Privacy and Integrity:“ Two Essences of Network Security”

Presenter

Prosanta Gope

AdvisorTzonelih

Hwang

Quantum Information and Network Security Lab, NCKU,2015

Flash Back

What we will do if the message size is shorter or larger than the block size?

Say, message Size =224 bit

Block Cipher Supported = 64 bit DES

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Modes of Operationpacket packetpacket

encrypted packet

encrypted packet

encrypted packet ….….

=

𝐾Encrypt

𝑃1

𝐾Encrypt

𝑃2

𝐾Encrypt

𝑃𝑁

…..

𝐶1 𝐶𝑁𝐶2

ex.ECB mode

BIG PLAINTEXTpacket

Modes of OperationWhy we need to use Modes of Operation?

I. Adapt cryptographic algorithm to applications. II. Increase the strength of a cryptographic algorithmIII. It is necessary to divide bigger plaintext into fixed sized blocks so that cipher can

work on it. (i.e.DES-64bit) i.e. Apply block cipher to a sequence of data blocks or data stream

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packet packetpacket ….….

DES DES DES

Conventional Modes of Operations

– Electronic Codebook Mode (ECB)– Cipher Block Chaining Mode (CBC) – Cipher Feedback Mode (CFB)– Output Feedback Mode (OFB)– CTR

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ECB: Electronic Codebook

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Notations.

– encryption key shared between two entities

- mathematical XOR

– Let P be the message to be encrypted, which is divided into { …}

– Let C be the message to be encrypted, which is divided into { …},

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Electronic Codebook Book (ECB)

Electronic Codebook Book (ECB)

• message is broken into independent blocks which are encrypted

• each block is a value which is substituted, like a codebook, hence name

• each block is encoded independently of the other blocks Ci = EK(Pi)

• uses: secure transmission of single values

8.11

The simplest mode of operation is called the electronic codebook (ECB) mode.

Electronic Codebook (ECB) Mode

Figure Electronic codebook (ECB) mode

Advantages and Limitations of ECB

message repetitions may show in ciphertext weakness is due to the encrypted message blocks being

independent main use is sending a few blocks of data

An Email between Alice and Bob with Following Header

Alice Bob

{Date: Tue, 21 Oct 2014 13: 34:47}EK

Trudy

Cipher-text Starts With the Encrypted Version of : “Date :Tue” Often Occur on Tuesday

So, Trudy Might be able to Guess Without Knowing the Plaintext that Such Messages are Email on Tuesday.

8.14

Electronic Codebook (ECB)

• Does not hide data patterns, unsuitable for long messages– Wiki example: pixel map using ECB

Plain text ECB mode Other modes

• Susceptible to replay attacks– Example: a wired transfer transaction can be replayed by resending the original message)

Other Issues?

Part 1 Cryptography 16

ECB Cut and Paste• Suppose plaintext is

Alice asks Bob. Trudy asks Tom.• Assuming 64-bit blocks and 8-bit ASCII:

P0 = “Alice as”, P1 = “ks Bob. ”,P2 = “Trudy as”, P3 = “ks Tom. ”

• Ciphertext: C0,C1,C2,C3

• Trudy cuts and pastes: C0,C3,C2,C1

• Decrypts asAlice asks Tom. Trudy asks Bob.

Questions?

Cipher Block Chaining (CBC)

Cipher Block Chaining

(CBC)

Cipher Block Chaining (CBC)

• message is broken into blocks • each previous cipher blocks is chained with current

plaintext block, hence name • use Initial Vector (IV) to start process

Ci = EK(Pi XOR Ci-1)

C-1 = IV

• uses: bulk data encryption, authentication

Weaknesses of CBC

A. Unable to eliminate the problem of some one modifying in transit. That to Leads security vulnerabilities “Bit Flipping Attack”.

B. Unable to detect “Rearranging of the Cipher Blocks”.C. Unable to assure “Integrity Protection”.

“Bit Flipping Attack”

ExampleName Position SalaryAdam President 785446Joe A Security Officer 541220Now Joe wants to Hike her salary and assuming in this case she has prior knowledge of final byte Say, P5.Hence She merely has to flip Penultimate bit of C4.

ResultName Position SalaryAdam President 785446Joe A Security Of#f87%$ 741220

A Human Can Detect this change,What About ,if it is just a Program???

What About if it ‘s a Computer??

Advantages and Limitations of CBC

a ciphertext block depends on all blocks before itneed 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 or must be sent encrypted in

ECB mode before rest of message

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In CBC mode, each plaintext block is exclusive-ored with the previous ciphertext block before being encrypted.

Cipher Block Chaining (CBC) Mode

Cipher block chaining (CBC) mode

An Email between Alice and Bob with Following Header

Alice Bob

{Date: Tue, 21 Oct 2014 13: 34:47}EK

Trudy

Plain-text Starts With the Encrypted Version of : “Date :Tue” Often Occur on Tuesday

Can Trudy Guess that?

Major Drawback of ECB and CBC is that, Encryption and Decryption can not start until a complete block of 64 bits of plaintext data is available.

Stream Modes of Operation• 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

Cipher FeedBack (CFB)• 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)Ci = Pi XOR EK(Ci-1)

C-1 = IV • uses: stream data encryption, authentication

s-bitCipher FeedBack

(CFB-s)

Encryption in CFB Mode

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Generating Key Stream for CFB

1

1 1

The input to the block cipher is a shift register ;

its value at stage is denoted as .

Initially, an initial vector (IV).

For 1, shift-left- -bits( ) .

Then, -mos

i

i i i

i

x

i x

x

i x s x C

K s

t-significant-bits(E ( )).K ix

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Advantages and Limitations of CFB

appropriate when data arrives in bits/bytes most common stream mode limitation is need to stall while do block encryption after

every n-bits note that the block cipher is used in encryption mode at

both ends errors propogate for several blocks after the error

Output Feed Back (OFB)

Output Feed Back (OFB)

Output Feed Back (OFB)

• 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

Oi = EK(Oi-1)

Ci = Pi XOR Oi

O-1 = IV

Advantages and Limitations of OFB

bit errors do not propagate uses: stream encryption on satellite communicationvulnerable to message stream modification

Counter Mode (CTR)

Output Feed Back (OFB)

Counter (CTR)

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 counter value for every plaintext block

(never reused)Oi = EK(i)

Ci = Pi XOR Oi

• uses: high-speed network encryptions

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

sender & receiver must remain in sync• Cannot ensure integrity support.

Summary of Encryption Modes