Secure e-mail in Java

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In previous sections of this chapter, we have examined fundamental issues in network security, including symmetric key and public key encryption, authentication, key distribution, message integrity and digital signatures In this section and the following two sections, we'll next examine how these techniques are being used to provide security in the Internet Being consistent with the general structure of this book, we begin at the top of the protocol stack and discuss application-layer security Our approach here is use a specific application, namely, e-mail, as a case study for application-layer security We then move down the protocol stack In Section 77 we examine the SSL protocol, which provides security at the transport layer for TCP And in Section 78, we'll consider IPsec, which provides security at the network layer Interestingly, it is possible to provide security services in any of the top four layers of the Internet protocol stack [Molva 1999] When security is provided for a specific application-layer protocol, then the application using the protocol will enjoy one or more security services, such as secrecy, authentication or integrity When security is provided for a transport-layer protocol, then all applications that use that protocol enjoy the security services of the transport protocol When security is provided at the network layer on a host-to-host basis, then all transport layer segments (and hence all application-layer data) enjoy the security services of the network layer When security is provided on a link basis, then all IP datagrams traveling over the link receive security services of the link One might wonder why security functionality is being provided at multiple layers in the Internet Wouldn't it suffice to simply provide the security functionality at the network layer, and be done with it There are two answers to this question First, although security at the network layer can offer "blanket coverage" by encrypting all the data in the datagrams (ie, all the transport-layer segments) and by authenticating all source IP addresses, it can't provide userlevel security For example, a commerce site can not rely on IP-layer security to authenticate a customer who is purchasing goods at the commerce site Thus, there is a need for security functionality at higher layers as well as blanket coverage at lower layers Second, in the Internet it is generally easier to deploy new services, including security services, at the higher-layers of the protocol stack While waiting for security to be broadly deployed at the network layer (which is arguably still many years in the future) many application developers "just do it" and introduce security functionality into to their favorite applications A classic example is PGP, which provides for encryption of email (and will be discussed later in this section) Requiring only client and server application code, PGP was one the first security technologies to be broadly used in the Internet Similarly, transport-layer security with SSL was broadly introduced into the Internet, as it too only required new code in the end systems However, IP-layer security -- socalled IPsec -- is taking much longer to broadly deploy, as it requires significant changes in the routers in the network core
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In this section we use many of the tools introduced in the previous section to create a high-level design of a secure email system We create this high-level design in an incremental manner, at each step introducing new security services When designing a secure e-mail system, let us keep in mind the racy example introduced in Section 71 -- the illicit love affair between Alice and Bob In the context of e-mail, Alice wants to send an e-mail message to Bob, and Trudy wants to intrude Before plowing ahead and designing a secure e-mail system for Alice and Bob, we should first consider which security features would be most desirable for them First and foremost is secrecy As discussed in Section 71, neither Alice nor Bob wants Trudy to read Alice's e-mail message The second feature that Alice and Bob would most likely want to see
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