Unformatted text preview: CHAPTER 2 SYMMETRIC ENCRYPTION AND
ANSWERS TO QUESTIONS
2.1 Plaintext, encryption algorithm, secret key, ciphertext, decryption
2.2 Permutation and substitution.
2.3 One secret key.
2.4 A stream cipher is one that encrypts a digital data stream one bit or
one byte at a time. A block cipher is one in which a block of plaintext
is treated as a whole and used to produce a ciphertext block of equal
2.5 Cryptanalysis and brute force.
2.6 In some modes, the plaintext does not pass through the encryption
function, but is XORed with the output of the encryption function. The
math works out that for decryption in these cases, the encryption
function must also be used.
2.7 With triple encryption, a plaintext block is encrypted by passing it
through an encryption algorithm; the result is then passed through the
same encryption algorithm again; the result of the second encryption is
passed through the same encryption algorithm a third time. Typically,
the second stage uses the decryption algorithm rather than the
2.8 There is no cryptographic significance to the use of decryption for the
second stage. Its only advantage is that it allows users of 3DES to
decrypt data encrypted by users of the older single DES by repeating
the key. -9- ...
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- Summer '11