Guidelines on Securing Public Web Servers

9.   The server sends a message to the client informing it that future messages from the 

server will be encrypted with the session key.  It then sends a separate (encrypted) 

message indicating that the server portion of the handshake is finished.  

10.   The SSL/TLS handshake is now complete, and the SSL/TLS session has begun.  The 

client and the server use the session keys to encrypt and decrypt the data they send to 

each other and to validate its integrity.  

7.5.4  SSL/TLS Encryption Schemes 

The SSL/TLS protocols support the use of a variety of different cryptographic algorithms for 

operations such as authenticating the Web server and Web client to each other, transmitting 

certificates, and establishing session keys.  Web clients and Web servers may support different 

cipher suites, or sets of ciphers; depending on factors such as the version of SSL/TLS they 

support, organizational policies regarding acceptable encryption strength; and government 

restrictions on export, import, and use of SSL/TLS enabled software.  Among its other 

functions, the SSL/TLS handshake protocols determine how the Web server and Web client 

negotiate which cipher suites they will use to authenticate each other, to transmit certificates, 

and to establish session keys.  Table 7.1 provides a list of Federal cipher suites, their 

recommended usage, and their relative strength [SSL98 and Cho02].   

Table 7.1: SSL/TLS Encryption Schemes with Diffie Hellman or RSA Key Exchange 

Recommended Use 

Cipher Suites 

Highest Security: 

Encryption: Advanced Encryption Standard (AES) 

256 bit encryption 

Authentication & Digest: Digital Signature Standard 

(DSS) or RSA with 2048 bit keys, and Secure Hash 

Algorithim 1 (SHA 1)

Security and Performance: 

Encryption: AES 128 bit encryption 

Authentication & Digest: DSS or RSA with 1024 bit 

keys, and SHA 1 

Security and Compatibility: 

Encryption: AES 128 bit encryption with fallback to 

Triple Data Encryption Standard (3DES) 168/112 

bit encryption (note: 3DES is considerably slower 

than AES) 

Authentication & Digest: DSS or RSA with 1024 bit 

keys, and SHA 1

Authentication and Tamper Detection 

Authentication & Digest: DSS or RSA with 1024 bit 

keys and SHA 1 

Choosing an appropriate encryption algorithm depends on several factors that will vary with 

organization.  Although at first glance it might appear that the strongest encryption available 

should always be used, that is not always true.  The higher the level of the encryption, the 

greater impact it will have on the Web server's resources and communications speed.

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Furthermore, a number of countries still maintain restrictions on the export, import, and/or use 

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 AES 128 is the exception to this rule, since it provides higher performance and security than Triple DES. 

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