Suppose that you receive a digital certificate that contains M and (h(M)]CA, where M = (Alice, Alice'spublic key) and "CA" is Certificate Authority.Assuming that you trust the CA, after verifying the signature on the certificate, what do you thenknow about the identity of the sender of the certificate?Remember that:[Xleob indicates encryption via Bob's private key to X (signature)h(X) indicates the cryptographic hash function applied to XO The sender is AliceO The sender is TrudyI cannot deduce the identity of the sender from the certificateThe sender is the Certificate Authority

Identifying the sender of certificate in a digital certificate The digital certificate contains M…

Answered: Suppose that you receive a digital…

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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Consider the case of Alice sending a message, m, to Bob. Both Alice and Bob use public key cryptography and each has a public and private key as described in the text. The figure attached below shows the operations that Alice must perform to provide confidentiality, authentication, and integrity when sending a message to Bob over the network. We can use either symmetric key or public key cryptography to encrypt a message. For our purposes, either technique will encrypt the message, and applying both doesn't make it "more secure". Also, we can assume that the session key would remain a secret so the fact that it is discarded does not make it "more secure". Why do we use a session key, Ks, instead of relying only on public key Cryptography? In other words, why do we use both public key and symmetric key cryptography?
How does Alice verify that a message M that was signed by Bob using RSA is really from Bob? Assume that eA and dA are the encryption and decryption exponents for Alice while eB and dB are those for Bob. The agreed upon hash function is h. All congruences are mod N and Alice has Bob's certificate. Question 10 options: She compares h(M) to DSdA mod N She compares h(M) to DSdB mod N She compares h(M) to DSeB mod N She compares h(M) to DSeA mod N
Consider the failed attempt of Alice to get Bob’s public key inFig. 8-25 . Suppose that Bob and Alice already share asecret key, but Alice still wants Bob’s public key. Is there now away to get it securely? If so, how?
Let Π = (Gen,Enc,Dec) be a private-key encryption scheme that has indistinguishable en- cryptions in the presence of an eavesdropper. Which of the following encryption schemes are also necessarily secure against an eavesdropper? If you think a scheme is secure, sketch a proof, if not, provide a counterexample. Here, for a bit string s, parity(s) is 1 if the number of 1’s in s is odd, and 0 otherwise. The || symbol stands for concatenation. So, for strings if x = 00 and y = 11, x||y = 0011. (a) Enc1k(m) = 0||Enck(m)(b) Enc2k(m) = Enck(m)||parity(m) (c) Enc3k(m) = Enck(m)||Enck(m)(d) Enc4k(m) = Enck(m)||Enck(m + 1). Here think of m as an integer. Please type answer note write by hend.
For the given scenario where Alice is trying to send encrypted data to Bob, and Bob is trying to decrypt it, using Elgamal encryption scheme, fill the values in the table
Consider a large institution that has 16891 employees. How many keys are necessary to allow any pair of employees to securely communicate using a symmetric cipher? Suppose that we instead use an asymmetric cryptographic scheme. How many keys are required in this case?
Given the following table describing the procedure for Alice to send a signed message with RSA signature to Bob, calculate the unknown entities and verify that Bob has received the correct message sent by Alice. Fill in the values of the "???" below. Alice Bob Public key (n,e) =(55,3); Private key (n,d)=(55,27) Send Public key (n,e ) to Bob: Receives Alice’s public key (n,e): Message to be sent is m=3 Computes signatures s for m = mdmod n = ??? Send (m,s) to Bob Receives (m,s) = ??? Compute m’ : se mod n = ??? b.Elgamal Signature Scheme: Given the following table describing the procedure for Alice to send a signed message with Elgamal signature to Bob, calculate the unknown entities and verify that Bob has received the correct message sent by Alice. Fill in the values of the "???" below. Alice Bob Chooses p=23 Chooses a primitive element α=7 Choose a random integer d=6 Compute β = αd mod p = ???…
Suppose we have the encoding that enables Carol to mount the cube root attack (see The Cube Root Problem). If Carol sends a message to Bob, supposedly signed by you, will there be anything suspicious and noticeable about the signed message, so that with very little additional computation Bob can detect the forgery? Is there anything Carol can do to make her messages less suspicious? Cuberoot problem: Let's say you pad on the right with random data. You chose that scheme so that there is a negligible probability that anything you sign will be smooth. However, if the public exponent is 3, this enables Carol to forge your signature on virtually any message she chooses! Let's say Carol wants your signature on some message. The message digest of that message is h. Carol pads h on the right with zeroes. She then computes its ordinary cube root and rounds up to an integer r. Now she has forged your signature, because re = r3 = (h padded on the right with a seemingly random number).
Suppose a key space of 230. Alice decides to encrypt a plaintext block 6 times with 6 different keys. What is the effective key space for Trudy?
Given Alice wants to communicate with Bob over an insecure network, you are required to• Design a protocol based on a shared secret key to allow them to authenticate each other; • Explain the limitation of using the shared secret key in this context
We consider ElGamal cryptosystem with public prime p = 53 and generator g = 2. Let h = 16 be Bob’s public-key (his private key x is unknown, h = g^x ). Alice encrypted message m with Bob’s public key and sent the corresponding ciphertext to Bob: (C1 ,C2 ) = (15, 50). You intercepted this ciphertext. Can you break the encryption and recover the original message m? Hint: Both g and h are very small numbers, can you deduce private key x?
Consider the encryption function for a symmetric cryptosystem E(k, m) described below, where K = M = {0, 1, 2, 3}, C = {0, 1, 2, 3}: E(k, m) = (2 ∗ k + m) mod 4 (a) Give the corresponding decryption function(b) Is this cryptosystem information-theoretically secure(or perfectly secure)? Why or why not?

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