Implementing Network Security ( Version 2.0) – CCNAS Chapter 7 Exam Online

Cryptology is the science of making and breaking secret codes. There are two separate disciplines in cryptology, cryptography and cryptanalysis. Cryptography is the development and use of codes. Cryptanalysis is the breaking of those secret (encrypted) codes.

DES uses a fixed length key. The key is 64-bits long, but only 56 bits are used for encryption. The remaining 8 bits are used for parity. A DES encryption key is always 56 bits long. When DES is used with a weaker encryption of a 40-bit key, the encryption key is 40 secret bits and 16 known bits, which make the key length 56 bits.

SEAL is a stream cipher that uses a 160-bit encryption key. It is a symmetric encryption algorithm that has a lower impact on the CPU resources compared to other software-based algorithms, such as software-based DES, 3DES, and AES.

DH is an asymmetric algorithm. AES, 3DES, and SEAL are all symmetric algorithms.

Digital signatures, generated by hash function, can provide the service for nonrepudiation of the transaction. Both public and private keys are used to encrypt data during the transaction. Shared secrets between the retailer and customers are not used.

The SSH protocol uses an asymmetric key algorithm to authenticate users and encrypt data transmitted. The SSH server generates a pair of public/private keys for the connections. Encrypting files before saving them to a storage device uses a symmetric key algorithm because the same key is used to encrypt and decrypt files. The router authentication with CHAP uses a symmetric key algorithm. The key is pre-configured by the network administrator. A VPN may use both an asymmetric key and a symmetric encryption algorithm. For example in an IPSec VPN implementation, the data transmission uses a shared secret (generated with an asymmetric key algorithm) with a symmetric encryption algorithm used for performance.

Nonrepudiation uses the unique characteristics of the sender of a message to confirm that the reputed sender is in fact the actual sender.

When data is encrypted, it is scrambled to keep the data private and confidential so that only authorized recipients can read the message. A hash function is another way of providing confidentiality.

Despite its advantages, AES is a relatively young algorithm. An important rule of cryptography is that a mature algorithm is always more trusted. 3DES is therefore a more trusted choice in terms of strength, because it has been tested and analyzed for 35 years. AES can be used in high-throughput, low-latency environments, especially when 3DES cannot handle the throughput or latency requirements. AES is available in a number of Cisco VPN devices as an encryption transform.

Diffie-Helman is not an encryption mechanism and is not typically used to encrypt data. Instead, it is a method to securely exchange the keys used to encrypt the data.

An asymmetric encryption algorithm uses two keys, namely a public key and a private key. A symmetric encryption algorithm uses an identical key for both encryption and decryption. A shared secret is an example of using symmetric algorithm.

In a brute-force attack, an attacker tries every possible key with the decryption algorithm knowing that eventually one of them will work. To defend against the brute-force attacks, modern cryptographers have as an objective to have a keyspace (a set of all possible keys) large enough so that it takes too much money and too much time to accomplish a brute-force attack. A security policy requiring passwords to be changed in a predefined interval further defend against the brute-force attacks. The idea is that passwords will have been changed before an attacker exhausts the keyspace.

Cryptographic encryption can provide confidentiality at several layers of the OSI model. For example, network layer protocols, such as the IPsec protocol suite, provide network layer confidentiality. Secure Sockets Layer (SSL) or Transport Layer Security (TLS), provide session layer confidentiality. MD5, Keyed MD5, and Secure Hash Algorithm 1 are examples of hash functions. They provide data integrity but not data confidentiality.

3DES is a good choice to protect data because it has an algorithm that is very trusted and has security strength.

DES, 3DES, and AES are examples of symmetric encryption algorithms (also known as shared secret key algorithms). The usual key length for symmetric algorithms is 80-256 bits. Asymmetric algorithms are relatively slow because they are based on difficult computational algorithms.

DH is a mathematical algorithm that allows two hosts to generate an identical shared secret on both systems without having communicated before. To start a DH exchange, both hosts must agree on two nonsecret numbers. The first number is a base number, also called the generator. The second number is a prime number that is used as the modulus. These numbers are usually public and are chosen from a table of known values.

Asymmetric algorithms are slow, so they are commonly used in low-volume transactions such as making online purchases or logging into a financial website.

Asymmetric algorithms use two keys, a public and a private key. Key management is simpler because one of the keys can be made public.​

Code signing is used to verify the integrity of executable files downloaded from a vendor website. Code signing uses digital certificates to authenticate and verify the identity of a website.

Data confidentiality is ensured through symmetric encryption algorithms, including DES, 3DES, and AES.

Digital signatures commonly use digital certificates that are used to verify the identity of the originator in order to authenticate a vendor website and establish an encrypted connection to exchange confidential data. One such example is when a person logs into a financial institution from a web browser.

Symmetric encryption algorithms use the same key (also called shared secret) to encrypt and decrypt the data. In contrast, asymmetric encryption algorithms (also called public-key) use a pair of keys, one for encryption and another for decryption.