Review Question.
Nishesh.Prasad
Member Posts: 185
in Security+
1. Assuming that Asymmetric Encryption is in place, if a message is encoded using the number 5, what will you use to decode it ?
a. 5
b. 1
c. 0
d. 1/5
*** can anyone point out the Answer with an explanation. Thanks.
a. 5
b. 1
c. 0
d. 1/5
*** can anyone point out the Answer with an explanation. Thanks.
MCITP: EA 2008 VCP4 MCSE 2003  CCNA  MCSA 2003: Security  MCDST  Security+  ITILV3
Comments

Gratefullaz Member Posts: 8 ■□□□□□□□□□really bad question and you do not have to worry about seeing it on the test as it oversimplifies and assumes.
based purely on best answer available with info given 1/5 for asymmetric
5 would be the answer for symmetric and would be a much better question.
In reality it would be based on the function of the algorithm and you could write a algorithm to use any one of those numbers to decrypt although 0 or 1 would seem unlikely as almost any number passed through it would decrypt and 5 would be symmetric.
Their logic seems to be that they use a private key value of 5 and the algo applies the same function used to encrypt to decrypt such as simple multiplication. If you multiply at both ends then the public key value of 1/5 brings back the value to 1.
You could have an algo that divides by the public key but then both values would be indentical which basically would reduce PKI to a shared secret.
You can write an algo that would decrypt with anyone of the answers provided so it is a bad question with the BEST answer being 1/5
PS. Be careful when studying practice test as anyone can create one and give an answer that is completely wrong which mine could possibly be above. I have seen the same questions on multiple practice exams all with a different answers in the key so make sure you understand the question and can back up your reasoning with information that you have learned and not simply spit out an answer that you see that someone else has given. I see that you are doing that by requesting the reasoning behind it. 
Nishesh.Prasad Member Posts: 185Thanks. After 40 Views & 0 Replies i almost gave up. I got this question in the Sybex S+.MCITP: EA 2008 VCP4 MCSE 2003  CCNA  MCSA 2003: Security  MCDST  Security+  ITILV3

Gratefullaz Member Posts: 8 ■□□□□□□□□□You would be better off knowing the functions of existing asymmetric technologies than some hypothetical.
Rivest Shamir Adleman (RSA) Uses a specific oneway function based on the difficulty of factoring N, a product of two large prime numbers (200 digits).
DiffieHellman Key Exchange Uses two system parameters (p and g).
• p is a prime number.
• g is an integer smaller than p generated by both parties.
El Gamal Extends DiffieHellman for use in encryption and digital signatures.
Elliptic Curve (EC) • Used in conjunction with other methods to reduce the key size.
• An elliptic curve key of 160 bits is equivalent to 1024bit RSA key, which means less computational power and memory requirements.
• Suitable for hardware applications (e.g.: smart cards and wireless devices).