Chapter 6-Block Cipher Operation

块密码和五种模式，块密码（Block Cipher）是对称加密的一种常见形式，通常用于加密大段数据。其基本原理是将明文分成多个等长的块（blocks），然后使用同一个密钥对每个块进行加密，得到密文块。

Chapter 6-Block Cipher Operation

double-DES



issue of reduction to single stage

have "meet-in middle" attack

encrypting P with all keys and store

since there is an 

decrypt C with all keys and match X value

can show takes O(2^56) steps

Triple DES

with 2 keys

structure

use 2 keys with E-D-E sequence

security

if k1=k2 then can work with single DES

no current known practical attacks, but several proposed impractical attacks might become basis of future attacks

with 3 keys

structure

application

has been adopted by some Internet applications

PGP

S

MIME

Modes of Operation

intro

block ciphers encrypt fixed size blocks: eg.DES encrypts 63 bits blocks with 56-bits key

NIST SP 800-38A defines 5 modes

Block Modes

Electronic Codebook Book(ECB)

uses

sending a few blocks of data

Limitations

现象

message repetition may show in ciphertext

particularly with data such graphics

原因

due to the encrypted message blocks independent

structure



Cipher Block Chaining(CBC)

structure

use Initial Vector(IV to start process)



uses

bulk data encryption,authentication

advantages

密文块的依赖性

a ciphertext block depends on all blocks before it

变动的影响

any change to a block affects all following ciphertext blocks

limitation

need Initiation Vector(IV)

must be known to sender & reciever

if sent in clear, attack can change bits of first block, and change IV to compensate(抵消)

sent encrypted in ECB mode

Message Padding(信息填充)

def

当信息末尾不足以构成一个加密块是需要采取措施处理它

方法

pad either with known non-data valus(eg nulls)

pad last block along with count od pad size



XTS-AES Mode

intro

for block oriented storage use

加密函数

T is tweak(调整值)，H is a Hash function

Stream Modes

Cipher Feedback(CFB)



uses

stream data encryption, authentication

process

1. message is treated as a stream of bits

2. added to the output of the block ciphert

3. result is fed back for next stage

standard allows any number of bit(1,8,64,128 etc)

structure

advantages

appropriate when data arrives in bit/bytes

block cipher is used in encryption mode at both ends to yield ps-random bitstream

errors propagate for several blocks after the error(but not indefinitely遥遥无期地)

Limitation

need to stall(暂停) while do block encryption after every-bits

Output Feed Back(OFB)

intro

outpu tof cipher is added to message

output is then feedback

feedback is independent of message

can be computed in advance

uses

stream encryption on noisy channels

structure

advantages

bit errors do not propagate

more vulnerable to message stream modification

Limitation

needs an IV which is unique for each use if ever reuse attacker can recover propagate

sender& reciever must remian in sync

Only full block feedback(ie CFB-64 or CFB-128) should be used

Counter(CTR)

intro

与OFB相似

similar to OFB

加密计数器值

but enctypts counter value rather than any feedback value

每块需要不同的counter value 和 key value，且不重用

加密方程



uses

high-speed network encryptions

structure

advantages

efficiency

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

can preprocess in advance of need

good for bursty high speed links

allow random access to encrypted data blocks

provable security(good as other modes)

Limitation

ensure never reuse key/counter values, otherwise could break(cf. OFB)

Feedback Characteristics