Applications of Cryptography
86 RSA BSAFE Crypto-C Developer’s Guide
public key is communicated to the server and an entry is made in the table maintained
by the server for the public keys.
As an alternative, the server can certify the public keys of the client nodes by
generating a digital certificate to be signed by the server’s private key. In this case, the
server only trusts messages from previously-certified keys. There is no table to
maintain because the digital certificate can be used to verify the identity of a node
each time a connection or request is needed.
There are two approaches to establishing a link between a client and the server.
In the first approach, the server and a client determine a session key using a Diffie-
Hellman key agreement protocol. The Diffie-Hellman parameters are established
once at the initial setup of the server, and communicated publicly to each client when
a secure connection is requested. The session key is used for bulk-data encryption; the
established client RSA key pair is used for authentication or for envelope
communications. Any block or stream cipher can be used for encryption with the
session key. For stream ciphers, a new key should be computed for each session; this
prevents attacks that compare blocks of data encrypted with the same key.
In the second approach, the server uses the client’s RSA public key (contained in the
digital certificate) to generate a digital envelope for the encrypted data sent from the
server to the client. Likewise, the client uses the server’s public key (known to all
nodes) to create a digital envelope. In addition, each message contains digital
signatures to authenticate the originator.
Peer-to-Peer Applications
Unlike a client/server application, a peer-to-peer network application provides each
node with access to any other node in the network. For example, users may wish to
communicate privately with other known or unknown users through secure email. In
a peer-to-peer situation, no single node is capable of authenticating other client nodes.
Digital signatures can be used to provide proof of authorship to any recipient. Each
node must generate its public/private key pair and obtain a digital certificate from
some approved central authority. VeriSign can provide details about how to obtain a
digital certificate.
Each message between any two or more nodes can be authenticated by attaching the
originator’s digital certificate to the message. The recipient can verify the authenticity
of the message and the originator by verifying the validity of the certificate.
Nodes on peer-to-peer applications can encrypt using digital envelopes. To do so, the
sender obtains the digital certificate of each recipient and extracts the public key.