A solution that uses the meet in the middle approach has a pretty good balance between memory resources and query execution. We issue two queries, one for the friends of the first user and one for the friends of the second user. Then we intersect the two sets. Now we use just 2 requests, the graph takes O(nk) space, and the reply sizes are O(k).

h2. Meet in the middle attack (not to be confused with man in the middle attack)

h2. Breaking 2DES

DES is a encryption standard which uses 56 bit keys. This was enough when the algorithm got invented but currently computers can use a brute force approach to break the encryption.  One simple idea to make the encryption more secure is to apply the encryption twice using two different keys. This approach is susceptible to the meet in the middle attack developed by Diffie Hellman. So currently 3DES is used which uses three keys and encrypts the message three times.