Fundamentals Elements of Network and Cyber Security

1
Fundamentals Elements of Network and Cyber
Security

• Hussein Abdel-Wahab, Ph.D.
• Professor and Graduate Program Director
• Departmet of Computer Science
• Old Dominion University
• wahab_at_cs.odu.edu
• www.cs.odu.edu/wahab

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General Concepts

• Players Alice, Bob and Trudy.
• How to communicate securely
• over an insecure medium?
• Alice should be able to send a message to Bob
• That Trudy can't understand or modify
• Bob is assured that Alice is the sender.

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Types of Attaches

• Passive Attacks
• The attacker eavesdrops and read/record
  messages in transit.
• Active Attacks The attacker may 
• Transmit new messages,
• Replay old messages,
• Modify/Delete  messages on transit.

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Fundamental Tenet of Cryptography

• If lots of smart people failed to solve a
  problem, then it probably won't be solved (soon).
  
• The time required to break  a code should be 
  longer than the time the encrypted data must
  remain secret.
• The value of most data decreases overtime.

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Cryptographic System  Algorithm Key

• It is perfectly OK to let everyone know the
  algorithm. Knowledge of the algorithm without the
  key does not help unmangle the information.
• Publishing the algorithm provides an enormous
  amount of free consulting to uncover weaknesses.

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Layers and Cryptography

• Application (e.g., PEM),
• Transport (e.g., SSL),
• Network (e.g., IPsec).

7
Traditional use of Cryptography

•  Plaintext gtgtgt Ciphertext gtgtgt Plaintext       
  (Encryption)       (Decryption)
• Cryptographer Invent clever secret codes.
• Cryptanalyst Attempt  to break these codes.

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Computational Difficulty

• Example combination lock
• Typically require 3 numbers between 1 and 40.
• If it takes 10 seconds for a good
  guy, it would take 10(403) seconds
• or about 1 week for the bad
  guy.
• By requiring 4 numbers
• If  it takes 13 seconds for
  the good guy,
• it would take  13(404)
  seconds
• or about 1 year for the bad
  guy.

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Example of Secret Codes

• Caesar cipher
• Substitute each letter with another
  letter
• which is 3 letters away in the alphabet.
• E.g., dozen -gt grcho.
• Mono-alphabetic cipher
• Arbitrary map one letter to another.
• There are 26!4(1026) possibilities. If
  each possibility takes 1 microsecond
• it would take 10 trillion years to try
  all possibilities.
• However statistical analysis of 
  language
• makes it much easier to break.

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Secret Key Cryptography (Symmetric Cryptography)

•                  (encryption)
• plaintext gtgtgt ciphertext
                                           
  key                       
• ciphertext gtgtgt plaintext
                (decryption)

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Uses of Symmetric Cryptography

• Transmission Over an Insecure Channel
• An eavesdropper will only see unintelligible
  data.
• Secure Storage on Insecure Media
• Forgetting the key makes the data irrevocably
  lost!
• Authentication Alice authenticating Bob
•         Alice           
          Bob
•   challenge      r gtgtgtgtgtgt
     r
• response       r Kc        ltltltltltltlt  
     cKr
•     

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Public Key Cryptography (Asymmetric
Cryptography)

• Each individual has two keys
• private key (not revealed to anyone)
• public key (make it known to everyone)
•    
• 
  (encryption) plaintext 
  gtgtgtgtgtgtgtgtgtgt ciphertext
                            
                public key
•                 private key    
  ciphertext  
  gtgtgtgtgtgtgtgtgt plaintext                 
  (decryption)  

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Digital Signature

•  
•            
  (signing) plaintext  
  gtgtgtgtgtgtgtgtgt ciphertext                        
                            
  private key  
•                   
• public key
                            
  ciphertext   gtgtgtgtgtgtgtgt plaintext
                   (verification)  

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Uses of Public KeySecret Key establishment

• Public key cryptographic algorithms are much
  slower than
• Secret key cryptographic algorithms.
• Thus they are normally used to establish
  temporary
• shared secret key for use during a given session.
  
• Alice                                
              Bob
•         K eB         gtgtgtgtgtgtgtgtgt            
  K dB
•             KmB          gtgtgtgtgtgtgtgtgt           
  KmB
•         KmA          ltltltltltltltltlt           
  KmA

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Uses of Public Key Authentication

• Alice authenticating Bob
• Alice
  Bob
• challenge c r eB gtgtgtgtgt c
• response r ltltltltlt r
  cdB

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Hash Algorithmsmessage-digest, finger-print,
one-way-function

• The hash of a message m,
• h H(m)
• has the following properties
• Given m, it is easy to compute h.
• Given h, it is hard to compute m.
• Given m, it is hard to find another m'
• such that H(m) H(m').
• It is hard to find m1 and m2
• such that H(m1) H(m2).

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Message Authentication/Integrity Code (MIC/MAC)

• Using Secret Key
•             Alice                    
      Bob
• m,h, where h H(mK)    gtgt    m,h , OK if h H
  (mK)
•       
• Bob is sure that Alice sent m, since she knows K.
• Bob can NOT prove to any one else that
• Alice sent him m, since he also knows K!

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Password Hashing

• UNIX stores the hash of passwords.
• For each user U with password P, there is a
  tuple
• ltU, hgt, where h H(P) 
• When user U types a password P,
• UNIX computes H(P)
• and the use is allowed to login if H(P) h

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The magic of XOR

•  
• 0 0 0 , 0 1 1, 1 0 1 1 1
  0
• Note that
•     a  a 0  
• a  bb a (since b b 0)
• A Simple XOR symmetric algorithm
• (P plain, C cipher, K key)
• Encrypt C P K
• Decrypt P C K (since (P K) K
  P)

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Secret Key CryptographyPrinciple

• Secret key cryptographic systems takes
• a key K and a data block M and generate a
  one-one mapping that looks completely random.
• I.e., any single bit change of K or M result
  in a totally independent random output.

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Secret Key Cryptography Transformation

• Substitution For small blocks of size k bits,
  specify for each of the 2k possible values of the
  input, the k-bit output.  
• Permutation Specify for each input bit, the
  output position to which it goes.
• Example DES (Data Encryption Standard)

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Hashes/Message-DigestsPrinciple

• Major Algorithms
• Ron Rivest Message Digest (MD2, MD4 and MD5)
  128-bit.
• NIST  Secure Hash Algorithm SHA-1 160-bit.
• Both takes an arbitrary-length string and map it
  to a
• fixed-length quantity that appears to be randomly
  chosen.
• They are easy to compute and are computed in
  rounds.
• It is computationally infeasible to find
• A message that has a given message digest.
• A different  message with the same message
  digest.
• Two messages that have the same message digest.

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Things to do with a Hash

• Authentication
•      Alice        
         Bob
•  challenge   r gtgt
  r   
• response   d ltlt
  dMDKr
• Alice computes MDKr and if equal d, then Bob
  knows K. 
• Computing a MAC
•      Alice                       
  Bob
•  m,d where d MD(Km) gtgt m,d, OK if d MD
  (Km)

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Encryption using Hash

• Generating one-time pad
• Both Alice and Bob knows he shared secret K
• and generates
•          b1 MD(K)      bi MD(Kbi-1), i2,3,
  .... 
•         
•    Alice                        Bob
• ci mi bi             gtgtgtgt      mi ci
  bi  

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Public Key CryptographyPrinciple

• Secret key algorithms  Hash algorithms similar.
• Public key algorithms are different from each
  other.
• What is common among all public key algorithms
  is
• each participant has two  keys, public and
  private,
• most of them are based on modular
  arithmetic
• x mod n is the remainder of x when divided by n.
• Example 24 mod 10 4

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Multiplication mod 10

•  
• Multiplication by 1, 3, 7 and 9 works as cipher
  since it performs 1-1 mapping.
• Each "1" is the intersection of k and k-1, e.g. k
  7, then k-1 is 3.
• Example  if k 7, then 1987 is encrypted to
  7369

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Totient Function

• What is so special about the set 1,3,7,9 ?
• These numbers are relatively prime to 10,
• i.e., they do not share with 10 any common
  factors other than 1.
• How many numbers lt n are relatively prime to n?
• This quantity is referred to as Ø(n) and is
  called the totient function
• If n is prime
• then 1,2, ..., n-1 are all relatively prime
  and Ø(n) n-1.
• If n p.q where p and q are two distinct primes,
  
• then Ø(n) (p-1)(q-1).
• Example  for n 10 2.5, Ø(10)
  (2-1).(5-1)1.44, which is the
  set 1,3,7,9.

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Exponentiation mod 10

• Examples  4 2 6, 8 8 6, 76  9
• An exponentiative  inverse of e is the number  d 
  such that
• e.d 1 mod Ø(n)
• Example For n 10, Ø(10)4
• e3 and d7 are exponentiative inverses since
  3.721 1 mod 4
• In public cryptography lte, ngt is public key   
  ltd,ngt is  private key

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Encrypt / Decrypt Sign / Verify

• Encrypt / Decrypt
• To encrypt m  compute  c me mod n
• To decrypt c   compute  m cd mod n
• Example 
• encrypt m 8 c 83 2
• decrypt c2  m 27 8
• Sign / Verify
• To sign m compute s md mod n
• To verify s compute m se mod n
• Example
• sign m 8 s 87 2
• verify  s2  m 23 8

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RSA Algorithm(Rivist, Shamir Adleman)

• Choose two  large primes p and q.   (typically
  256 bits each keep them secret).
• Compute n p.q Ø(n) (p-1)(q-1).   (it
  is very hard to factor n into p q).
• Choose a number e relatively prime to Ø(n).
• Find d as multiplicative inverse of 
• e mod Ø(n), i.e., e.d 1 mod Ø(n)).
• public key  lte,ngt     private key  ltd,ngt.

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RSA works

• Encrypt/Decrypt
• To encrypt a message m (ltn) c me mod n
• To decrypt c m cd mod n
• This works since
• cd mod n (me)d mod n  me.d mod n            
    m mod  n   // since e.d 1 mod Ø(n)
                 m                 //
  since m lt n
• Sign/Verify
• To sign a message m (ltn) s md mod n
• To verify s m se mod n
• This also works since
• se mod n me.d mod n m mod n m

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Why is RSA Secure?

• Every one  knows the public key  lte, ngt.
• To find the private key ltd,ngt  you  need to know
  Ø(n)
• since e.d 1 mod Ø(n).
• To know Ø(n) you need  to know p and q
• since Ø(n) (p-1).(q-1).
• Thus to break RSA you should be able to factor n
  to find  p and q.
• Factoring a big number is hard. (the best
  technique  to factor 512 bit number will take
  30,000 MIPS-years!)
• Popular RSA keys values
• public key lt365537, ngt private key ltd ,
  ngt.

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Efficiency of  RSA Operations

• How to compute 12354 mod 678?
• 1232 123.123 15129 213 mod 678 1233
  123.213 26199 435 mod 678 1234 123.435
  53505 621 mod 678 ...... 12354     
  ......                 87  mod 678
• This requires 54  multiplications and 54
  divisions.  
• How to compute 12332 mod 678?
• 1232   123.123 15129       213 mod 678
  1234    213.213 45369       621 mod 678
  1238    621.621 385641     537 mod 678
  12316  537.537 288369     219 mod 678
  12332  219.219 47961       501 mod 678
• This requires 5  multiplications and 5 divisions
  instead of 32.
• To efficiently compute  12354 54 in binary as
• 1          1                   0         
      1                 1             
  0            
•                      
                                 
• ((((  (1232) 123      )2        
       )2 123      )2123   )2
• This requires 8  multiplications and 8 divisions.

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Diffie-Hellman Key agreement Protocol

• Alice and Bob agree on  p (large prime)   g lt
  p.  
•              Alice                                
                                 Bob
• Pick SA  (512-bit random number)     Pick SB 
  (512-bit random number)
• Compute TA ( gSA) mod p       Compute TB
  (gSB) mod p
•             send    TA       gtgtgtgtgtgtgtgt
  ltltltltltltltltlt      send TB   
• Compute  X  TB SA mod p     Compute Y TA
  SB mod p 
• X is the same as Y, why?
•        X   TBSA  gSBSA    Y   TASB  gSASB
• No one can compute  g (SASB ) by knowing  g (SA
  )  g (SB )

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Email Security Protocols

• PEM (Privacy Enhanced Mail)
• Add encryption, authentication and integrity
• to ordinary text messages.
• MIME (Multipurpose Internet Mail Extensions)
• Is a standard for encoding arbitrary data in
  email
• (images, video, etc.).
• S/MIME Incorporated many principles of PEM 
  into MIME.

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PEM MIC-CLEAR

• From Alice To Bob Subject Colloquium
  Date Tue Oct 26, 2005
• -----BEGIN PRIVACY ENHANCED MESSAGE-----
  Originator-ID-Asymmetric  ltcertificategt
  MIC-Info RSA-MD5, RSA, ltMICgt
• Dear Bob I would like to invite you to give
  a colloquium next Fall, If you accept, let us
  talk about the details. Alice -----END PRIVACY
  ENHANCED MESSAGE-----

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PEM ENCRYPTED

• From Alice To Bob Subject
  Colloquium Date Tue Oct 26, 2005
• -----BEGIN PRIVACY ENHANCED MESSAGE-----
  DEK-Info DES-CBC, IV MIC-Info RSA-MD5, RSA,
  ltMICgt Recipient-ID-Asymmetric ltRecipient
  certificategt Key-Info RSA, ltkey encrypted with
  recipient  public keygt
• ltencoded encrypted message using DES-CBCgt
  
• -----END PRIVACY ENHANCED MESSAGE-----     

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SSL/TLSSecure Socket Layer, Netscape Transport
Layer Security, IETF

• Run as a user-level processes on top of  TCP/IP.
• Alice        
  
           Bob
• I want to talk, ciphers I support, Ra
  --------------------- gt lt ----------------------
  ------ crtificate, cipher I choose, Rb choose
  secret S, compute K f (S,Ra,Rb) SBob ,
  keyed hash of handshake msgs  -------------- gt
                                                  
  compute K f(S,Ra,Rb) lt------------------------ 
    keyed hash of handshake msgs
• lt--     data protected with keys derived
  from K   --gt
• Ra and Rb are 32 octets long, the first 4 are the
  time
• This ensures that Rs are always different.

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SSL Keys

• Alice chooses a random number S,
• called pre-master secret.
• It is shuffled with Ra Rb to produce
• a master secret K.
• The master secret is shuffled with Ra Rb to
  produce 6 keys Three for each side for 
  encryption, integrity, and IV.
• Note that Alice has authenticated Bob,
• but Bob has no idea to whom he's talking!
  Normally the server authenticates the user
  using
• ltname, passwordgt sent securely over the ssl
  connection.

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Authentication SystemsPassword-based

• Its not who you are, Its what you know
• On-line Password attack Easy to defend, e.g.,
  limit  and slow down the number of guesses.
• Off-line Password attack Capture a quantity X
  derived from the password and take your time to
  guess the password that produces X.
• (e.g., use a dictionary)

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Authentication SystemsAddress-based

• It's not what  you know. It's where you are
• In Unix /etc/hosts.equiv
• Contains a list of computers that have
  identical user accounts to allow users on these
  hosts to rlogin without providing passwords.

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Trusted Intermediaries

•  
• For N entities, if each keeps N -1 secrets,
• then adding a new entity involves adding N new
  secrets. Clearly not practical for large N.
• KDC (Key Distribution Center) 
• Keeps N keys, and adding one key for each
  new entity.
• Alice                        
  KDC               Bob Need to talk to
  Bob  --------------gt                             
  generate random R, R KAX lt---------  X
  KAR , Y KBR  -------gt  R KBY
• C1  RM1 -----------------------------
  ---------gt  M1 RC1 M2 RC2 
  lt------------------------------------   C2 
  RM2
• Disadvantages of KDC
• If compromised, all Keys are compromised.
• Single point of failure
• Performance bottleneck.

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CA Certificate Authority

• Each entity keeps its private key.
• The CA  certifies  (sign) that the public key
  belong to the entity.
• All public key certificates may be kept in one
  place or each  entity keeps its own.
• Certifies expire after a reasonable period (1
  year). It can be revoked and the CA periodically
  publish a CRL (certificate revocation list) .
• Clients should check the latest CRL before
  trusting a certificate.

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Delegation

•  
• It's not who you are. It's who you're working for
  
• Sometime it is necessary to have some entity act
  on your behave.
• This is achieved using delegation
• Generate  a special  message, signed by you
• (using public key cryptography, or through KDC),
  specifying
• To whom you are delegating the rights,
• Which rights are being delegated
• For how long.

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Off-Line Password Guessing

• Obtaining a  hash of a  password h, an attacker
  can guess
• the password  w and checks to see if h MD (w).
• If some one obtains  a file F containing the
  hashes of
• many passwords, e.g., /etc/passwd
• he can perform a dictionary attack  
•  for each word w in dictionary D  do
•    compute h MD (w)
•   for each e in F do
•      if e h  then w as a password
• done
• done
• The number of  performed hashes is D

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Off-Line Password Guessing Adding Salt

• Storing a random number salt (s) with  h MD
  (ws)
• makes it harder for  a dictionary attack  
• for each entry lts, hgt in F  do        for each 
  word w  in the dictionary  D do
•      compute e   MD (ws)     
  if e h  then w as a password done
• done
• The number of performed hashes is D.F

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Mutual Authentication

• Shared Secret Alice                            
                         Bob
• I'm Alice -------------------------------------
  --gt lt -------------------------------------------
  ----- Rb f(K, Rb) -------------------------------
  ---------gt Ra  ----------------------------------
  ------------gt lt----------------------------------
  -------- f(K, Ra)

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Reducing number of Messages

•    Packing more information into each message
•   Alice                                         
                 Bob
• I'm Alice, Ra   ------------------------------
  -------gt lt---------------------------------------
  ---  Rb, f(K, Ra) f(K, Rb)   --------------------
  ------------------------gt  

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Reflection Attack!

• Trudy can impersonate Alice to Bob by
• opening a second connection to Bob
• Session1
• Trudy                                          
               Bob
• I'm Alice, Ra   ----------------------------------
  --------------gt
• lt-------------------------------------------------
  ----- Rb, f(K, Ra)
• suspend session 1......
• Session 2 Trudy                             
                         Bob
• I'm Alice, Rb  -------------------------------
  ---------------gt lt-------------------------------
  -----------------   Rb', f(K, Rb) abort session
  2.......
• continue  session 1......
• f(K, Rb)  ----------------------------------------
  -----------------gt

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Password Guessing Attack!

• Trudy                                             
                      Bob
• I'm Alice, Ra   ----------------------------------
  -----------gt
• lt-------------------------------------------------
  Rb, f(K, Ra)
• .........
• suspend session and use
• Ra, and f(K,Ra) to guess K.

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Using Time Stamps

• We can use time stamps to reduce the number of
• messages to two  
• Alice                          
                                   Bob
• I'm Alice, f(K, timestamp)  -------------------
  ------gt lt--------------------------------------
  f(K, timestamp)

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Mediated AuthenticationNeedham-Shroeder Protocol

•    
• Alice                            
  KDC                                Bob  
• N1, Alice wants Bob  ----------gt
• lt--------------  KA N1,"Bob", KAB, ticket to
  Bob,             where ticket to Bob KB KAB,
  "Alice"
• ticket to Bob,  KABN2  -------------------------
  ----------------------gt
• lt ------------------------------------------------
  ------------ KABN2--, N3
• KAB N3--  --------------------------------------
  --------------------------gt
• N is a nonce, a number that is used only once
  (e.g., random number).
• N1 to prevent Trudy from impersonating KDC and
• replaying old replies to Alice.
• N2 and N3 are challenges for mutual
  authentication.
•