Python Quiz

1.What is output for − 'search'. find('S') ?

s -1 ‘ ‘ None of the above

2.What is the output of following code −[ (a,b) for a in range(3) for b in range(a) ]

[ (1,0),(2,1),(3,2)] [ (0,0),(1,1),(2,2)] [(1,0),(2,1),(2,1)] [ (1,0),(2,0),(2,1)]

3.How can we generate random numbers in python using methods?

random.uniform () random.randint() random.random() All of the above

4.Suppose we have a set a = {10,9,8,7}, and we execute a.remove(14) what will happen ?

We cannot remove an element from set. Method is executed but no exception is raised. Key error is raised. There doesn’t exist such method as remove.

5.Which code is used to open a file for binary writing?

''w'' ''wb'' ''r+'' ''a''

The Key Distribution Center

public key encryption has its own difficulties, in particular the problem of obtaining someone's true public key. Both of these problems – determining a shared key for symmetric key cryptography, and securely obtaining the public key for public key cryptography – can be solved using a trusted intermediary. For symmetric key cryptograghy , the trusted intermediary is called a Key Distribution Center (KDC), which is a single, trusted network entity with whom one has established a shared secret key. We will see that one can use the KDC to obtain the shared keys needed to communicate securely with all other network entities. For public key cryptography, the trusted intermediary is called a Certification Authority (CA). A certification authority certifies that a public key belongs to a particular entity (a person or a network entity). For a certified public key, if one can safely trust the CA that the certified the key, then one can be sure about to whom the public key belongs. Once a public key is certified, then it can be distributed from just about anywhere, including a public key server, a personal Web page or a diskette.

The Key Distribution Center

Suppose once again that Bob and Alice want to communicate using symmetric key cryptography. They have never met (perhaps they just met in an on-line chat room) and thus have not established a shared secret key in advance. How can they now agree on a secret key, given that they can only communicate with each other over the network? A solution often adopted in practice is to use a trusted Key Distribution Center (KDC).

The KDC is a server that shares a different secret symmetric key with each registered user. This key might be manually installed at the server when a user first registers. The KDC knows the secret key of each user and each user can communicate securely with the KDC using this key. Let's see how knowledge of this one key allows a user to securely obtain a key for communicating with any other registered user. Suppose that Alice and Bob are users of the KDC; they only know their individual key, K_A-KDC and K_B-KDC, respectively, for communicating securely with the KDC. Alice takes the first step, and they proceed as illustrated in Figure 7.5-1.

 Setting up a one-time session key using a Key Distribution Center

• Using K_A-KDC to encrypt her communication with the KDC, Alice sends a message to the KDC saying she (A) wants to communicate with Bob (B). We denote this message, K_A-KDC (A,B) . As part of this exchange, Alice should authenticate the KDC (see homework problems), e.g., using an authentication protocol (e.g., our protocol ap4.0) and the shared key K_A-KDC .

Digital Signature Algorithm

DSA is a United States Federal Government standard for digital signatures. It was proposed by the National Institute of Standards and Technology (NIST) in August 1991 for use in their Digital Signature Standard (DSS), specified in FIPS 186 in 1993.

The first part of the DSA algorithm is the public key and private key generation, which can be described as:

• Choose a prime number q, which is called the prime divisor.
• Choose another primer number p, such that p-1 mod q = 0. p is called the prime modulus.
• Choose an integer g, such that 1 < g < p, g**q mod p = 1 and g = h**((p–1)/q) mod p. q is also called g's multiplicative order modulo p.
• Choose an integer, such that 0 < x < q.
• Compute y as g**x mod p.
• Package the public key as {p,q,g,y}.
• Package the private key as {p,q,g,x}.

The second part of the DSA algorithm is the signature generation and signature verification, which can be described as:

To generate a message signature, the sender can follow these steps:

• Generate the message digest h, using a hash algorithm like SHA1.
• Generate a random number k, such that 0 < k < q.
• Compute r as (g**k mod p) mod q. If r = 0, select a different k.
• Compute i, such that k*i mod q = 1. i is called the modular multiplicative inverse of k modulo q.
• Compute s = i*(h+r*x) mod q. If s = 0, select a different k.
• Package the digital signature as {r,s}.