120 lines
5.1 KiB
Markdown
120 lines
5.1 KiB
Markdown
# Symmetric Cryptography
|
||
|
||
- Symmetric encryption gives us confidentiality
|
||
- Implemented using block ciphers or stream ciphers
|
||
- Lightweight and fast
|
||
- Used for general communication
|
||
|
||
|
||
|
||
### Stream Cipher
|
||
|
||
- Stream ciphers use an initial seed key to generate an infinite keystream of random looking bits
|
||
- The message and keystream are usually combined using an `xor` ($\oplus$) which is reversible if applied twice
|
||
|
||
- However, using the same keystream to encrypt two messages makes messages easy to break
|
||
- A random *number used once* nonce is added as an additional seed
|
||
- The nonce is not a secret, it simply ensures the keystream is new
|
||
|
||
##### Pros and Cons
|
||
|
||
✅ Encrypting long continuous streams, possibly of unknown length
|
||
|
||
✅ Extremely fast with low memory footprint, ideal for low-power battery devices
|
||
|
||
✅ If designed well, can seek to any location in the stream
|
||
|
||
❌ The keystream must appear statistically random
|
||
|
||
❌ You must **never** reuse a key & nonce
|
||
|
||
❌ Stream ciphers do not protect the cipher-text
|
||
|
||
#### Block Ciphers
|
||
|
||
- Block ciphers use a key to encrypt a fixed size block of plain text into a *fixed-sized block* of cipher-text
|
||
- Changing and permuting the bits of the block depending on the key
|
||
- Different lengths of messages can be handled by splitting the message up, and padding
|
||
|
||
##### SP-Network
|
||
|
||
- Repeated substitution and permutation
|
||
|
||
|
||
|
||
- This round will be run multiple times (around 10-15 times)
|
||
|
||
###### Key Mixing
|
||
|
||
- Mixing in the key prevents attackers from reversing the process
|
||
|
||
|
||
|
||
- To decrypt, reverse the process
|
||
|
||
#### Symmetric Algorithms
|
||
|
||
- `DES` was used from 1970s to 2000s
|
||
- `3DES` (using DES 3 times) is sometimes found in legacy systems
|
||
- `AES` and `ChaCha20` are the only two ciphers used in `TLS 1.3`
|
||
|
||
| Algorithm | Cipher type | Design | Block Size (bits) | Speed | Memory Footprint | Safe Implementation Difficulty | Key Sizes |
|
||
| ---------- | ----------- | ------------ | ----------------- | --------- | ---------------- | ------------------------------ | --------- |
|
||
| `DES` | Block | `Feistel` | 64 | Fast | Low | Easy | 56 |
|
||
| `3DES` | Block | `Feistel` | 64 | Slow | Low | Easy | 112 |
|
||
| `AES` | Block | `SP-Network` | 128 | very fast | medium | Hard | 128/192 |
|
||
| `ChaCha20` | Stream | add-xor-rot | N/A | Very fast | very low | Easy | 256 |
|
||
|
||
### Attack Models
|
||
|
||
1. Brute force
|
||
- Weakest attack, guessing the key
|
||
- If the key is $2^{128}$, on a supercomputer it would take $10^9$ years
|
||
2. Cipher text only
|
||
- Static analysis on the cipher text, frequency analysis etc
|
||
- e.g. looking at the Enigma machine and recognising a letter cannot be itself
|
||
3. Known plaintext
|
||
- Where you know some plaintext and the corresponding ciphertext
|
||
- e.g. Enigma being broken using “heil hitler”
|
||
4. Chosen plaintext
|
||
- Seeing if certain plain-texts take the algorithm longer/shorter
|
||
5. Chosen ciphertext
|
||
6. Related-key attack
|
||
- Get the same message encrypted in different keys
|
||
- More of a theoretical attack
|
||
|
||
Modern algorithms are expected to overcome these attacks trivially
|
||
|
||
## Asymmetric Encryption
|
||
|
||
- Two keys, a public & private key
|
||
- Public-key asymmetric cryptography hinges upon the premise that:
|
||
- It is computationally infeasible to calculate a private key from a public key
|
||
- In practice this is achieved through intractable mathematical problems
|
||
|
||
#### Key Exchange
|
||
|
||
- Diffie-Hellman key exchange allows two parties to mathematically agree a shared secret over an insecure channel
|
||
|
||
|
||
|
||
It is extremely easy to go from a -> A but extremely difficult to go backwards.
|
||
|
||
- Encryption performed by the **public** key can only be decrypted by the corresponding **private** key
|
||
|
||
#### Public key Encryption
|
||
|
||
- Client encrypts message with the server’s public key; now only the server’s private key can be used to read it.
|
||
- The authenticity of signatures generated by the private key can be verified by the public key
|
||
|
||
|
||
|
||
##### Public key Algorithms
|
||
|
||
| Algorithm | Key Exchange | Encryption | Digital Signatures | Mathematical Problem | Elliptic Curves | Typical Key Size |
|
||
| :------------- | :----------: | :--------: | :----------------: | --------------------- | :-------------: | ---------------- |
|
||
| Diffie-Hellman | ✅ | ❌ | ❌ | Discrete Logs | ✅ | 256 |
|
||
| `RSA` | ❌ | ✅ | ✅ | Integer Factorisation | ❌ | 2048/4096 |
|
||
| `Elgamal` | ❌ | ✅ | ✅ | Discrete Logs | ✅ | 2048 |
|
||
| `DSA` | ❌ | ❌ | ✅ | Discrete Logs | ✅ | 256 |
|