A hash table is all the most likely codes needed to encrypt a key so if you think of the old punch cards with holes that would line up with a column of text to reveal a coded message contained within the body of text. A hash table is all the possible (within certain preprogammed limits as for keylength and characterset) different cards all ready for your decryption program to overlay onto the encoded text and check whether they decrypt the hidden message. A little overly simplified explanation, but essentially correct. A hash table can only be used to decrypt one thing (file, or section of) at once in any single process. Without the hash table precompiled the process would take ages as it would have to process each hash, test it, process the next hash, test it, ect. So its basically a crypto "cheat sheet".

WHy do you need to know?

a) number of keys to be stored is greater than the number of records to be stored.

b) a large number of keys are duplicate keys

c) number of keys actually stored at any point of time is small as compared to the number of possible keys

d) none of these

Answer is c). Can you please explain how since as per my understanding of hashing it is used for direct access and faster retrieval..then how could the answer be c)...Please explain....