Computer Security

1.1.1. Computer Security

1.1.2. Network Security

1.1.3. Internet Security

2.1.1. Data Confidentiality

2.1.2. Privacy

2.2.1. Data Integrity

2.2.2. System Integrity

3.1.1. Security Threat

3.1.2. Two types of Attacks

1. Active Attack
   
   • An attacker tries to modify the content of the messages.
   • The data stream is directly modified.
   • This is a threat for System Integrity and Availability.
   • You can’t prevent an active attack. You get to know about it only after the content of the messages is modified.
   • Examples include Password Tracking, Replay Attack and Viruses.
2. Passive Attack
   
   • An attacker observes the messages, and uses them for malicious purposes information.
   • Here, the data stream is just monitored, not modified.
   • This is a threat for Confidentiality.
   • You can prevent and detect a passive attack. A passive attack becomes an active attack when data is modified.
   • Examples include snooping, sniffing and eavesdropping.

3.2.1. Definitions by different standards

1. X.800
   
   • It’s a
   1. Authentication
      
   2. Access Control (Data Privacy)
      
   3. Confidentiality
      
   4. Data Integrity
      
   5. Non-Repudiation (Rejection)
      
      • Protection against Denial of Service.

3.2.2. Notorization

5.2.1. Types of attacks on access controls

1. Brute-Force Attacks
   
   • You keep trying all combinations of characters in all possible lengths, until you actually crack the password.
2. Dictionary Attacks
   
   • Guessing passwords from a predefined dictionary of common words, phrases and passwords,

5.2.2. Layered Security Tools/Devices

5.2.3. Zero-Day Attack

6.1.1. Virus

6.1.2. Trojan Horse

6.1.3. Trapdoors/Backdoor

6.1.4. Logic Bomb

6.2.1. Worms

6.2.2. Zombie

7.2.1. Based on Encryption Techniques

1. Substitution
   
   1. Classical Substitution Cipher
      
      • Every occurrence of a plain text symbol is replaced by a corresponding ciphertext character.
      1. Ceaser/Shift Cipher
         

         This is the earliest known subsitution cipher. \[ C_{1} = e(m_{1}) = m_{1} + k\mod(s) \]

         1. For example, let m = college, k = 3 and s = 26.
            
            • C₁[1] = (position of ’c’) + 3mod(26) = f
            • C₁[2] = (position of ’o’) + 3mod(26) = r
            • C₁[3] = (position of ’l’) + 3mod(26) = o
            • C₁[4] = (position of ’l’) + 3mod(26) = o
            • C₁[5] = (position of ’e’) + 3mod(26) = h
            • C₁[6] = (position of ’g’) + 3mod(26) = j
            • C₁[7] = (position of ’e’) + 3mod(26) = h
            • Hence, C₁ = e(college) = froohjh
         2. In general, for alphabets, Ceasar Cipher is given as
            

            Encryption:	\(E(P) = (P+K)\mod(26)\)
            Decryption:	\(D(C) = (C-K)\mod(26)\)

         3. For example, Encrypt KHOOR with k=4
            
            • C₁[1] = (position of ’K’) + 4mod(26) = O
            • C₁[2] = (position of ’H’) + 4mod(26) = L
            • C₁[3] = (position of ’O’) + 4mod(26) = S
            • C₁[4] = (position of ’O’) + 4mod(26) = S
            • C₁[5] = (position of ’R’) + 4mod(26) = V
            • Hence the string is OLSSV
         4. Decrypt Ciphertext ABC, k=3
            
            • A: \((0-3)\mod26 = -3\mod(26) = (-3+26)\mod(26) = 23\)
         5. Cryptanalysis
            
            1. Break the ciphertext WKLV using cryptanalysis
               

               The program given below is a very simple Python Script to generate all possible plaintexts for a given ciphertext.

               def breakCaesar(x):
                   for k in range(26):
                       for c in x.upper():
                           shift = (ord(c) - ord('A') - k)%26 
                           cipher_char = chr(ord('A')+shift)
                           print(cipher_char, end="")
                       print(end=", ")
                       
               breakCaesar("WKLV")
               
               

               WKLV, VJKU, UIJT, THIS, SGHR, RFGQ, QEFP, PDEO, OCDN, NBCM, MABL, LZAK, KYZJ, JXYI, IWXH, HVWG, GUVF, FTUE, ESTD, DRSC, CQRB, BPQA, AOPZ, ZNOY, YMNX, XLMW,
               

               Explanation: For each alphabet, this is what’s happening:

               • ord(c) - ord('A') is basically the difference between the ASCII value of the character in the string, and the ASCII value of the letter ’A’. This difference is in the range 0-25. ord(c) - ord('A') is the value of \(C\) in \(D(C) = (C-K)\mod(26)\).
               • - k is doing the \((C-K)\) part in \(D(C) = (C-K)\mod(26)\).
               • % 26 is doing the \(\mod(26)\) part in \(D(C) = (C-K)\mod(26)\).
               • We add ord('A') to the shift so that we can convert this number back to ASCII.
               • To convert the ASCII value back into a number, we have to typecast it into a character, by passing it into the chr() function.
            2. Break the ciphertext GWTBS
               

               breakCaesar("GWTBS")
               

               GWTBS, FVSAR, EURZQ, DTQYP, CSPXO, BROWN, AQNVM, ZPMUL, YOLTK, XNKSJ, WMJRI, VLIQH, UKHPG, TJGOF, SIFNE, RHEMD, QGDLC, PFCKB, OEBJA, NDAIZ, MCZHY, LBYGX, KAXFW, JZWEV, IYVDU, HXUCT,
               

      2. Railfence Cipher
         

         \[ \]

         n

         k

         .

         o

         i

         e

         b

         t

         l

         _

         o

         _

         b

   2. Simple Substitution/ Monoalphabetic
      
      • Rather than shifting the alphabets, you shuffle the alphabets randomly.
      • Every plain text letter maps to a random cipher text letter, hence there are 26 keys.
      • Given a ciphertext, there are \(26!\) possible plain texts.
      • The relative letter frequencies never change.
   3. Polygram
      
      • A block of plain text symbols is replaced by a corresponding ciphertext block of characters.
      • Sequence of two plain text characters are known as digrams, replaced by other digrams.

      • For example, the word

        CO LL EG Ex
        

        This can become, say (random, just for example):

        xy ab up ld
        

      1. Playfair Cipher
         
         1. Explanation
            
            • Make a \(5 \times 5\) matrix. In these 25 spaces, you’ll have to fill all 26 alphabets. To accommodate the extra alphabet, we merge I and J in one box, by convention.
            • You start by filling the alphabets of the keyword given (without repetition).
            • After you’re done with that, start with the rest of the alphabet, and fill in the rest of the gaps in order without repetition.
            1. Example we’ll be using to learn the rules
               
               • keyword: CRYPTOGRAPHY CLASS SPRING

               C	R	Y	P	T
               O	G	A	H	L
               S	I/J	N	B	D
               E	F	K	M	Q
               U	V	W	X	Z

            2. How to Encrypt a given plain text
               
               1. If letters of a pair are both same, then add an x after the first letter and encrypt the new pair.

                  • For example: If you have the word H E L L O, you’d resolve it as:

                    HE LX LO
                    

                    and not

                    HE LL O
                    

               2. If letters appear on the same row, replace them with the letters to their immediate right with wrapping, according to the direction.

                  • For example, consider one row of the matrix to be:

                    O

                    G

                    A

                    H

                    L

                    For HA, the corresponding plain text could be LH or AG. In our course, we’ll stick to AG.

               3. If letters appear on the same column, replace them with the letters immediately below, also with wrapping according to the direction.

                  • For example, consider one column of the matrix to be:

                    Y

                    A

                    N

                    K

                    W

                    • For NK, the corresponding plain text would be KW.
                    • For YW, the corresponding plain text would be AY.
               4. If letters are not on the same row/column, replace them with the letters on the same row respectively but add the other pair of corners of the rectangle, by the original pair.
            3. Demonstration of the plain text being encrypted
               
               • Consider the plain text “Hello find the solution”.

               • This will be broken into:

                 HE   LX   LO   FI   ND   TH   ES   OL   UT   IO   NZ
                 

               • And the encrypted text will look like:

                 OQ   OZ   OG   IG   BS   PL   SO   GO   ZC   SG   DW             
                 

         2. Solving another Example
            

            • Keyword: KEYWORD

              K	E	Y	W	O
              R	D	A	B	C
              F	G	H	I/J	L
              M	N	P	Q	S
              T	U	V	X	Z

            1. Plain Text: Why don’t you
               

               • This will be broken into:

                 WH   YD   ON   TY   OU
                 

               • And the encrypted text will look like:

                 KL   OR   KS   ZK   KZ
                 

            2. Plain Text: Come to the window
               

               • This will be broken into:

                 CO   ME   TO   TH   EW   IN   DO   WX
                 

               • And the encrypted text will look like:

                 OZ   NK   ZK   VF   YO   GQ   CE   BW
                 

   4. Homophonic
      
      • If a letter \(x\) appears for \(y\%\) of all characters, we assign \(y\) number of symbols to represent it.
   5. Polyalphabetic
      
2. Transposition
   
3. Product
   

7.2.2. Based on Number of Keys

1. Single/Private Key
   
   • It’s called symmetric key encryption.
2. Two-key or Public Key cryptography
   
   • It’s called assymmetric key encryption.

7.2.3. Based on how plaintext is processed

1. Block
   
   • Block Cipher transforms a fixed length of plaintext data into a block of ciphertext of the same length, with the help of a secret key.
   • The most commonly used length of text (size of the block) is 64 bits.
   • Two types are:
     • Transposition
     • Substitution
2. Stream