CCNA Security Chapter 7 Exam Answers (CCNAS v1.2)

Explanation: 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.

Explanation: 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.

Explanation: There are many cipher methods developed for message encryptions. In transposition ciphers, no letters are replaced; they are simply rearranged. An example of this type of cipher is known as the rail fence cipher. In this transposition, the words are spelled out as if they were a rail fence, meaning some are in front and some in back across several parallel lines.

Explanation: Digital signatures provide three basic security services:
Authenticity of digitally signed data – Digital signatures authenticate a source, proving that a certain party has seen and signed the data in question.
Integrity of digitally signed data – Digital signatures guarantee that the data has not changed from the time it was signed.
Nonrepudiation of the transaction – The recipient can take the data to a third party, and the third party accepts the digital signature as a proof that this data exchange did take place. The signing party cannot repudiate that it has signed the data.

Explanation: 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.

Explanation: A keyed-hash message authentication code (HMAC or KHMAC) is a type of message authentication code (MAC). HMACs use an additional secret key as input to the hash function, adding authentication to data integrity assurance.

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

Explanation: A digital certificate class is identified by a number. The higher the number, the more trusted the certificate. The classes include the following:
Class 0 is for testing purposes in which no checks have been performed.
Class 1 is for individuals with a focus on verification of email.
Class 2 is for organizations for which proof of identity is required.
Class 3 is for servers and software signing for which independent verification and checking of identity and authority is done by the issuing certificate authority.
Class 4 is for online business transactions between companies.
Class 5 is for private organizations or governmental security.

Explanation: When two users must authenticate each other using digital certificates and CA, both users must obtain their own digital certificate from a CA. They submit a certificate request to a CA, and the CA will perform a technical verification by calling the end user (out-of-band). Once the request is approved, the end user retrieves the certificate over the network (in-band) and installs the certificate on the system. After both users have installed their certificate, they can perform authentication by sending their certificate to each other. Each site will use the public key of the CA to verify the validity of the certificate; no CA is involved at this point. If both certificates are verified, both users can now authenticate each other.

Explanation:

To make a key strong, there are several essential characteristics of key management that should be considered:

• Generation – The use of good random number generators is needed to ensure that all keys are likely to be equally generated so that the attacker cannot predict which keys are more likely to be used.
• Verification – Almost all cryptographic algorithms have some weak keys that should not be used. With the help of key verification procedures, these keys can be regenerated if they occur.
• Exchange – Key management procedures should provide a secure key exchange mechanism that allows secure agreement on the keying material with the other party, probably over an untrusted medium.
• Revocation and Destruction – Revocation notifies all interested parties that a certain key has been compromised and should no longer be used. Destruction erases old keys in a manner that prevents malicious attackers from recovering them.

Explanation: Typical usage of cryptographic keys includes the following:

• Symmetric keys, which can be exchanged between two routers supporting a VPN.
• Digital signatures, which are used when connecting to a secure website.
• Hash keys, which are used in symmetric and asymmetric key generation, digital signatures, and other types of applications.

Explanation: The Diffie-Helman (DH) algorithm is the basis of most modern automatic key exchange methods. It is a mathematical algorithm that allows two computers to generate an identical shared secret on both systems without having communicated before. DH is commonly used when data is exchanged using an IPsec VPN.

Explanation: 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.

Explanation: A cryptographic hash function should have the following properties:The input can be any length.
The output has a fixed length.
The hash value is relatively easy to compute for any given input.
The hash is one way and not reversible.
The hash is collision free, meaning that two different input values will result in different hash values

Explanation: A Keyed-hash message authentication code (HMAC and KHMAC) is a type of message authentication code that uses an additional secret key as input to the hash function. This adds authentication to integrity assurance. When two parties share a secret key and use HMAC functions for authentication, the received HMAC digest of a message indicates that the other party was the originator of the message (non-repudiation), because it is the only other entity possessing the secret key. 3DES is an encryption algorithm, and MD5 and SHA-1 are hashing algorithms.

Explanation: 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.

Explanation: 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.

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

Explanation: In a hierarchical CA topology, CAs can issue certificates to end users and to subordinate CAs, which in turn issue their certificates to end users, other lower level CAs, or both. In this way, a tree of CAs and end users is built in which every CA can issue certificates to lower level CAs and end users. Only the root CA can issue a self-signing certificate in a hierarchical CA topology.

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

Explanation: 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.

Explanation: 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.

Explanation: 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.

Explanation: 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.

Explanation: 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.