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fix: broken image links have been fixed

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Sungchan Yi
2024-02-09 14:25:14 +09:00
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_posts/Lecture Notes/Internet Security/2023-09-18-symmetric-key-cryptography-2.md
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@@ -63,7 +63,7 @@ $$
#### The Feistel Function
![is-03-feistel-function.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-feistel-function.png#)
![is-03-feistel-function.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-feistel-function.png#)
The Feistel function takes $32$ bit data and divides it into eight $4$ bit chunks. Each chunk is expanded to $6$ bits using a P-box. Now, we have 48 bits of data, so apply XOR with the key for this round. Next, each $6$-bit block is compressed back to $4$ bits using a S-box. Finally, there is a (straight) permutation at the end, resulting in $32$ bit data.
@@ -179,7 +179,7 @@ AES, DES use fixed block size for encryption. How do we encrypt longer messages?
### Electronic Codebook Mode (ECB)
![is-03-ecb-encryption.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ecb-encryption.png#)
![is-03-ecb-encryption.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ecb-encryption.png#)
- Codebook is a mapping table.
- For the $i$-th plaintext block, we use key $k$ to encrypt and obtain the $i$-th ciphertext block.
@@ -198,7 +198,7 @@ Since the same key is used for all blocks, once a mapping from plaintext to ciph
### Cipher Block Chaining Mode (CBC)
![is-03-cbc-encryption.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-cbc-encryption.png#)
![is-03-cbc-encryption.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-cbc-encryption.png#)
- Two identical messages produce to different ciphertexts.
- This prevents chosen plaintext attacks
@@ -248,7 +248,7 @@ Since the same key is used for all blocks, once a mapping from plaintext to ciph
### Cipher Feedback Mode (CFB)
![is-03-cfb-encryption.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-cfb-encryption.png#)
![is-03-cfb-encryption.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-cfb-encryption.png#)
- The message is treated as a stream of bits; similar to stream cipher
- **Result of the encryption is fed to the next stage.**
@@ -283,7 +283,7 @@ Since the same key is used for all blocks, once a mapping from plaintext to ciph
### Output Feedback Mode (OFB)
![is-03-ofb-encryption.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ofb-encryption.png#)
![is-03-ofb-encryption.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ofb-encryption.png#)
- Very similar to stream cipher.
- Initialization vector is used as a seed to generate the key stream.
@@ -316,7 +316,7 @@ Since the same key is used for all blocks, once a mapping from plaintext to ciph
### Counter Mode (CTR)
![is-03-ctr-encryption.png](../../../assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ctr-encryption.png#)
![is-03-ctr-encryption.png](/assets/img/posts/Lecture%20Notes/Internet%20Security/is-03-ctr-encryption.png#)
- Without chaining, we use a counter (typically incremented by $1$).
- Counter starts from the initialization vector.