# Solution Manual For Computer Security: Principles And Practice, 3rd Edition

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CHAPTER 2 CRYPTOGRAPHIC TOOLS
ANSWERS TO QUESTIONS
2.1 Plaintext, encryption algorithm, secret key, ciphertext, decryption
algorithm.
2.2 One secret key.
2.3 (1) a strong encryption algorithm; (2) Sender and receiver must have
obtained copies of the secret key in a secure fashion and must keep the
key secure.
2.4 Message encryption, message authentication code, hash function.
2.5 An authenticator that is a cryptographic function of both the data to be
authenticated and a secret key.
2.6 (a) A hash code is computed from the source message, encrypted using
symmetric encryption and a secret key, and appended to the message.
At the receiver, the same hash code is computed. The incoming code is
decrypted using the same key and compared with the computed hash
code. (b) This is the same procedure as in (a) except that public-key
encryption is used; the sender encrypts the hash code with the sender’s
private key, and the receiver decrypts the hash code with the sender’s
public key. (c) A secret value is appended to a message and then a
hash code is calculated using the message plus secret value as input.
Then the message (without the secret value) and the hash code are
transmitted. The receiver appends the same secret value to the
message and computes the hash value over the message plus secret
value. This is then compared to the received hash code.
2.7
1. H can be applied to a block of data of any size.
2. H produces a fixed-length output.
3. H(x) is relatively easy to compute for any given x, making both
hardware and software implementations practical.
4. For any given value h, it is computationally infeasible to find x such
that H(x) = h.
5. For any given block x, it is computationally infeasible to find y โ x
with H(y) = H(x).
-9-
6. It is computationally infeasible to find any pair (x, y) such that H(x)
= H(y).
2.8 Plaintext: This is the readable message or data that is fed into the
algorithm as input. Encryption algorithm: The encryption algorithm
performs various transformations on the plaintext. Public and private
keys: This is a pair of keys that have been selected so that if one is
used for encryption, the other is used for decryption. The exact
transformations performed by the encryption algorithm depend on the
public or private key that is provided as input. Ciphertext: This is the
scrambled message produced as output. It depends on the plaintext and
the key. For a given message, two different keys will produce two
different ciphertexts. Decryption algorithm: This algorithm accepts
the ciphertext and the matching key and produces the original plaintext.
2.9 Encryption/decryption: The sender encrypts a message with the
recipient’s public key. Digital signature: The sender “signs” a message
with its private key. Signing is achieved by a cryptographic algorithm
applied to the message or to a small block of data that is a function of
the message. Key exchange: Two sides cooperate to exchange a
session key. Several different approaches are possible, involving the
private key(s) of one or both parties.
2.10 The key used in conventional encryption is typically referred to as a
secret key. The two keys used for public-key encryption are referred
to as the public key and the private key.
2.11 A digital signature is an authentication mechanism that enables the
creator of a message to attach a code that acts as a signature. The
signature is formed by taking the hash of the message and encrypting
the message with the creator’s private key. The signature guarantees
the source and integrity of the message.
2.12 A pubic-key certificate consists of a public key plus a User ID of the
key owner, with the whole block signed by a trusted third party.
Typically, the third party is a certificate authority (CA) that is trusted
by the user community, such as a government agency or a financial
institution.
2.13 Several different approaches are possible, involving the private key(s)
of one or both parties. One approach is Diffie-Hellman key exchange.
Another approach is for the sender to encrypt a secret key with the
recipient’s public key.
ANSWERS TO PROBLEMS
-10-
2.1 Yes. The eavesdropper is left with two strings, one sent in each
direction, and their XOR is the secret key.
2.2 a.
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b. The two matrices are used in reverse order. First, the ciphertext is
laid out in columns in the second matrix, taking into account the
order dictated by the second memory word. Then, the contents of
the second matrix are read left to right, top to bottom and laid out in
columns in the first matrix, taking into account the order dictated by
the first memory word. The plaintext is then read left to right, top to
bottom.
c. Although this is a weak method, it may have use with time-sensitive
information and an adversary without immediate access to good
cryptanalysis t(e.g., tactical use). Plus it doesn’t require anything
more than paper and pencil, and can be easily remembered.
-11-
2.3 a. Let -X be the additive inverse of X. That is -X
P = (C + โK1) โ K0
b. First, calculate โC’. Then โC’ = (P’ โ K0)
+ X = 0. Then:
+ (โ K1). We then have:
+ โC’ = (P โ K0) + (P’ โ K0)
However, the operations + and โ are not associative or distributive
C
with one another, so it is not possible to solve this equation for K0.
2.4 a. The constants ensure that encryption/decryption in each round is
different.
-12-
b. First two rounds:
L0
R0
K0
<>5
L1
R1
K2
<>5
L2
R2
-13-
c. First, let’s define the encryption process:
L2 = L0 + [(R0 << 4) + K0] โ [R0 +
R2 = R 0
+ [(L2 <> 5) + K3]
ฮด1] โ [(R0 >> 5)
Now the decryption process. The input is the ciphertext (L2, R2), and
the output is the plaintext (L0, R0). Decryption is essentially the same
as encryption, with the subkeys and delta values applied in reverse
order. Also note that it is not necessary to use subtraction because
there is an even number of additions in each equation.
R0 = R 2
+ [(L2 <> 5) + K3]
L0 = L2
+ [(R0 <> 5) + K1]
-14-
d.
L0
R0
K0
<>5
L1
R1
K2
<>5
L2
R2
2.5 a. Will be detected with both (i) DS and (ii) MAC.
b. Wonโt be detected by either (Remark: use timestamps).
c. (i) DS: Bob simply has to verify the message with the public key
from both. Obviously, only Aliceโs public key results in a successful
verification.
-15-
(ii) MAC: Bob has to challenge both, Oscar and Bob, to reveal their
secret key to him (which he knows anyway). Only Bob can do that.
d. (i) DS: Alice has to force Bob to prove his claim by sending her a
copy of the message in question with the signature. Then Alice can
show that message and signature can be verified with Bobโs public
key ) Bob must have generated the message.
(ii) MAC: No, Bob can claim that Alice generated this message.
2.6 The statement is false. Such a function cannot be one-to-one because
the number of inputs to the function is of arbitrary, but the number of
unique outputs is 2n. Thus, there are multiple inputs that map into the
same output.
-16-
2.7 a. Overall structure:
N ร 16 letters
Message
16 letters
16 letters
M1
M2
16
IV = 4
0000
F
Padding
16 letters
โขโขโข
MN
16
H1
16
H2
F
F
โขโขโข
HN =
hash
code
Compression function F:
Hiโ1
Mi
Column-wise mod 26 addition
row-wise
rotations
Column-wise mod 26 addition
Hi
b. BFQG
c. Simple algebra is all you need to generate a result:
AYHGDAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAA
-17-
2.8
5
1
a. M3 = 3
4
2
2
4
1
3
5
1
3
2
4
5
4
2
5
1
3
5
2
3
4
1
b. Assume a plaintext message p is to be encrypted by Alice and sent to
Bob. Bob makes use of M1 and M3, and Alice makes use of M2. Bob
chooses a random number, k, as his private key, and maps k by M1
to get x, which he sends as his public key to Alice. Alice uses x to
encrypt p with M2 to get z, the ciphertext, which she sends to Bob.
Bob uses k to decrypt z by means of M3, yielding the plaintext
message p.
c. If the numbers are large enough, and M1 and M2 are sufficiently
random to make it impractical to work backwards, p cannot be found
without knowing k.
2.9 We show the creation of a digital envelope:
Sender’s
private
key
H
E
Hash code
Message
Digital
signature
E
Encrypted
message
Random
symmetric
key
Digital
envelope
E
Receiver’s
public
key
Encrypted
symmetric
key
-18-

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