Introduction to Symmetric
Block Cipher
Jing Deng
Based on
Prof. Rick Han’s Lecture Slides
Dr. Andreas Steffen’s Security
Tutorial
Cryptography
plaintext
ciphertext
Encryption
plaintext
Decryption
• Encryption algorithm also called a cipher
• Cryptography has evolved so that modern
encryption and decryption use secret keys
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Cryptographic algorithms can be openly published
Only have to protect the keys
plaintext
ciphertext
Encryption
Key KA
plaintext
Decryption
Key KB
Symmetric-Key Cryptography
plaintext
ciphertext
Encryption
Key KA
plaintext
Decryption
Key KB=KA
Secure Key Distribution
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Both sender and receiver keys are the same: KA=KB
The keys must be kept secret and securely
distributed
• Thus, also called “Secret Key Cryptography”
Data Encryption Standard (DES)
Cryptanalysis
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Brute force: try every key
Ciphertext-only attack:
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Attacker knows ciphertext of several messages encrypted
with same key (but doesn’t know plaintext).
Possible to recover plaintext (also possible to deduce key) by
looking at frequency of ciphertext letters
Known-plaintext attack:
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Attackers observes pairs of plaintext/ciphertext encrypted
with same key.
Possible to deduce key and/or devise algorithm to decrypt
ciphertext.
Cryptanalysis (2)
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Chosen-plaintext attack:
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Attacker can choose the plaintext and look at the paired
ciphertext
Attacker has more control than known-plaintext attack and
may be able to gain more info about key
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Adaptive Chosen-Plaintext attack:
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Examples
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Attacker chooses a series of plaintexts, basing the next
plaintext on the result of previous encryption
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Differential cryptanalysis – DES is resistant it
Linear cryptanalysis
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Lossless compression before encryption removes
redundancy
• Cryptanalysis attacks often exploit the
redundancy of natural language
Examples
• Simple and non-secure ciphers
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–
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Shift Cipher – Caesar Cipher
Affine Cipher
Vigenere Cipher
Hill Cipher
• Information-secure cipher
– One-Time Pad
Confusion and Diffusion
plaintext
ciphertext
Encryption
Key KA
plaintext
Decryption
Key KB
• Terms courtesy of Claude Shannon, father of
Information Theory
• “Confusion” = Substitution
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a -> b
Caesar cipher
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abcd -> dacb
DES
• “Diffusion” = Transposition or Permutation
Confusion and Diffusion (2)
• “Confusion” : a classical Substitution Cipher
Courtesy:
Andreas
Steffen
• Modern substitution ciphers take in N bits and
substitute N bits using lookup table: called SBoxes
Confusion and Diffusion (3)
• “Diffusion” : a classical Transposition cipher
Courtesy:
Andreas
Steffen
• modern Transposition ciphers take in N bits and
permute using lookup table : called P-Boxes
Block Cipher
•
Divide input bit stream into n-bit sections, encrypt
only that section, no dependency/history between
sections
Courtesy:
Andreas
Steffen
•
In a good block cipher, each output bit is a function
of all n input bits and all k key bits
Example: DES
• Data Encryption Standard (DES)
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Encodes plaintext in 64-bit chunks using a 64-bit
key (56 bits + 8 bits parity)
Uses a combination of diffusion and confusion to
achieve security
Was cracked in 1997
• Parallel attack – exhaustively search key space
Decryption in DES – it’s symmetric! Use KA again as
input and then the same keys except in reverse
order
Example: DES (2)
• DES
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64-bit input is permuted
16 stages of identical
operation
• differ in the 48-bit
key extracted from
56-bit key - complex
• R2= R1 is encrypted
with K1 and XOR’d
with L1
• L2=R1, …
Final inverse permutation
stage
Example: DES (3)
Beyond DES
• Triple-DES: put the output of DES back as
input into DES again with a different key,
loop again: 3*56 = 168 bit key
• Advanced Encryption Standard (AES)
– Requirements:
• shall be designed so that the key length may be
increased as needed.
• block size n = 128 bits, key size k = 128, 192, 256 bits
– Candidates: MARS, twofish, RC6, Serpent,
Rijndael
– successor (Rijndael)
Encryption Mode (ECB)
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Electronic Code Book (ECB) mode for block
ciphers of a long digital sequence
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Vulnerable to replay attacks: if an attacker thinks block
C2 corresponds to $ amount, then substitute another Ck
Attacker can also build a codebook of <Ck, guessed Pk>
pairs
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Encryption Mode (CBC)
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Cipher Block Chaining (CBC) mode for block
ciphers
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Inhibits replay attacks and codebook building: identical
input plaintext Pi =Pk won’t result in same output code due
to memory-based chaining
IV = Initialization Vector – use only once
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Stream Cipher
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Stream ciphers
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Rather than divide bit stream into discrete blocks, as
block ciphers do, XOR each bit of your plaintext
continuous stream with a bit from a pseudo-random
sequence
At receiver, use same symmetric key, XOR again to
extract plaintext
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Encryption Mode (OFB)