Review Questions
19.1 What are the five principal services provided by PGP?
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19.2 What is the utility of a detached signature?
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19.3 Why does PGP generate a signature before applying compression?
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19.4 What is R64 conversion?
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19.5 Why is R64 conversion useful for an e-mail application?
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19.6 How does PGP use the concept of trust?
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19.7 What is RFC 5322?
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19.8 What is MIME?
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19.9 What is S/MIME?
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19.10 What is DKIM?
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Problems
19.1 PGP makes use of the cipher feedback (CFB) mode of CAST-128, whereas most
symmetric encryption applications (other than key encryption) use the cipher block
chaining (CBC) mode. We have
CBC: Ci = E(K, [Ci-1 Pi]); Pi = Ci-1 D(K, Ci)
CFB: Ci = Pi E(K, Ci-1); Pi = Ci E(K, Ci-1)
These two appear to provide equal security. Suggest a reason why PGP uses the CFB
mode.
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19.2 In the PGP scheme, what is the expected number of session keys generated before a
previously created key is produced?
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19.3 As discussed in Appendix P, a PGP user may have multiple public keys. So that a
recipient knows which public key is being used by a sender, a key ID, consisting of the
least significant 64 bits of the public key, is sent with the message. What is the probability
that a user with N public keys will have at least one duplicate key ID?
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19.4 As discussed in Appendix P, the first 16 bits of the message digest in a PGP signature
are translated in the clear. This enables the recipient to determine if the correct public
key was used to decrypt the message digest by comparing the plaintext copy of the
first two octets with the first two octets of the decrypted digest.
a. To what extent does this compromise the security of the hash algorithm?
b. To what extent does it in fact perform its intended function, namely, to help determine
if the correct RSA key was used to decrypt the digest?
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19.5 For this problem and the next, consult Appendix P. In Figure P.2, each entry in the
public-key ring contains an Owner Trust field that indicates the degree of trust associated
with this public-key owner. Why is that not enough? That is, if this owner is
trusted and this is supposed to be the owner’s public key, why is that trust not enough
to permit PGP to use this public key?
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19.6 What is the basic difference between X.509 and PGP in terms of key hierarchies and
key trust?
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19.7 Phil Zimmermann chose IDEA, three-key triple DES, and CAST-128 as symmetric
encryption algorithms for PGP. Give reasons why each of the following symmetric
encryption algorithms described in this book is suitable or unsuitable for PGP: DES,
two-key triple DES, and AES.
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19.8 Consider radix-64 conversion as a form of encryption. In this case, there is no key.
But suppose that an opponent knew only that some form of substitution algorithm
was being used to encrypt English text and did not guess that it was R64. How effective
would this algorithm be against cryptanalysis?
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19.9 Encode the text “plaintext” using the following techniques. Assume characters are
stored in 8-bit ASCII with zero parity.
a. Radix-64
b. Quoted-printable
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