PRNGs, RC4 and Block-Cipher Modes of Operation

Stream Cipher & RC4 Modes Of Operation

Number Generators (PRNGs)

Stream Ciphers and rc4

properly designed, can be as secure as a block cipher with same key length but usually simpler & faster however must never reuse same key

• otherwise cryptanalysis is quite simple

Example: RC4

a proprietary cipher owned by RSA DSI

• another Ron Rivest design, simple but effective
• variable key size, byte-oriented stream cipher
• widely used (web SSL/TLS, wireless WEP/WPA)
• key forms random permutation of all 8-bit values
• uses that permutation to scramble input info
• processed a byte at a time

encryption continues shuffling array values sum of shuffled pair selects "stream key" value from permutation (k=S[t]) XOR S[t] with next byte of message to en/decrypt

i = j = 0 for each message byte M

i

i = (i + 1) (mod 256) j = (j + S[i]) (mod 256) swap(S[i], S[j]) t = (S[i] + S[j]) (mod 256) C = M XOR S[t]

i

i

claimed secure against known attacks

• have some analysis, but none practical since RC4 is a stream cipher, must never reuse a key have a concern with WEP, but due to the way keys are generated for use as input to RC4 rather than RC4 algorithm itself

Modes of Operation

block ciphers encrypt fixed size blocks

• eg. DES encrypts 64-bit blocks with 56-bit key need some way to en/decrypt arbitrary amounts of data in practise e.g. a message of 1000 bytes ⚫

NIST SP 800-38A defines 5 modes have block and stream modes to cover a wide variety of applications can be used with any block cipher

at end of message must handle a possible last short block

• which is not as large as blocksize of cipher
• pad either with known non-data value (eg nulls)
• or pad last block along with count of pad size
  • eg. [ b1 b2 b3 0 0 0 0 5]
  • means have 3 data bytes, then 5 bytes pad+count
• this may require an extra entire block over those in message there are other modes, which avoid the need for an extra block

message is broken into independent blocks which are encrypted Each block encrypted using the same key each block is a value which is substituted, like a codebook, hence name each block is encoded independently of the other blocks

C = E (P)

i K i

uses

: secure transmission of single values

Electronic Codebook Book (ECB)

message repetitions may show in ciphertext

• if aligned with message block
• particularly with data such graphics
• or with messages that change very little, which become a code-book analysis problem weakness is due to the encrypted message blocks being independent main use is sending a few blocks of data. e.g. Session encryption keys

message is broken into blocks linked together in encryption operation each previous cipher blocks is chained with current plaintext block, hence name use Initial Vector (IV) to start process

C = E (P XOR C)

i K i

i-1

C = IV

-1

uses: bulk data encryption, authentication

Cipher Block Chaining (CBC)

a ciphertext block depends on all blocks before it any change to a block affects all following ciphertext blocks need Initialization Vector (IV)

• which must be known to sender & receiver
• if sent in clear, attacker can change bits of first block, and change IV to compensate
• hence IV must either be a fixed value
• (as in EFTPOS Electronic Funds Transfer at Point Of Sale)
• or must be sent encrypted in ECB mode before rest of message

block modes encrypt entire block may need to operate on smaller units

• real time data convert block cipher into stream cipher
• cipher feedback (CFB) mode
• output feedback (OFB) mode
• counter (CTR) mode use block cipher as some form of pseudo-random number generator

message is treated as a stream of bits added to the output of the block cipher result is feed back for next stage (hence name) standard allows any number of bit (1,8, 64 or 128 etc) to be feed back

• denoted CFB-1, CFB-8, CFB-64, CFB-128 etc most efficient to use all bits in block (64 or 128)

C = P XOR E (C)

i

i

K i-1

C = IV

-1

uses

: stream data encryption, authentication

s-bit Cipher Feed Back (CFB-s)

appropriate when data arrives in bits/bytes most common stream mode errors propagate for several blocks after the error

message is treated as a stream of bits output of cipher is added to message output is then feed back (hence name) feedback is independent of message can be computed in advance

O = E (O)

i

K i-1

C = P XOR O

i

i

i

O = IV

-1

: stream encryption on noisy channels

uses

Output FeedBack (OFB)

needs an IV which is unique for each use

• if ever reuse attacker can recover outputs

Advantage: bit errors do not propagate Disadvantage: more vulnerable to message stream modification sender & receiver must remain in sync only use with full block feedback

• subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used

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 value for every plaintext block (never reused)

O = E (i)

i K

C = P XOR O

i i

i

uses: high-speed network encryptions

Counter (CTR)

efficiency

• can do parallel encryptions in h/w or s/w
• can preprocess in advance of need
• can process blocks of plaintext or ciphertext in a random-access fashion provable security (good as other modes) but must ensure never reuse key/counter values, otherwise could break (cf OFB)

Network Security Essentials: Applications and Standards, chapter 2.

Network Security: Private Communication in a Public World, chapter 3.