Chapter 2: Cryptography Technique

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Chapter 2 Cryptography Technique

• "You can have everything in life you want if
  you'll just help enough other people to get what
  they want!" -- Zig Ziglar
• Prepared by
• SITI ZAINAH ADNAN
• If you do have any feedback or comment,
• please feel free to email me at
• sitizai_at_hotmail.com
• Your cooperation is very much appreciated !

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Chapter 2 Cryptography Technique

• TOPICS
• Introduction
• Terminology Basic Encryption Methods
• Secret Code
• Monoalphabetic Ciphers
• Cryptanalysis of a Monoalphabetic Cipher
• Polyalphabetic Substitution Cipher
• Transpositions
• Stream and Block Ciphers

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References

• Book (available at the Informatics library)
• CHAPTER 2, 3 and 4 Security In Computing,
  Charles P.Pfleeger, Prentice Hall International
• Notes (available at IVC)

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Encryption

• Transforms data so that it is unintelligible to
  the outside observer
• To maintain secure data in an insecure
  environment

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Encryption
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Encryption With One Key

• Symmetric encryption (Single-key encryption OR
  Private Key encryption)

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Encryption with Two Keys

• Asymmetric encryption (Two-key encryption OR
  Public-Key encryption)

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Terminology

• Encryption
• A process of encoding a message so that its
  meaning is not obvious. Also known as
  encode/encipher
• Decryption
• Is the reverse process of encryption.Also known
  as decode/decipher
• Cryptosystem
• A system for encryption and decryption.
• Plaintext
• The original form of a message.

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Terminology

• Ciphertext
• The encrypted form of an original message.
• Secret Code
• Use of simple secret code to represent the
  original message.
• Example, apple means go ahead.

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Terminology

• Cryptography
• The science of using mathematics to encrypt and
  decrypt data
• Enables user to store sensitive information or
  transmit it across insecure networks so that it
  cannot be read by anyone except the intended
  recipient
• Cryptanalysis
• The science of analyzing and breaking secure
  communication
• Done by cryptanalyst (attackers)
• Cryptology
• The research into and study of encryption and
  decryption include cryptography and cryptanalyst

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Terminology

• Key
• A value that is used together with the plaintext
  as input into encryption algorithms to produce
  ciphertext
• Allows different encryption of a plaintext
• Provide additional security
• If the encryption algorithm is revealed, future
  message can still be kept secret because the
  interceptor will not know the key value
• Basically really big numbers, measured in bits
  e.g. 1024 bit key

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Cryptosystem model
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• An opponent, observing Y but not having access to
  K or X, may attempt to recover X or K or both X
  and K - assume that opponent knows the encryption
  E and decryption D algorithms
• To recover X and K, plaintext estimate X and
  estimate K are recovered

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• Input are message X and the encryption key K,
  the encryption algorithm forms the chipertext as
  
• Y Y1, Y2....Yn OR Y Ek (X)
• Y is produced by encryption algorithm E as a
  function of plaintext X, which determined by key
  K value
• The intended receiver, in possession of the key,
  is able to invert the transformation
• X Dk (Y)

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Encryption algorithms

• Two basic methods
• Substitution
• Monoalphabetic ciphers
• Polyalphabetic ciphers
• Transposition (permutation)

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Monoalphabetic Ciphers

• Each letter is translated or substituted by a
  fixed letter after it in the alphabet.
• E.g. Caesar Cipher uses a shift 3.
• The plaintext letter pi is encipher as ciphered
  letter ci by the rule.
• ci E(pi) pi 3
• A translation chart of the Caesar cipher is as
  shown below.
• Plain text ABCD E FG H IJKLMNOPQRSTUVWXYZCipher
  text defg h ij k lmnopqrstuvwxyzabc
• E.g. Happy New Year would be kdssb qhz bhdu

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Monoalphabetic Cipher Cryptanalysis

• Some letters are used more often than others.
• Example of the clues
• The letter E, T and A occur more than J, Q and Z
• The break between the two words, the SS can be
  translated to VV
• Therefore cryptanalysis uses such occurrence
  patterns to decipher the cipher text easily
• Count do help to narrow the possibilities
• The frequent occurring letters in the ciphertext
  are likely to be among the more frequently
  occurring letters in English

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Monoalphabetic Cipher Cryptanalysis
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Polyalphabetic Cipher

• The weakness of monoalphabetic cipher is that
  their frequency distribution reflects the
  distribution of the underlaying alphabet.
• A cipher that is more cryptographically secure
  would display flat distribution.
• One way to flatten the distribution is to combine
  distribution that are high with that are low.
• Uses multiple mappings between plaintext and
  ciphertext and it is not just a single mapping

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Polyalphabetic Cipher

• Example of mapping
• Odd position mapping
• ABCDEFGHIJKLMNOPQR S T UVWXYZ..
• Adgjmpsvybehknqtwz c f ilorux
• Even position mapping
• ABCDEFGHIJKLMN O PQR S TUVWXYZ..
• Nsxchmrwbglqva f kpu z ejotydi
• Example of text
• TREAT YIMPO SSIBL E ? fumnf dyvtf czysh h

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Polyalphabetic Cipher

• As compared with monoalphabetic cipher
• E.g. encryption using Odd position mapping
• TREATY IMPOSSIBLE ? fzmafu yktq cc ydhm

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Transposition

• Rearrange the order of bits, characters or blocks
  of characters that are being encrypted or
  decrypted.
• The original letters of the plaintext are
  preserved only their positions change.
• E.g. Columnar Transposition
• The rearrangement of the characters of the
  plaintext into columns
• The resulting ciphertext is formed by traversing
  the columns

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Columnar Transposition
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Columnar Transposition

• The plaintext is
• THIS IS A MESSAGE TO SHOW HOW A COLUMNAR
  TRANSPOSITION IS WORKING
• The ciphertext is
• TAGO CASIR HMEW ORPOK IETH LTONS SSOO URSS
  ISSW MAIW SAHA NNTO

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Stream Ciphers

• ISSOPMI wdhuw
• Plaintext cihertext
• Encryption

• It converts one symbol of plaintext immediately
  into a symbol of ciphertext
• The transformation depends only on the symbol,
  the key, and control information of the
  encipherment algorithm
• E.g. substitution encryption

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Stream Ciphers - Advantages

• Speed of transformation
• Each symbol is encrypted without regard for any
  other plaintext symbols, each symbol can be
  encrypted as soon as it is read
• Low error propagation
• Each symbol is separately encoded, an error in
  the encryption process affects only that character

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Stream Ciphers - Disadvantages

• Low diffusion
• Each symbol is separately enciphered
• Susceptible to malicious insertions and
  modifications
• Since, each symbol is separately enciphered, an
  active interceptor who has broken the code can
  splice together pieces of previous messages and
  transmit a spurious message that may look
  authentic

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Block Ciphers

• OITPYR
• Encryption ba
• qc
• kd
• em

• It encrypts a group of plaintext symbols as one
  block , e.g. 64 bits or more
• It works on blocks of plaintext and produce
  blocks of ciphertext
• E.g. transposition encryption

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Block Ciphers - Advantages

• Diffusion
• Information from plaintext is diffused into
  several ciphertext symbols
• One ciphertext block may be depend on several
  plaintext letters
• Immunity of insertions
• Since blocks of symbols are enciphered, it is
  impossible to insert a single symbol into one
  block
• The length of the block would then be incorrect,
  and decipherment would reveal the insertion

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Block Ciphers - Disadvantages

• Slowness of encryption
• Block ciphers must wait until an entire block of
  plaintext symbols has been received before
  sorting the encryption process
• Error propagation
• An error will affect the transformation of all
  other characters in the same block

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Key Management Protocols

• Attack to Remote communications
• When two remote systems transfer messages along
  communication medium, several potential attack
  scenarios arise. There are three main types of
  attacks
• Disclosure to an unauthorised listener.
• Receipt of a message from a masquerading sender.
• Corruption or blocking of sent messages.

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Key Management Protocols

• a. Disclosure to an unauthorised listener
• Involves a malicious entity or an intruder
  gaining access to the message by simply observing
  and interpreting the data travelling along the
  communication medium.
• b. Receipt of a message from a masquerading
  sender
• This attack involves an unauthorised masquerading
  entity claiming to be some authorised entity.
• The result is that a bad message might be sent
  from an intruder.

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Key Management Protocols

• c. Corruption or blocking of sent messages
• Refers to messages sent by a sender being
  corrupted or blocked by an unauthorised intruder.
• Intruders corrupt the good message and replace
  it with a bad message.

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Key Management Protocols

• Encryption systems provide an important tool in
  computer security, it gives a user the ability to
  transmit information in a concealed form.
• It is very useful to transmit documents and data
  over a channel that may be intercepted.
• Protocol is established to orderly sequence the
  steps of the encryption by two or more parties
  in their communication
• Three type of encryption protocols
• Private key protocol
• Public key protocol
• Arbitrated protocol

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Private Key Protocol

• A protocol that may be used between communicating
  entities to authenticate systems and hide
  messages.
• Assumed that communicating entities have access
  to encryption routines M that encrypts and
  decrypts using some key K.
• Both sender and receiver knows K.
• Sender Receiver
• message Mk Mkk message
• encrypt decrypt

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Encryption With Private Key
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Private Key Protocol - Advantages

• Fast
• Useful for encrypting data for a short distance

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Private Key Protocol - Disadvantages

• If the key is revealed (stolen etc.), the
  interceptors can decrypt all the information in
  both direction.
• Distribution of keys becomes a problem, as keys
  must be transmitted with utmost security.
• The number of keys increases with the square of
  the number of users exchanging secret
  information.

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Public Key Protocol

• Deviced by Whitfield Diffie and Martin Hellman in
  1996.
• This scheme overcomes the exchange of private key
  problem.
• Makes use of two encryption keys, public key and
  private key.
• Allows remote entities to advertise part of the
  encryption key, that is public key.
• Requires both public key and private key to
  encrypt and decrypt a message.
• Addresses several problems of key distribution
  and key loss issues.

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Encryption with Public Keys
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Public Key Protocol

• Based on mathematical functions rather than on
  substitution and permutation
• Asymmetric involving the use of two separate
  keys, in contrast to symmetric conventional
  encryption, which uses only one key
• Has profound consequences in the area of
  confidentiality, key distribution, and
  authentication

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Public Key Cryptosystem

• Each end system in a network generates a pair of
  keys to be used for encryption and decryption of
  messages that it will receive
• Each system publishes its encryption key by
  placing it in a public register or file. This is
  the public key. The companion key is kept private
  (eg. Verisign provide security service for
  international e-commerce, DigiCert mainly for
  local e-commerce)
• If A wishes to send a message to B, it encrypts
  the message using Bs public key
• When B receives the message, B decrypts it using
  Bs private key. No other recipient can decrypt
  the message because only B knows Bs private key.

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Public Key - Advantages

• All participants have access to public keys and
  private keys are generated locally by each
  participant and therefore need never be
  distributed
• As long as system controls its private key, its
  incoming communication is secure
• At any time,a system can change its private key
  and publish the companion public key to replace
  its old public key
• It is computationally infeasible to deduce the
  private key from the public key
• Anyone who has a public key can encrypt
  information but cannot decrypt it
• Only the person who has the corresponding private
  key can decrypt the information

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Public Key Examples

• Elgamal (named for its investor, Taher Elgamal)
• RSA (named for its investors Ron Rivest, Adi
  Shamir and Leonard Adleman)
• Diffie-Hellman (named for its investors)
• DSA Digital Signature Algorithm (invented by
  David Kravitz)

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Public Key Cryptosystem Encryption
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Public Key Cryptosystem Authentication
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Public Key Cryptosystem Applications
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Public Key Protocol

• Sender (B)
  Receiver (A)
• Message MSBPA
  MSBPASAPB Message Encrypt
  Decrypt
• Legend
• SBSecret key of B PBPublic key of B
• SASecret key of A PAPublic key of A

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Arbitration Protocols with Third Party

• An alternative to the point key management
  protocols between senders and receivers.
• Protocol that uses third party to ensure
  authentication between communicating entities.
• Two types of arbitrated key management protocols
  might be envisioned.

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Arbitration Protocols with Third Party

• 1) All data communication is performed through a
  third party, so that A might communicate with C
  through arbiter B. This requires that A and B
  agree on some key management protocol and that B
  and C agree on a protocol as well.
• A B C
• Arbitrator

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Arbitration Protocols with Third Party

• 2) The third party arbiter establishing
  authentication between the sender and receiver
  e.g. visiting prisoner at prison, have to go thru
  the prison warden

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Reasons why an arbiter is not desirable

• Difficult to find a trusted arbitrator e.g.
  recognize and valid authority
• Cause delay in communication e.g. have to go thru
  arbiter for any communication
• Costly in maintaining an arbiter e.g. fees
  payment, service charges
• Bottleneck happens if many users access to same
  arbiter e.g. delay in communication
• Not secure since arbiter has access to sensitive
  information e.g. no 100 confidentiality

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Key Distribution

• This scheme involves distribution of public and
  secret keys by transmission over a secure
  channel.
• Uses private key management protocol between
  remote systems and the KDC - Key Distribution
  Centre.
• This approach allows two entities to receive keys
  from KDC for their communication.
• Secrecy of their transmission to the KDC is
  assured by the distribution process.

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Pretty Good Privacy (PGP)

• It is a hybrid cryptosystem by Philip Zimmerman
• Fast ,easy and free ported to most popular
  computer platforms, PCs and Macs as well to
  protect messages on the Internet
• Used in email e.g. Netscape Messager, MS Outlook
  Express and Qualcomm Eudora Pro

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PGP Encryption

• PGP compresses the plaintext to save transmission
  time and disk space and to strengthen
  cryptographic security
• Compression reduces plaintext pattern and
  enhances resistance to cryptanalysis
• PGP creates session key, one-time-only secret
  key from the random movement of users mouse and
  keystrokes type
• Using the IDEA algorithm with the session key,
  the message is encrypted.
• The session key is encrypted with the RSA
  algorithm and the recipients public key
• The encrypted message and session key is then
  bundled together and ready to be mailed or stored

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PGP Encryption
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PGP Decryption

• The recipients copy of PGP uses his/her the
  private key to recover the temporary session key
• Then, PGP uses it to decrypt the ciphertext

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PGP Decryption
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Secure Socket Layer (SSL)

• Developed by Netscape in 1994
• Provides two functions
• Encrypting the information flow between client
  and server
• Client/server authentication
• Latest version is SSL 3.0 (1996)
• Supported by
• Client applications (Netscape Navigator,
  Microsoft Internet Explorer)
• Server applications (Netscape, Microsoft, Apache,
  Oracle, NSCA etc.)
• Certification Authorities (VeriSign)

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Secure Socket Layer (SSL)

• Comes with two strength 40-bit and 128-bit which
  refer to the length of the session key
  generated by every encrypted transaction
• HTTPS - Hypertext Transfer Protocol Secure
• https// represent secure site using SSL services
  (HTTP SSL)

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Secure Socket Layer (SSL)

• Fundamental concerns about communication over
  the Internet and other TCP/IP networks
• SSL server authentication allows a user to
  confirm a server's identity
• SSL client authentication allows a server to
  confirm a user's identity
• An encrypted SSL connection requires all
  information sent between a client and a server to
  be encrypted by the sending software and
  decrypted by the receiving software, thus
  providing a high degree of confidentiality