148 lines
4.7 KiB
Markdown
148 lines
4.7 KiB
Markdown
# Data Encryption Standard (DES)
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### Pseudorandom Permutations
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- A pseudorandom permutation is a function that cannot be distinguished from a random permutation
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- Maps a set of values $\{0,1\}^n \times \{0,1\}^s \rightarrow \{0,1\}^n$ such that:
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- For any key, the function F is a *bijection* (1:1)
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- The key just changes the mapping
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- There is an *efficient algorithm* to calculate $F(x)$ for all keys and all messages
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**Confusion**: Obscure the relationship between plaintext, key and ciphertext
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- Often achieved through substitution operations
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- Using lookup tables
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**Diffusion**: Influence of each plaintext and key bit is distributed throughout the ciphertext
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- Achieved via permutation
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- Swapping or otherwise mixing bits/bytes
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Shannon called a cipher like this a **product cipher**
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### Feistal Network
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- A Feistal Network is one mechanism used to create block ciphers
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- Developed by Horst Feistal while he worked at IBM
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- Underpins DES, GOST, Blowfish, Twofish and numerous others.
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- To decrypt, we run the encrypted bits through the network again
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##### A Single Feistal Round
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- During each round, only half of the block is encrypted
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Very similar to a stream cipher.
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###### Round $i$
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###### Round $i+1$
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Note - the last round does a final swap so the left and right are in the correct places.
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###### Decrypting
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Basically the `xor`s cancel themselves out, the most important part is choosing a good function $f$
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#### About Feistal Networks
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- 1 or 2 rounds is not sufficient
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- Luby and Rackoff show that if $f$ is a cryptographically secure pseudorandom function then:
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- 3 rounds are sufficient to make a pseudorandom permutation
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- 4 rounds are sufficient to make a strong pseudorandom permutation
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- Balanced Feistal networks
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- L and R are equal sizes
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- Unbalanced feistal networks
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- L and R can be different sizes
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- e.g. `skipjack`, `OAEP`
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### DES
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1972: NIST put out a call for a US standard for encryption
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1974: IBM propose DES
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1976: NIST accepts an altered version of DES following consultation with NSA
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- Feistal network with 64-bit block size
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- 56-bit key
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- The most studied cipher in history
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- Hasn’t been broken for over 46 years
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This speeds up loading bits into registers
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##### The F function
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$S_1, S_2....$ are called s-boxes. These substitute 6 bits input to 4 bits output based on lookup tables. The lookup tables for each s-box is different.
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##### Expansion
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- Adds *diffusion*
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- Increases from 32 to 48 bits to match the round key
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> Half the input bits are connected to two output positions
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##### Substitution Boxes
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- Add confusion
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- The s-boxes map 6 bit inputs to 4-bit outputs
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- There are 8 s-boxes in total, each is different
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- This s-box is not random, very carefully designed
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- s-boxes need to be highly **non-linear**: $S(a) \oplus S(b) \neq S(a\oplus b)$
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- This prevents simple systems of linear equations such as we saw in LFSRs.
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- The formula needed to represent DES is too complicated
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- Key design principles
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1. No output bit should be too close to a linear combination of input bits
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2. 1-bit change input should lead to at least 2-bits output
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3. If you only change the 4 middle bits, each output must occur exactly once
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4. If the first two bits are different but the last two are identical, the output must differ
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5. For any non-zero difference in input, no more than 8 of the 32 inputs exhibiting this difference should share the same output difference
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- We want to limit the number of predictable swaps
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6. A collision (zero difference) is only possible for 3 adjacent s-boxes
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##### Permutation
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- At the end of $f()$ is a permuatation
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- This moves bits between s-boxes on the next round
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- Blue showing how $S_1$ output bits are diffused
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#### The Avalanche Effect
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If you input all 0s, we will see a random cipher text
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However if we change one 0 to a 1, how does this effect the result.
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- On average, if you change one (first) bit in $R$, one bit will change in the expansion
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- Due to the way the s-boxes are setup, at least 2 of the 4 bits in the output will be different
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- Now when the permutation happens, these two changes are spread to other s-boxes
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- Now next round we’ll get 4 changes, then 8, then 16 …
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For DES the worst case scenario when one bit is changed (with 5 rounds) is there will be an effect on every bit on the output. |