Symmetric and Asymmetric Encryption

Multiple Choice Questions:

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

1. 3DES (TripleDES)
2. RSA
3. RC5


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?

1. Not true, the message can also be decrypted with the Public Key.

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

3. The Public Key contains a special function which is used to encrypt the message and which can only be reversed by the appropriate Private Key.
4. The encrypted message contains the function for decryption which identifies the Private Key.


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?

1. First, the message is encrypted with symmetric encryption and aferwards it is encrypted assymmetrically together with the key.
2. The secret key is symmetrically transmitted, the message itself assymmetrically.
3. First, the message is encrypted with assymmetric encryption and aferwards it is encrypted symmetrically together with the key.
4. The secret key is assymmetrically transmitted, the message itself symmetrically.


Answer 4 is correct.

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

1. More complex and therefore more time-consuming calculations.
2. Problem of the secure transmission of the Secret Key.
3. Less secure encryption function.
4. Isn't used any more.


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?

1. PGP uses assymmetric encryption.
2. In the world wide web, primarly symmetric Encryption is used.
3. Symmetric encryption is applied in the transmission of PIN numbers from the EC automat to the server of the bank for example.
4. PGP uses combined encryption.


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

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

1. The key indicates which funcion is used for encryption. Thereby it is more difficult to decrypt a intercepted message as the function is unknown.
2. The key contains the secret function for encryption including parameters. Only a password can activate the key.
3. All functions are public, only the key is secret. It contains the parameters used for the encryption resp. decryption.
4. The key prevents the user of having to reinstall the software at each change in technology or in the functions for encryption.


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).