FIT2093 Chapter Notes - Chapter 6: General Idea, Link Encryption, Key Distribution Center
31 May 2018
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FIT2093 W6 - Text Notes!
Symmetric Key Cryptography II!
1. What is the purpose of the S-boxes in DES?!
DES: Data Encryption Standard, s-box = substitution-box basic component of symmetric key
algorithms which performs substitutions, because DES is a block cipher the s-boxes are used to
obscure the relationship between the key and the cipher text!
-a block cipher, processed in 64-bit blocks!
-Non linear !
2. What is triple encryption?!
A mode of the DES encryption algorithm that encrypts data 3 times, three 64-bit keys are used,
instead of one, for an overall key length of 192 bits,!
First encryption is encrypted with the second key, and the resulting cipher text is again encrypted
with a third key)!
3. What is a meet-in-the-middle attack?!
A plaintext attack that targets block cipher cryptographic functions which performs multiple
encryption operations in sequence!
- have to know some parts of the plaintext and their cipher texts, using the MITM attack it is
possible to break ciphers, which have two or more secret keys for multiple encryption using the
same algorithm, eg. 3DES!
4. Why is it not desirable to reuse a stream cipher key?!
If two plaintext are encrypted with the same key using a stream cipher then cryptanalysis is often
quite simple, if the two cipher text streams are XORed together the result is the XOR of the
original plaintext !
XOR: digital logic gate that gives a true output when the number of true inputs is odd, implements
and exclusive or; that is, a true output results if one, and only one, of the inputs to the gate is true!
5. !
6. Why do some block cipher modes of operation only use encryption while others use both
encryption and decryption?!
In some modes, the plaintext does not pass through the encryption functions, but is XORed with
the output of the encryption function. For decryption in these cases, the encryption function must
also be used!
7. For each of the modes, ECB, CBC, CTR shown in figures 6.3, 6.4 and 6.7 respectively:!
"a) identify which decrypted plaintext blocks Px will be corrupted if there is an error in block
"C4 of the transmitted ciphertext!
"ECB: only C4 as they are idenpendent!
"CBC: only the connecting block would be corrupted, so 2 blocks, all the other would be
" " correct!
"CTR: only C4 would be corrupted!
"b)!
8. No it is not possible to perform encryption in parallel as the feedback from the first encryption
needs to be completed before you encrypt the next, partial parallel is possible though as you can
perform the XOR operation !
9. After 8 consecutive blocks shifting, you will get the correct transmission!
What are the essential ingredients of a symmetric cipher?!
+ Plaintext!
+ Encryption algorithm!
find more resources at oneclass.com
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Document Summary
Des: data encryption standard, s-box = substitution-box basic component of symmetric key algorithms which performs substitutions, because des is a block cipher the s-boxes are used to obscure the relationship between the key and the cipher text. A block cipher, processed in 64-bit blocks. A mode of the des encryption algorithm that encrypts data 3 times, three 64-bit keys are used, instead of one, for an overall key length of 192 bits, A plaintext attack that targets block cipher cryptographic functions which performs multiple encryption operations in sequence. If two plaintext are encrypted with the same key using a stream cipher then cryptanalysis is often quite simple, if the two cipher text streams are xored together the result is the xor of the original plaintext. In some modes, the plaintext does not pass through the encryption functions, but is xored with the output of the encryption function.
