CIS 6930/4930 Computer and Network Security. Topic 3.1 Secret Key Cryptography (Cont d)
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1 CIS 6930/4930 Computer and Network Security Topic 3.1 Secret Key Cryptography (Cont d) 1
2 Principles for S-Box Design S-box is the only non-linear part of DES Each row in the S-Box table should be a permutation of the possible output values Output of one S-box should affect other S- boxes in the following round 3
3 Desirable Property: Avalanche Effect Roughly: a small change in either the plaintext or the key should produce a big change in the ciphertext Better: any output bit should be inverted (flipped) with probability 0.5 if any input bit is changed What is the expected number of different bits between two length-n random bit streams? 0.5n (Bernoulli distribution) f function must be difficult to un-scramble should achieve avalanche effect output bits should be uncorrelated 4
4 DES Avalanche Effect: Example 2 plaintexts with 1 bit difference: 0x and 0x encrypted using the same key: 0x016B24621C181C32 Resulting ciphertexts differ in 34 bits (out of 64) Similar results when keys differ by 1 bit 5
5 Example (cont d) An experiment: number of rounds vs. number of bits difference Round # Bits changed
6 DES: Keys to Avoid Using Weak keys : These are keys which, after the first key permutation, are: 28 0 s followed by 28 0 s 28 1 s followed by 28 1 s 28 0 s followed by 28 1 s 28 1 s followed by 28 0 s Property of weak keys Easy clue for brute force attacks. Sixteen identical subkeys. Encrypting twice produces the original plaintext. 7
7 Weak keys Alternating ones + zeros (0x ) Alternating 'F' + 'E' (0xFEFEFEFEFEFEFEFE) 0xE0E0E0E0F1F1F1F1 0x1F1F1F1F0E0E0E0E DES weak keys produce sixteen identical subkeys 8
8 More Keys to Avoid! Semi-weak keys : These are keys which, after the first key permutation, are: s followed by alternating 0 s and 1 s s followed by alternating 1 s and 0 s 12. Alternating 1 s and 0 s followed by alternating 1 s and 0 s Property of semi-weak keys For a semi-weak key pair (K 1, K 2 ), K 1 {K 2 {m}} = m 9
9 Semi-weak key pairs 0x011F011F010E010E and 0x1F011F010E010E01 0x01E001E001F101F1 and 0xE001E001F101F101 0x01FE01FE01FE01FE and 0xFE01FE01FE01FE01 0x1FE01FE00EF10EF1 and 0xE01FE01FF10EF10E 0x1FFE1FFE0EFE0EFE and 0xFE1FFE1FFE0EFE0E 0xE0FEE0FEF1FEF1FE and 0xFEE0FEE0FEF1FEF1 K 1 {K 2 {m}} = m 10
10 DES Key Size 56 bits is currently too small to resist brute force attacks using readily-available hardware Ten years ago it took $250,000 to build a machine that could crack DES in a few hours Now? 11
11 Cryptanalysis of DES Differential cryptanalysis exploits differences between encryptions of two different plaintext blocks provides insight into possible key values Linear cryptanalysis requires known plaintext / ciphertext pairs, analyzes relationships to discover key value No attacks on DES so far are significantly better than brute force attacks, for comparable cost 12
12 Advanced Encryption Standard (AES) Selected from an open competition, organized by NSA winner: Rijndael algorithm, standardized as AES Some similarities to DES (rounds, round keys, alternate permutation+substitution) but not a Feistel cipher Block size = 128 bits Key sizes = 128, 192, or
13 AES-128 Overview 128-bit Input Round bit K bit K bit key Generate round keys Round bit K bit Output 14
14 AES Assessment No known successful attacks on full AES Best attacks work on 7 9 rounds For brute force attacks, AES-128 needs much more effort than DES 15
15 Attacks on AES Differential Cryptanalysis: based on how differences in inputs correlate with differences in outputs Linear Cryptanalysis: based on correlations between input and output Side Channel Attacks: Implementations of the cipher on systems inadvertently leak data Timing Attacks: measure the time it takes to do operations 16
16 CIS 6930/4930 Computer and Network Security Topic 3.2 Modes of Operation 17
17 Processing with Block Ciphers Most ciphers work on blocks of fixed (small) size How to chain cipher text together? Modes of operation ECB (Electronic Code Book) CBC (Cipher Block Chaining) OFB (Output Feedback) CFB (Cipher Feedback) CTR (Counter) 18
18 Issues for Block Chaining Mode Ciphertext manipulation Can an attacker modify ciphertext block(s) in a way that will produce a predictable/desired change in the decrypted plaintext block(s)? Note: assume the structure of the plaintext is known, e.g., first block is employee #1 salary, second block is employee #2 salary, etc. Information leakage Does it reveal info about the plaintext blocks? 19
19 Issues (Cont d) Parallel/Sequential Can blocks of plaintext (ciphertext) be encrypted (decrypted) in parallel? Error propagation If there is an error in a plaintext (ciphertext) block, will there be an encryption (decryption) error in more than one ciphertext (plaintext) block? 20
20 Electronic Code Book (ECB) Plaintext M 1 M 2 M 3 M padding Key E E E E Ciphertext C 1 C 2 C 3 C 4 The easiest mode of operation; each block is independently encrypted 21
21 ECB Decryption M 1 M 2 M 3 M padding Key D D D D C 1 C 2 C 3 C 4 Each block is independently decrypted 22
22 ECB Properties Does information leak? Can ciphertext be manipulated? Parallel processing possible? Do ciphertext errors propagate? M 1 M 24 M 3 M 42 Key padding D D D D C 1 C 4 C 3 C 2 C 1 C 2 C 3 C 4 23
23 Cipher Block Chaining (CBC) Initialization Vector Key M 1 M 2 M 3 M padding E E E E C 1 C 2 C 3 C 4 Chaining dependency: each ciphertext block depends on all preceding plaintext blocks 24
24 Initialization Vectors Initialization Vector (IV) Used along with the key; not secret For a given plaintext, changing either the key, or the IV, will produce a different ciphertext Why is that useful? IV generation and sharing Random; may transmit with the ciphertext Incremental; predictable by receivers 25
25 CBC Decryption M 1 M 2 M 3 M 4 Initialization Vector padding Key D D D D C 1 C 2 C 3 C 4 How many ciphertext blocks does each plaintext block depend on? 26
26 CBC Properties Does information leak? Identical plaintext blocks will produce different ciphertext blocks Can ciphertext be manipulated predictably???? Parallel processing possible? no (encryption), yes (decryption) Do ciphertext errors propagate? yes (encryption), a little (decryption) 27
27 Output Feedback Mode (OFB) Initialization Vector one-time pad Key 64 E Pseudo-Random E Number EGenerator E M 1 M 2 M 3 M padding C 1 C 2 C 3 C 4 28
28 OFB Decryption IV one-time pad Key 64 E E E E M 1 M 2 M 3 M padding C 1 C 2 C 3 C 4 No block decryption required! 29
29 OFB Properties Does information leak? identical plaintext blocks produce different ciphertext blocks Can ciphertext be manipulated predictably???? Parallel processing possible? yes (generating pad), yes (XORing with blocks) Do ciphertext errors propagate???? 30
30 OFB (Cont d) If you know one plaintext/ciphertext pair, can easily derive the one-time pad that was used i.e., should not reuse a one-time pad! Conclusion: IV must be different every time 31
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