Symmetric and Asymmetric Encryption

Multiple Choice Questions:

1. Which of the following Algorithms belong to symmetric encryption?

Solution:

Answers 1, 3 and 4 are correct. 3DES is a modified version the DES-algorithm (dates Encryption standard), which was introduced in 1974 and became ANSI-standard. Up to now, no weakness could be found except the insufficient length of the key (8 bytes, expanded to 32 bytes in 3DES). RC5 was thought as a successor for DES and is considered to be secure. Also IDEA (internationally dates Encryption Algorithm) could be a successor for DES. So far it hasn't been cracked, but its security is still disputed because it hasn't been tested properly yet. RSA was developed 1977 and is used in PGP, for example.

2. Assymmetric Encryption: Why can a message encrypted with the Public Key only be decrypted with the receiver's appropriate Private Key?

A so called "one way function with back door" is applyed for the encryption.

Answer 2 is correct. An one-way function is a function which a computer can calculate quickly, but whose reversal would last months or years. An one-way function with back door can be reversed with the help of a couple of additional information (the back door), but scarcely without this information. The information for the back door is contained in the private Key.

3. In which way does the Combined Encryption combine symmetric and assymmetric encryption?

Answer 4 is correct.

4. Which is the largest disadvantage of the symmetric Encryption?

Answer 2 is correct. As there is only one key in the symmetrical encryption, this must be known by both sender and recipient and this key is sufficent to decrypt the secret message. Therefore it must be exchanged between sender and receiver in such a manner that an unauthorized person can in no case take possesion of it.

5. Which of the following statements are correct?

Answers 1 and 3 are correct. See PGP Corporation or International PGP Homepage

6. Which is the principle of the encryption using a key?

Answer 3 is correct. The encoding of a message is calculated by an algorithm. If always the same algorithm would be used, it would be easy to crack intercepted messages. However, it isn't possible to invent a new algorithm whenever the old one was cracked, therefor the possibility to parameterize algorithms is needed and this is the assignment of the key. All algorithms must be public, only the keys are secret (principle of Kerckhoff, Dutch cryptographer during 19th century).