The Official (ISC)2 SSCP CBK Reference by Mike Wills

Author:Mike Wills
Language: eng
Format: epub
ISBN: 9781119602002
Publisher: Wiley
Published: 2019-10-29T16:00:00+00:00

This is shown with small values for all numbers in Figure 5.11.

FIGURE 5.11 Diffie-Hellman-Merkle shared key generation (conceptual)

What about Eve, sitting along the sidelines of this conversation? Suppose Eve is, well, eavesdropping on Bob and Carol’s key exchange; she somehow is trapping packets going back and forth and recognizes that they’ve agreed to an algorithm and its control parameters; she recognizes the exchange of Bob’s and Carol’s public keys for what they are. As long as Eve does not have a secret key that participated in the computation of the session key, she does not have anything that lets her read the traffic that Bob and Carol encrypt with the session key. Eve is left to using brute-force, side channel, or other attacks to attempt to break the session encryption.

Ted, on the other hand, is someone Bob and Carol want to include in a three-way secure conversation (still keeping Eve out in the cold, of course). The process previously shown in steps 1 through 5 can easily be expanded to include three or more parties who share the choices about algorithms and parameters, who then compute their own public keys and share them; they then use everybody else’s public keys to compute their own copy of the session key.

Obviously, this simplified description of the Diffie-Hellman key exchange process has some vulnerabilities. It doesn’t actually authenticate that Bob is Bob, or Carol is Carol, thus tempting Ted to be the “man in the middle” who masquerades to be the other party from the initial handshake and key generation through to the end of the session. The choice of trapdoor function, and the control values for it, can also present exploitable vulnerabilities. But in its simplest form, this is where the public key infrastructure (PKI) got its start.

Building a public key infrastructure starts with the algorithms used to generate the shared secret keys used to establish trustworthy communications. Those algorithms have to be implemented in some combination of software and hardware, which are then made available to users to incorporate into their systems or use as standalone messaging apps. These apps themselves and the software and hardware distribution channels (wholesale, retail, original equipment manufacturer (OEM), or other) all have to be part of a network of trust relationships if two end users are going to trust in such apps to protect their communication with each other. Thus, the problem of building a public key infrastructure must also embrace the problem of trusted software (and hardware) distribution and update. This will be explored further in the “Understand Public Key Infrastructure Systems” section of this chapter.

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