Review Questions
7.1 What are the five principal services provided by PGP? Get this solution
7.2 What is the utility of a detached signature? Get this solution
7.3 Why does PGP generate a signature before applying compression? Get this solution
7.4 What is R64 conversion? Get this solution
7.5 Why is R64 conversion useful for an e-mail application? Get this solution
7.6 How does PGP use the concept of trust? Get this solution
7.7 What is RFC 5322? Get this solution
7.8 What is MIME? Get this solution
7.9 What is S/MIME? Get this solution
7.10 What is DKIM? Get this solution
Problems
7.1 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. Get this solution
7.2 In the PGP scheme, what is the expected number of session keys generated before a previously created key is produced? Get this solution
7.3 In PGP, what is the probability that a user with public keys will have at least one duplicate key ID? Get this solution
7.4 The first 16 bits of the message digest in a PGP signature are translated in the clear.
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? Get this solution
7.5 In Figure 7.4, 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? Get this solution
7.6 What is the basic difference between X.509 and PGP in terms of key hierarchies and key trust? Get this solution
7.7 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. Get this solution
7.8 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? Get this solution
7.9 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 Get this solution
