Cryptography Reference
In-Depth Information
The SSL Record Protocol specifies the process for using these keys to protect traffic
exchanged between the client and the server. For example, for data sent from the
client to the server, the process is:
1. compute a MAC on the data (and various other inputs) using key
K
MCS
;
2. append the MAC to the data and then pad, if necessary, to a multiple of the
block length;
3. encrypt the resulting message using key
K
ECS
.
Upon receipt of the protected message, the server decrypts it using
K
ECS
and
then verifies the recovered MAC using
K
MCS
. It is interesting to note that SSL
uses a variant of the MAC-then-encrypt construction that we referred to in
Section 6.3.6.
12.1.5
SSL key management
We now consider SSL key management issues by examining some of the phases
of the key management lifecycle.
KEY MANAGEMENT SYSTEM
SSL essentially relies on two 'separate' key management systems:
Public-key management system
. Since SSL is designed for use in open
environments, it relies on an external key management system that governs
the public-key pairs that are required by SSL users (which is
all users
if mutual
entity authentication is required, or just users who are
servers
if only unilateral
entity authentication is required). This key management system is beyond the
scope of an SSL specification and is relied on to establish and maintain public-
key certificates and information concerning their validity. If this system fails
then the security provided by SSL is undermined.
Symmetric key management system
. Within SSL is a self-contained symmetric
key management system. SSL is used to generate symmetric sessions keys,
which are designed to have limited lifetimes.
Of course these are not really independent key management systems, since there
are aspects of the key management lifecycle where these two systems overlap.
However, we separate them to indicate that there are certain aspects of key
management that are beyond the scope of SSL itself.
KEY GENERATION
There are two types of keys deployed in SSL:
Asymmetric keys
. These are generated using the public-key management system,
which is not governed by the specification of SSL.
Symmetric keys
. These are all generatedwithin SSL. As described in Section 12.1.4,
the session keys are all derived from the master secret that is established
