Docs/blockcipher.md (#84)

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* docs/blockcipher.md

---------

Co-authored-by: Yijun Lee <119404328+yijun-lee@users.noreply.github.com>

Author: hunjyeong

…ntent/Basic Cryptography/Block cipher.md …ryptography/block cipher/Block cipher.mdcontent/Basic Cryptography/Block cipher.md renamed to content/Basic Cryptography/block cipher/Block cipher.md content/Basic Cryptography/Block cipher.md renamed to content/Basic Cryptography/block cipher/Block cipher.md

@@ -18,7 +18,7 @@ If the length of the plaintext to be encrypted is longer than the block size, a
 
 | Confidentiality only                                                                                                           | Authenticated Encryption with Additional Data(AEAD)                                                                                                       |
 | ------------------------------------------------------------------------------------------------------------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| - ECB(Electric Codebook)<br>- CBC(Cipher-block Chaining)<br>- CFB(Cipher Feedback)<br>- OFB(Output Feedback)<br>- CTR(Counter) | - GCM(Galois counter)<br>- CCM(Counter with cipher block chaining message authentication code)<br>- SIV(Synthetic initialization vector)<br>- AES-GCM-SIV |
+| - [[ECB(Electric Codebook)]]<br>- [[CBC(Cipher-block Chaining)]]<br>- [[CFB(Cipher Feedback)]]<br>- [[OFB(Output Feedback)]]<br>- [[CTR(Counter)]] | - GCM(Galois counter)<br>- CCM(Counter with cipher block chaining message authentication code)<br>- SIV(Synthetic initialization vector)<br>- AES-GCM-SIV |
 
 ## Comparison with [[Stream cipher]]
 |                  | [[Stream Cipher]]                     | Block Cipher                                          |

content/Basic Cryptography/block cipher/CBC(Cipher-block Chaining).md

@@ -0,0 +1,40 @@
+## CBC(Cipher-block Chaining)
+
+In CBC mode, each plaintext block is XORed with the previous ciphertext block before being encrypted.
+
+As a result, each ciphertext block depends on all the plaintext blocks processed up to that point.
+
+If the previous block is not encrypted, the next block cannot be encrypted, so encryption must be performed sequentially, making parallel processing impossible. Similarly, decryption cannot be performed in parallel.
+
+By setting the IV (Initialization Vector) randomly, even if the same plaintext or key is used, different ciphertexts can be generated.
+
+<br>
+
+### CBC mode Encrytion
+![[block_cipher_CBC_Enc.png]]
+
+
+$C_i = E_K(P_i \oplus C_{i-1})$
+
+$C_0 = IV$
+
+
+<br>
+
+### CBC mode Decrytion
+![[block_cipher_CBC_Dec.png]]
+
+
+$P_i = D_K(C_i) \oplus C_{i-1}$
+
+$C_0 = IV$
+
+<br>
+
+**Definitions**
+
+- $P_i$: Plaintext block $i$
+- $C_i$: Ciphertext block $i$
+- $E_K$: Encryption function with key $K$
+- $D_K$: Decryption function with key $K$
+- $IV$: Initialization Vector

content/Basic Cryptography/block cipher/CFB(Cipher Feedback).md

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+## CFB(Cipher Feedback)
+
+CFB mode makes a block cipher operate like a synchronous stream cipher. Since it functions as a stream cipher, padding is not required even if the data size does not align perfectly with the block size.
+
+Because each block uses the previous ciphertext, encryption in CFB mode cannot be processed in parallel, similar to CBC.
+
+While similar to CBC, CFB uses XOR with the encrypted feedback value. Unlike CBC, decryption in CFB mode can be performed in parallel.
+
+<br>
+
+### CFB mode Encrytion
+![[block_cipher_CFB_Enc.png]]
+
+$C_i = \begin{cases} IV, & i = 0 \\ E_K(C_{i-1}) \oplus P_i, & \text{otherwise} \end{cases}$
+
+<br>
+
+### CBC mode Decrytion
+![[block_cipher_CFB_Dec.png]]
+
+$P_i = E_K(C_{i-1}) \oplus C_i$
+
+<br>
+
+**Definitions**
+
+- $P_i$: Plaintext block $i$
+- $C_i$: Ciphertext block $i$
+- $E_K$: Encryption function with key $K$
+- $IV$: Initialization Vector

content/Basic Cryptography/block cipher/CTR(Counter).md

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+## CTR(Counter)
+
+Like OFB, CTR mode makes a block cipher function as a stream cipher.
+
+It uses an incrementing counter value for each block, encrypts it, and then XORs the result with the plaintext to generate the ciphertext.
+
+CTR mode supports parallel processing and does not require padding, making it highly efficient.
+
+Since each block is processed independently, any corruption or modification of a ciphertext block does not affect other blocks.
+
+<br>
+
+### CTR mode Encrytion
+![[block_cipher_CTR_Enc.png]]
+
+$C_i = P_i \oplus E_K(\text{Nonce} \parallel \text{Counter}_i)$
+
+$\text{Counter}_{i+1} = \text{Counter}_i + 1$
+
+<br>
+
+### CTR mode Decrytion
+![[block_cipher_CTR_Dec.png]]
+
+$P_i = C_i \oplus E_K(\text{Nonce} \parallel \text{Counter}_i)$
+
+$\text{Counter}_{i+1} = \text{Counter}_i + 1$
+
+<br>
+
+**Definitions**
+
+- $P_i$: Plaintext block $i$
+- $C_i$: Ciphertext block $i$
+- $E_K$: Encryption function with key $K$

content/Basic Cryptography/block cipher/ECB(Electric Codebook).md

@@ -0,0 +1,35 @@
+## ECB(Electric Codebook)
+
+ECB is a method that divides a message into multiple blocks and encrypts each block individually.
+
+Since encryption and decryption operations are independent of each other, they can be processed in parallel.
+
+However, because all blocks use the same encryption key, it is vulnerable to security risks.
+
+<br>
+
+### ECB mode Encrytion
+![[block_cipher_ECB_Enc.png]]
+
+- Divide the plaintext $P$ into fixed block sizes (e.g., 128 bits).
+- Represent each block as $P_1, P_2, \dots, P_n$.
+- Encryption: $C_i = E_K(P_i)$
+- Combine all ciphertext blocks in order to generate the final ciphertext $C$.
+
+<br>
+
+### ECB mode Decrytion
+![[block_cipher_ECB_Dec.png]]
+
+- Divide the ciphertext $C$ into fixed block sizes.
+- Represent each block as $C_1, C_2, \dots, C_n$.
+- Decryption: $P_i = D_K(C_i)$
+- Combine the decrypted plaintext blocks to generate the final plaintext $P$.
+
+
+<br>
+
+**Definitions**
+
+- $E_K$: Encryption function with key $K$
+- $D_K$: Decryption function with key $K$

content/Basic Cryptography/block cipher/OFB(Output Feedback).md

@@ -0,0 +1,39 @@
+## OFB(Output Feedback)
+
+OFB mode makes a block cipher operate like a synchronous stream cipher.
+
+Unlike CBC or CFB, it performs encryption using only the encrypted feedback values, independent of the previous ciphertext or plaintext.
+
+Both encryption and decryption can be processed in parallel.
+
+<br>
+
+### OFB mode Encrytion
+![[block_cipher_OFB_Enc.png]]
+
+$C_i = P_i \oplus E_K(I_i)$
+
+$I_i = E_K(I_{i-1})$
+
+$I_0 = IV$
+
+<br>
+
+### OFB mode Decrytion
+![[block_cipher_OFB_Dec.png]]
+
+$P_i = C_i \oplus E_K(I_i)$
+
+$I_i = E_K(I_{i-1})$
+
+$I_0 = IV$
+
+<br>
+
+**Definitions**
+
+- $P_i$: Plaintext block $i$
+- $C_i$: Ciphertext block $i$
+- $E_K$: Encryption function with key $K$
+- $I_i$: Input block $i$
+- $IV$: Initialization Vector