For the remaining questions, consider a 4-bit block cipher, described in hexadec
ID: 3815475 • Letter: F
Question
For the remaining questions, consider a 4-bit block cipher, described in hexadecimal by the following table:
Plaintext
Ciphertext
Plaintext
Ciphertext
0
a
8
e
1
c
9
d
2
f
a
0
3
6
b
7
4
3
c
5
5
8
d
b
6
4
e
9
7
2
f
1
You can think of this as a simple substitution cipher for hexadecimal digits. There is no “key” other than the table itself.
Encrypt the 4-block message face using this cipher for each digit individually.
Decrypt the message baab using this cipher.
Why is it insecure to use a block cipher in this way?
Plaintext
Ciphertext
Plaintext
Ciphertext
0
a
8
e
1
c
9
d
2
f
a
0
3
6
b
7
4
3
c
5
5
8
d
b
6
4
e
9
7
2
f
1
Explanation / Answer
Given plain text is face , from the table
chiper text corresponds to plain text f is 1
chiper text corresponds to plain text a is 0
chiper text corresponds to plain text c is 5
chiper text corresponds to plain text e is 9
Hence the cipher text is 1059.
Given chiper text is baab, from the table
plain text corresponds to chiper text b is d
plain text corresponds to chiper text a is 0
plain text corresponds to chiper text a is 0
plain text corresponds to chiper text b is d
Hence the plain text is d00d.
This way of encryption is insecure as we are substituing one letter at a time any
hacker can easily decrypt the message.
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