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cleanup of itsec md

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Tom Zuidberg
2026-02-19 19:47:59 +01:00
parent e6e974f3df
commit fd986c324b
2 changed files with 14 additions and 114 deletions
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ObsidianNotes/.obsidian/workspace.json vendored
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ObsidianNotes/IT-Security.md
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# It Security # It Security
## Security Goals - CIA ## Security Goals - CIA
- Confidentiality - Confidentiality
- only authorized entities can access assets in a system - only authorized entities can access assets in a system
- Attacks: - Attacks:
@@ -18,72 +17,45 @@
- authorized entities can access assets in a system as intended - authorized entities can access assets in a system as intended
- Attack: Denial of Service- flooding a server with fake requests, jam signal with stronger singal on the same frequency, enter password wrongly to get the account blocked - Attack: Denial of Service- flooding a server with fake requests, jam signal with stronger singal on the same frequency, enter password wrongly to get the account blocked
 
## Encryption Scheme definition ## Encryption Scheme definition
Noted as a tuple (P, C, K, E, D): Noted as a tuple (P, C, K, E, D):
- P = plaintexts - P = plaintexts
- C = ciphertexts - C = ciphertexts
- K = keys - K = keys
- E = encryption functions - E = encryption functions
- D = decryption functions - D = decryption functions
 
For any K_1 in K, there is a K_2 in K such that for all p in P, D_K_2(E_K_1(p)) = p For any K_1 in K, there is a K_2 in K such that for all p in P, D_K_2(E_K_1(p)) = p
For symmetric encryption, K_1 = K_2 For symmetric encryption, K_1 = K_2
This definition doesn't cover any notion of security This definition doesn't cover any notion of security
## Symmetric Encription scheme ## Symmetric Encription scheme
Properties: Properties:
- Bob and Alice share the same key in advance - Bob and Alice share the same key in advance
- Decription is *difficult* without the key - Decription is *difficult* without the key
 
## Caesar Cipher ## Caesar Cipher
= Letter shift by k amount = Letter shift by k amount
vulnerable to Brute force attacks (exhaustive search attacks) vulnerable to Brute force attacks (exhaustive search attacks)
 
## Monoalphabetic Substitution Cipher ## Monoalphabetic Substitution Cipher
= replace each letter by a fixed permutation of the alphabet = replace each letter by a fixed permutation of the alphabet
key space is very large -> No brute force, however: key space is very large -> No brute force, however:
vulnerable to frequency analysis, as Monoaplhabetic Substitution preservers letter frequencies vulnerable to frequency analysis, as Monoaplhabetic Substitution preservers letter frequencies
 
## Perfect Secrecy ## Perfect Secrecy
Defintion: Defintion:
> An encryption scheme is said to provide **perfect secrecy** iff given a probability distribution Pr on P, and Pr(P) > 0 for all plaintexts p and for each p in P, c in C and k in K chosen uniformly at random Pr(p|c) = Pr(p) > An encryption scheme is said to provide **perfect secrecy** iff given a probability distribution Pr on P, and Pr(P) > 0 for all plaintexts p and for each p in P, c in C and k in K chosen uniformly at random Pr(p|c) = Pr(p)
Meaning: Whether or not c is observed, p is as likely as its occurrence in the plaintext space Meaning: Whether or not c is observed, p is as likely as its occurrence in the plaintext space
A cipher providing perfect secrecy cannot be broken by an attacker. Not even by one with infinite computational resources and infinite time. (Shannon'S Theorem) A cipher providing perfect secrecy cannot be broken by an attacker. Not even by one with infinite computational resources and infinite time. (Shannon'S Theorem)
 
## One-Time-Pad (OTP) ## One-Time-Pad (OTP)
*aka Vernam Cipher or Vernam's One-Time-Pad* *aka Vernam Cipher or Vernam's One-Time-Pad*
for each encryption, chose a key uniformly at random. for each encryption, chose a key uniformly at random.
Encryption: C = P xor K Encryption: C = P xor K
Decryption: C xor K = P xor K xor K = P Decryption: C xor K = P xor K xor K = P
- Advantages: - Advantages:
@@ -97,18 +69,13 @@ Decryption: C xor K = P xor K xor K = P
- does not guarantee **integrity** - does not guarantee **integrity**
- insecure if keys are reused - insecure if keys are reused
  ==**Learn the Prove for perfect secrecy by heart!**==
**Learn the Prove for perfect secrecy by heart!**
## Computational Security ## Computational Security
= An encryption scheme is called computationally secure iff all known attacks against the cipher are computationally infeasible within any reasonable amout of time/resources = An encryption scheme is called computationally secure iff all known attacks against the cipher are computationally infeasible within any reasonable amout of time/resources
 
## Attacker Models ## Attacker Models
- Ciphertext only attack - Ciphertext only attack
- attacker knows only cipher text - attacker knows only cipher text
- known plaintext attack - known plaintext attack
@@ -119,38 +86,26 @@ Decryption: C xor K = P xor K xor K = P
- can obtain plaintext for ciphertexts of his choice before target ciphertext is known - can obtain plaintext for ciphertexts of his choice before target ciphertext is known
Security in a chosen-ciphertext setting is hardest to achieve Security in a chosen-ciphertext setting is hardest to achieve
Ciphertext-only setting is more difficult for the attacker -> easier to achieve Ciphertext-only setting is more difficult for the attacker -> easier to achieve
 
## Stream Ciphers ## Stream Ciphers
Idea: Idea:
- Replace K with PRBG: - Replace K with PRBG:
- Seed of PRBG with a truly random key K - Seed of PRBG with a truly random key K
- PRBG should be cryptographically secure, though there is no proof - PRBG should be cryptographically secure, though there is no proof
- new initialization vector for each P - new initialization vector for each P
 
> For each plaintext P select a fresh IV and set C = E_K(P) = IV || P xor PRBG(IV, K) > For each plaintext P select a fresh IV and set C = E_K(P) = IV || P xor PRBG(IV, K)
> >
> PRBG(IV, K) is referred to as *key stream*. The same key K is used for multiple plaintexts > PRBG(IV, K) is referred to as *key stream*. The same key K is used for multiple plaintexts
 
Weakness: Weakness:
If IV is reused with the same key, Stream Cipher is vulnerable to known-plaintext attacks (cf Chap 2 slide 32) If IV is reused with the same key, Stream Cipher is vulnerable to known-plaintext attacks (cf Chap 2 slide 32)
E.g. used to attack WPA2 (KRACK attack) E.g. used to attack WPA2 (KRACK attack)
 
examples: examples:
- broken - broken
-
- A5/1 - A5/1
- E0 - E0
- unbroken - unbroken
@@ -158,118 +113,78 @@ examples:
- CHACHA20 - CHACHA20
- blockciphers in CTR mode - blockciphers in CTR mode
 
## Block Ciphers ## Block Ciphers
Operate on plaintext blocks of a specific length Operate on plaintext blocks of a specific length
- called the block length b of the cipher - called the block length b of the cipher
- plaintext space P = ciphertext space C = {0,1}^b - plaintext space P = ciphertext space C = {0,1}^b
 
Examples: Examples:
- broken - broken
- DES - DES
- IDEA - IDEA
- unbroken - unbroken
- KASUMI - KASUMI
- AES - AES
Camellia - Camellia
 
## Advanced Encryption Standard (AES) ## Advanced Encryption Standard (AES)
more secure and efficient than 3DES, block length of 128 bit, regardless of key length
more scure and efficient than 3DES, block length of 128 bit, regardless of key length
 
Operates on rounds: Operates on rounds:
input and output of each round represented as 4x4 byte matrices input and output of each round represented as 4x4 byte matrices
Operations: Operations:
- Substitute Byte (SB) - substitutes one byte - Substitute Byte (SB) - substitutes one byte
- Round Key Addition (KA) - XOR byt with corresponding key - Round Key Addition (KA) - XOR byt with corresponding key
- Shift Row (SR) - Shift a row by different amounts - Shift Row (SR) - Shift a row by different amounts
- Mix Column (MC) - Multiplication of a column by a given matrix - Mix Column (MC) - Multiplication of a column by a given matrix
 
Overall Operation: Overall Operation:
plaintext -> KA -> SB -> SR -> MC* -> KA -> ciphertext & next round continuing after first KA operation plaintext -> KA -> SB -> SR -> MC* -> KA -> ciphertext & next round continuing after first KA operation
*MC not done in the last round! *MC not done in the last round!
Number of rounds depends on key size: Number of rounds depends on key size:
- 128 bit key -> 10 rounds - 128 bit key -> 10 rounds
- 192 -> 12 - 192 -> 12
- 256 -> 14 - 256 -> 14
 
Modes of encryption: Modes of encryption:
- Electronic Code Book (ECB) - Electronic Code Book (ECB)
- Cipher Block Chaining (CBC) - Cipher Block Chaining (CBC)
- Counter (CTR) - Counter (CTR)
- Output Feedback (OFB) -> covered exercises - Output Feedback (OFB) -> covered exercises
 
### Electronic Codebook Mode (ECB) ### Electronic Codebook Mode (ECB)
- Encryption: C_i = E_k(P_i) for i = 1, ..., n - Encryption: C_i = E_k(P_i) for i = 1, ..., n
- Decryption: P_i = D_k(C_i) for i = 1, ..., n - Decryption: P_i = D_k(C_i) for i = 1, ..., n
- Requires padding of P_n to b bit - Requires padding of P_n to b bit
 
Problem: Problem:
- Same P_i leads to same C_i -> Patterns are visible - Same P_i leads to same C_i -> Patterns are visible
-> ECB should not be used! -> ECB should not be used!
 
### Cipher Block Chaining Mode (CBC) ### Cipher Block Chaining Mode (CBC)
- IV = C_0 - IV = C_0
- Encryption: C_i = E_k(P_i xor C_i-1) for i = 1, ..., n - Encryption: C_i = E_k(P_i xor C_i-1) for i = 1, ..., n
- Decryption: P_i = D_k(C_i) xor C_i-1 for i = 1, ..., n - Decryption: P_i = D_k(C_i) xor C_i-1 for i = 1, ..., n
- Requires padding of P_n to b bit - Requires padding of P_n to b bit
  Requires a fresh IV for each plaintext to encrypt!
- Requires a fresh IV for each plaintext to encrypt!
If same IV is reused on P and P*, then C_1 and C_1* reveal whether P_1 = P_1* If same IV is reused on P and P*, then C_1 and C_1* reveal whether P_1 = P_1*
- Vulnerable to a padding-oracle attack - Vulnerable to a padding-oracle attack
Should not be used anymore Should not be used anymore
 
### Counter Mode (CTR) ### Counter Mode (CTR)
- IV public, fresh for each plaintext - IV public, fresh for each plaintext
- Encryption: C_i = E_k(IV+i) xor P_i for i = 1, ..., n - Encryption: C_i = E_k(IV+i) xor P_i for i = 1, ..., n
- Decryption: P_i = C_i xor E_k(IV+i) for i = 1, ..., n - Decryption: P_i = C_i xor E_k(IV+i) for i = 1, ..., n
 
Properties: Properties:
- CTR does not require padding - CTR does not require padding
- Ciphertext has the same size as plaintext - Ciphertext has the same size as plaintext
- CTR turns a block cipher into stream cipher - CTR turns a block cipher into stream cipher
- CTR encryption and decryption can be parallelized - CTR encryption and decryption can be parallelized