Cryptography Reference
In-Depth Information
Problems
10.1.
In Sect. 10.1.3 we state that sender (or message) authentication always implies
data integrity. Why? Is the opposite true too, i.e., does data integrity imply sender
authentication? Justify both answers.
10.2.
In this exercise, we want to consider some basic aspects of security services.
1. Does privacy always guarantee integrity? Justify your answer.
2. In which order should confidentiality and integrity be assured (should the entire
message be encrypted first or last)? Give the rationale for your answer.
10.3.
Design a security service that provides data integrity, data confidentiality and
nonrepudiation using public-key cryptography in a two-party communication sys-
tem over an insecure channel. Give a rationale that data integrity, confidentiality
and nonrepudiation are achieved by your solution. (Recommendation: Consider the
corresponding threats in your argumentation.)
10.4.
A painter comes up with a new business idea: He wants to offer custom paint-
ings from photos. Both the photos and paintings will be transmitted in digital form
via the Internet. One concern that he has is discretion towards his customers, since
potentially embarrassing photos, e.g., nude photos, might be sent to him. Hence,
the photo data should not be accessible for third parties during transmission. The
painter needs multiple weeks for the creation of a painting, and hence he wants to
assure that he cannot be fooled by someone who sends in a photo assuming a false
name. He also wants to be assured that the painting will definitely be accepted by
the customer and that she cannot deny the order.
1. Choose the necessary security services for the transmission of the digitalized
photos from the customers to the painter.
2. Which cryptographic elements (e.g., symmetric encryption) can be utilized to
achieve the security services? Assume that several megabytes of data have to be
transmitted for every photo.
10.5.
Given an RSA signature scheme with the public key (
n
= 9797
,
e
= 131),
which of the following signatures are valid?
1. (
x
= 123
,
sig(
x
)=6292)
2. (
x
= 4333
,
sig(
x
)=4768)
3. (
x
= 4333
,
sig(
x
)=1424)
10.6.
Given an RSA signature scheme with the public key (
n
= 9797
,
e
= 131),
show how Oscar can perform an existential forgery attack by providing an example
of such for the parameters of the RSA digital signature scheme.
10.7.
In an RSA digital signature scheme, Bob signs messages
x
i
and sends them
together with the signatures
s
i
and her public key to Alice. Bob's public key is the
pair (
n
,
e
); her private key is
d
.
