Test 3 Review B

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Problem R3b.1.

In storing passwords, what is “salt,” and how does it help keep the passwords secure?

Salt is a random sequence of characters that is generated for each user separately, and it is stored along with the cryptographic hash of the result of appending the password onto the hash. To verify a password entered by a user logging in, we read the user's salt from the database, append the password to it, perform a cryptographic hash, and then confirm that this result matches what is stored in the database.

This is much more secure than simply storing the hash of the password, since doing the latter makes it far easier to perform a dictionary attack: The attacker can perform a single hash of a frequently-used password and compare it to all hashed values, whereas with a salted system, the attacker must try each individual salt value along with the frequently-used password, slowing the decryption process down considerably.

Problem R3b.3.

What is displayed when the below program is executed?

nums = [7, 9, 6, 7, 8]

map = {}

for i in range(5):

    n = nums[i]

    map[n] = i

for key in map:

    print('{0} to {1}'.format(key, map[key]))

7 to 3          (Lines may be reordered in any way.)
9 to 1
8 to 4
6 to 2

Problem R3b.4.

What is displayed by the following Python fragment?

words = 'The quick brown fox jumps over the lazy dog'

map = {}

for word in words.split():

    map[len(word)] = word

for k in range(3, 6):

    print('{0} {1}'.format(k, map[k]))

3 dog
4 lazy
5 jumps

Problem R3b.5.

Suppose census is a dictionary mapping state names to populations. Complete the following so that it displays the total population of all the states.





for state in census:







print total_pop

total_pop = 0

for state in census:

    total_pop += census[state]

print total_pop

Problem R3b.7.

We have a dictionary identified by word_counts in which all keys are strings. What is wrong with the following program to remove all keys that start with p, and what can be done to repair it?

for key in word_counts:

    if len(key) > 0 and key[0] == 'p':

        word_counts.pop(key)

While we are iterating through the keys of a dictionary (as with for…in), Python will not allow us to change that very same dictionary, as pop does.

The best way to repair this would be to create a secondary list into which we place all the keys we wish to remove. Then, after completing our iteration through the list, we would have a second loop through this secondary list in which we could remove each key that was added into this list.