1. Secret key encryption algorithm system
2. Asymmetric cipher system
3. Public key encryption algorithm system
4. Symmetric cipher system
a. 1 and 4
b. 2 and 3
c. 1 and 2
d. 3 and 4
187. b. The Rivest, Shamir, and Adelman (RSA) scheme uses a public key encryption algorithm and is an asymmetric cipher system. The data encryption standard (DES) uses a secret key encryption algorithm and is a symmetric cipher system. RSA uses two keys (private and public), whereas DES uses one key (private).
188. What is the most common attack against cryptographic algorithms?
a. Ciphertext-only attack
b. Birthday attack
c. Chosen plain text attack
d. Adaptive chosen plain text attack
188. a. Exploiting a weakness is called an attack. In a ciphertext-only attack, an attacker has some ciphertext encrypted with an algorithm. He does not know the plain text or the key, but he knows the algorithm. His goal is to find the corresponding plain text. This is the most common attack.
A birthday attack is an attack against message digest 5 (MD5), a hash function. The attack is based on probabilities where it finds two messages that hash to the same value (collision) and then exploits it to attack. The attacker is looking for “birthday” pairs of two messages with the same hash values. This attack is not feasible given today’s computer technology.
In a chosen plain text attack, the attacker knows the plain text and the corresponding ciphertext and algorithm but does not know the key. This type of attack is harder but still possible. The adaptive chosen plain text attack is a variation of the chosen plain text attack where the selection of the plain text is changed based on the previous attack results.
189. A message authentication code can protect against which of the following combinations of actions?
1. Authorized, accidental data modification
2. Authorized, intentional data modification
3. Unauthorized, accidental data modification
4. Unauthorized, intentional data modification
a. 2 and 4
b. 2 and 3
c. 3 and 4
d. 1 and 4
189. c. A message authentication code, a type of cryptographic checksum, can protect against both accidental and intentional, but unauthorized, data modification. Ordinary error detecting codes such as cyclic redundancy codes are not adequate because they cannot detect intentional modification. A message authentication code is initially calculated by applying a cryptographic algorithm and a secret value, called the key, to the data. The initial code is retained. The data is later verified by applying the cryptographic algorithm and the same secret key to the data to produce another, second code; this second code is then compared to the initial code. If the two codes are equal, then the data is considered authentic. Otherwise, an unauthorized modification is assumed. Any party trying to modify the data without knowing the key would not know how to calculate the appropriate code corresponding to the altered data.
190. Which of the following encryption algorithms or schemes is absolutely unbreakable?
a. Data encryption standard
b. One-time pad
c. International data encryption algorithm
d. Rivest cipher 2 and 4
190. b. One-time pad is unbreakable given infinite resources. Each random key in the one-time pad is used exactly once, for only one message, and for only a limited time period. The algorithm for a one-time pad requires the generation of many sets of matching encryption keypads. Each pad consists of a number of random key characters, not generated by a cryptographic key generator. Each key character in the pad is used to encrypt one and only one plain text character; then the key character is never used again. The number of random keypads that need to be generated must be at least equal to the volume of plain text messages to be encrypted. Due to the number of random keypads to be generated, this approach is not practical for high-speed communication systems. This is the reason the one-time pad is absolutely unbreakable.
Brute force attack is possible with the data encryption standard (DES) and international data encryption algorithm (IDEA). The key length in Rivest cipher 2 and 4 (RC2 and RC4) is variable, and details of their algorithms are unknown because they are new proprietary algorithms. IDEA is a new algorithm and works as a double-DES (2DES). DES is in the public domain so that anyone can use it. IDEA is patented and requires a license for commercial use. RC2 and RC4 are unpatented but are trade secrets.
191. Which of the following statements is true about one-way hash function and encryption algorithm?
a. They both convert a plain text into an unintelligent text.
b. They both can reverse from output to input.
c. They both do not destroy information.
d. They both operate on a key.
191. a. A hash function can detect modification of a message, independent of any connection with signatures. That is, it can serve as a cryptographic checksum. It is a solution to the problem of signing long messages. A one-way hash function converts an arbitrary-length message into a fixed-length hash. Like an encryption algorithm, a one-way hash function converts a plain text message into an unintelligent text. This is where the similarity stops. However, unlike an encryption algorithm, there is no way to go backward with a one-way hash function. It is impossible to reverse a one-way hash function to get the original input from the output value. An encryption algorithm does not destroy any information. A one-way hash function destroys information and does not have a key. No secrecy is involved in the one-way hash function; the security is in the lack of ability to reverse itself. This property makes it a useful way to identify a message.
192. What do controls to protect against malicious changes to a message include?
a. Data checksums and cyclic redundancy code
b. Message integrity code and message authentication code
c. Message integrity code and cyclic redundancy code
d. Data checksums and message authentication code
192. b. A message integrity code uses a secret key to produce a fixed length hash code that is sent with the message. Integrity codes are used to protect the integrity of large interbank electronic funds transfers. A message authentication code is a hashed representation of a message and is computed by the message originator as a function of the message being transmitted and the secret key. If the message authentication code computed by the recipient matches the authentication code appended to the message, the recipient is assured that the message was not modified. Both integrity codes and authentication codes are cryptographic checksums, which are stronger than non-cryptographic checksums.
Cryptography can effectively detect both intentional and unintentional modification; however, cryptography does not protect files from being modified. Both secret key and public key cryptography can be used to ensure integrity. When secret key cryptography is used, a message authentication code is calculated and appended to the data. To verify that the data has not been modified at a later time, any party with access to the correct secret key can recalculate the authentication code. The new authentication code is compared with the original authentication code. If they are identical, the verifier has confidence that an unauthorized party has not modified the data.