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For instance the cipher suite “RSA-RC4-MD5” means that RSA certiﬁcates are used for both authentication and key exchange,

while RC4 is used as the bulk encryption cipher, and MD5 is used for digest computation.

SSL and the OSI Model

The SSL protocol is a security protocol that sits on top of TCP at the transport layer. In the OSI model, application layer

protocols such as HTTP or IMAP, handle user application tasks such as displaying web pages or running email servers.

Session layer protocols establish and maintain communications channels. Transport layer protocols such as TCP and UDP,

handle the ﬂow of data between two hosts. Network layer protocols such as IP and ICMP provide hop-by-hop handling of data

packets across the network.

SSL operates independently and transparently of other protocols so it works with any application layer and any transport layer

protocol. This allows clients and servers to establish secure SSL connections without requiring knowledge of the other

party’s code.

SSLSSL

cc=8634-2782-6787-5931

cc=’?#o_-#’8_-,-h’”+z#’?!

cc=8634-2782-6787-5931

cc=’?#o_-#’8_-,-h’”+z#’?!

SSL Encrypted Tunnel

Application

HTTP,Telnet,FTP,etc.

Session/Transport

SSL

TCP,UDP

Network

IP,ICMP

Data Link

Device Driver

Interface Card

Physical

Interface Card

Application

HTTP,Telnet,FTP,etc.

Session/Transport

SSL

TCP,UDP

Network

IP,ICMP

Data Link

Device Driver

Interface Card

Physical

Interface Card

Internet

Figure 3 - How SSL functions in the OSI model

Figure 3 illustrates how SSL functions in the OSI model:

An application layer protocol hands unencrypted data to the session/transport layer, SSL encrypts the data and hands it down

through the layers. When the server receives the data at the other end, it passes it up through the layers to the session layer

where SSL decrypts it and hands it off to the application layer. Since the client and the server have gone through the key

negotiation handshake, the symmetric key used by SSL is the same at both ends.

The Cost of Encryption

While SSL solves the problem of securely transferring private data, it introduces another problem: HTTPS trafﬁc poses a major

security risk to enterprises.

Technology Primer: Secure Sockets Layer (SSL)

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