Symmetric key cryptography

Question 1

what are the strengths of symmetric key cryptography?

Answer 1

Speed/ Efficient Algorithms – much quicker
than Asymmetric
Hard to break when using a large Key Size
Ideal for bulk encryption / decryption

Question 2

what are the weaknesses of symmetric key cryptography?

Answer 2

Poor Key Distribution (must be done out of
band – ie phone, mail, etc)
Poor Key Management / Scalability (each user
needs a unique key)
Cannot provide authenticity or non-repudiation
– only confidentiality

Question 3

why was AES introduced?

Answer 3

Clearly, a replacement for DES was needed
– have theoretical attacks that can break it
– have demonstrated exhaustive key search attacks
•Can use Triple-DES – but slow with small blocks

Question 4

what are the problems with ECB?

Answer 4

•Repetitions in message may show in ciphertext if
aligned with message block particularly with data such
graphics
•Messages that change very little
•Weakness due to encrypted message blocks being
independent
•Main use is sending a few blocks of data

Question 5

what are uses of CBC?

Answer 5

bulk data encryption , authentication

Question 6

what is the advantage of CBC?

Answer 6

Each ciphertext block depends on all preceding message
blocks thus a change in the message affects all ciphertext
blocks after the change as well as the original block

Question 7

what is the weakness of CBC?

Answer 7

Need Initial Value (IV) known to sender & receiver
however if IV is sent in the clear, an attacker can change
bits of the first block, and change IV to compensate hence
either IV must be a fixed value or it must be sent encrypted
in ECB mode before rest of message

Question 8

Compare CFB and OFB

Answer 8

CFB - Errors will propagate in this mode
- No IV required

OFB - Errors will not propagate in this mode
- IV required

Question 9

what is the use of CTR

Answer 9

high-speed network encryptions

Question 10

what are the advantages and limitations of CTR

Answer 10

can do parallel encryptions in h/w or s/w
•can preprocess in advance of need
•good for high speed links
•random access to encrypted data blocks
•provable security (good as other modes)
•but must ensure never reuse key/counter
values, otherwise could break

Question 11

what is the use of key escrow

Answer 11

Key escrow is a data security measure in which a cryptographic key is entrusted to a third party (i.e., kept in escrow).