Rijndael Cipher: Key Generation and Mode of Operation, Slides of Computer Security

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Rijndael Cipher

For the first round, we use the original key.

For the subsequent rounds, we need to buildthe key.

Rijndael Cipher

W

W

are given from the key

i could be from 4 to 44

There are two cases:

When i is a multiple of 4 and is < 44.

When i is not a multiple of 4 and is <

Rijndael Cipher

From the resultant (b, c, d, a) vector , construct (e, f, g, h) using the16x16 S-boxNow the only remaining unknown variable is r

(i)

Rijndael Cipher

r

(i) is calculated using the following formula:

r

(i) = (00000010)

(i-4)/

in GF (

wait till 2 slides for the maths to come

Other Block Ciphers

Cipher Block Chaining

Cipher Block Chaining

Cipher Feed-back Mode

Cipher Feed-back Mode

Output Feed-back Mode

Counter (CTR)

A “new” mode, though proposed early on.

Similar

to

OFB

but

encrypts

counter

value

rather than any feedback value

Must have a different key & counter pair forevery plaintext block (never reused)

uses: high-speed network encryptions

Counter (CTR)

Advantages and Limitations of CTR

Efficiency

can do parallel encryptions in h/w or s/w

can preprocess in advance of need

good for bursty high speed links

Random access to encrypted data blocks

Provable security (good as other modes)

But must ensure to never reuse key/counterpair values, otherwise could break.