General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

on a UNIX system, you can create a certificate (and associated private key) e.g. with
openssl ca -in req.pem -out newcert.pem
c○: Michael Kohlhase 386
Building on the notion of a public key certificate, we can build secure variants of the applicationlevel
protocols. Of course, we could do this individually for every protocol, but this would duplicate
efforts. A better way is to leverage the layered infrastructure of the Internet and build a generic
secure transport-layer protocol, that can be utilized by all protocols that normally build on TCP
or UDP.
Building Security in to the WWWeb Infrastructure
Idea: Build Encryption into the WWWeb infrastructure (make it easy to use)
Secure variants of the application-level protocols that encrypt contents
Definition 582 Transport layer security (TLS) is a cryptographic protocol that encrypts
the segments of network connections at the transport layer, using asymmetric cryptography
for key exchange, symmetric encryption for privacy, and message authentication codes for
message integrity.
TLS can be used to make application-level protocols secure.
c○: Michael Kohlhase 387
Let us now look at bit closer into the structure of the TLS handshake, the part of the TLS protocol
that initiates encrypted communication.
A TLS Handshake between Client and Server
Definition 583 A TLS handshake authenticates a server and provides a shared key for
symmetric-key encryption. It has the following steps
1. Client presents a list of supported encryption methods
2. Server picks the strongest and tells client (C/S agree on method)
3. Server sends back its public key certificate (name and public key)
4. Client confirms certificate with CA (authenticates Server if successful)
5. Client picks a random number, encrypts that (with servers public key) and sends it to
server.
6. Only server can decrypt it (using its private key)
7. Now they both have a shared secret (the random number)
8. From the random number, both parties generate key material
Definition 584 A TLS connection is a transport-layer connection secured by symmetric-key
encryption. Authentication and keys are established by a TLS handshake and the connection
is encrypted until it closes.
c○: Michael Kohlhase 388
The reason we switch from public key to symmetric encryption after communication has been
initiated and keys have been exchanged is that symmetric encryption is computationally more
efficient without being intrinsically less secure.
But there is more to the integration of encryption into the WWWeb, than just enabling secure
transport protocols. We need to extend the web servers and web browsers to implement the
secure protocols (of course), and we need to set up a system of certification agencies, whose public
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