computer Security and Cryptography

18.6 WHO CAN YOU TRUST?: KOHNFELDER'S CERTIFICATES

Kohnfelder writes in Part I, Section D, Weaknesses in Public-Key Cryptosystems of his thesis,

Although the enemy may eavesdrop on the key transmission system, the key must be sent via a channel in such a way that the originator of the transmission is reliably known.

Kohnfelder observed that all public-key cryptosystems are vulnerable to a spoofing attack if the public keys are not certified; User_ID[C] pretending to be User_ID[A] to User_ID[B] by providing User_ID[C]'s public-key (in place of User_ID[A]'s public key) to User_ID[B]. Unless User_ID[B] has some way of checking the correspondence between ID[A] and PuK(ID[K]), this type of spoofing attack is possible.

Kohnfelder proposed a method to make spoofing more difficult in Part III of his thesis. He postulates the existence of a public file that contains (in my notation) pairs {(ID[A]), PuK([ID[A])} for each user in the system. Although it might be possible for User_ID[C] to contact to ask for a copy of User_ID[A]'s public key, the public file solution suffers from the same operational defects as a network-wide key server:

• What entity will maintain and certify a large database that is continually changing?
• The public file will need to be replicated to prevent severe access ...