Cryptography

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Cryptography

• Jill Tolentino

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Introduction

• Cryptography in Greek means secret writing.
• Today, it refers to the science and art of
  transforming messages to make them secure and
  immune to attacks.
• Cryptography is used to achieve
• confidentiality .............. only authorized
  persons can access information
• integrity ...................... information that
  was sent is what was received
• authentication ............. guarantee of
  originator and of electronic transmission
• non-repudiation ........... originator of
  information cannot deny content or transmission

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Cryptography components

• Original message is called plaintext.
• An encryption algorithm transforms the plaintext
  to ciphertext.
• A decryption algorithm tranforms the ciphertext
  back to plaintext.
• Sender uses an encryption algorithm and the
  receiver uses a decryption algorithm.

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Encryption and Decryption

• An encryption/decryption algorithm is referred to
  as ciphers.
• Through the use of public ciphers with secret
  keys, one cipher can serve millions of
  communicating pairs.
• A key is a number (value) that the cipher, as an
  algorithm, operates on.
• The encryption and decryption algorithms are
  public the keys are secret.

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Cryptography

• Cryptography algorithms are divided into two
  groups
• Symmetric-key (secret-key) cryptography
  algorithms
• Public-key(asymmetric) cryptography algorithms

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Symmetric-key Cryptography

• The same key is shared and used by the sender and
  receiver to encrypt and decrypt the message.
• The decryption algorithm is the inverse of the
  encryption algorithm.
• Example If encryption algorithm uses a
  combination of addition and multiplication, the
  decryption algorithm uses a combination of
  division and subtraction.

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Symmetric-key Cryptography

• Advantages
• Efficient- takes less time to encrypt a message
  using a symmetric-key algorithm than a public-key
  algorithm.
• Key is smaller
• Can encrypt and decrypt long messages

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Symmetric-key Cryptography

• Disadvantages
• Each pair of users must have a unique symmetric
  key.This means if N people in the world want to
  use this method, there needs to be N(N-1)/2
  symmetric keys. For example, for 1 million
  people to communicate, 500 billion symmetric keys
  are needed.
• Distribution of the keys between two parties can
  be difficult.

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Symmetric-key Cryptography

• Traditional ciphers use a character or symbol as
  the unit of encryption/decryption.
• Block ciphers use a block of bits as the unit of
  encryption/decryption.
• Traditional ciphers involve either substitution
  or transposition.
• Substitution Cipher bit-level encryption method
  in which n bits substitute for another n bits as
  defined by P-boxes, encoders, and decoders.
• Transposition Cipher A character-level
  encryption method in which the position of the
  character changes.

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Examples
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Public-key Cryptography

• There are two keys a private key and a public
  key.
• The sender uses the public key to encrypt the
  message and the receiver uses the private key to
  decrypt the message.

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Public-key Cryptography

• Advantages
• More efficient for short messages.
• Removes the restriction of a shared symmetric key
  between two entities.
• Each entity is independent, and the pair of keys
  created can be used to communicate with any other
  entity.
• Number of keys needed is reduced For 1 million
  users to communicate, only 2 million keys are
  needed, not 500 billion, as in symmetric-key
  cryptography.

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Public-key Cryptography

• Disadvantages
• Complexity of the algorithmthe algorithm needs
  large numbers to be effective.
• Association between entity and its public key,
  must be verified. (Can be overcome using a
  certification authority)

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Public-key Cryptography

• The most common public-key algorithm is called
  the RSA method after its inventors (Rivest,
  Shamir, and Adleman).
• The sender uses the following algorithm to
  encrypt the message CPe mod N
• The receiver uses the following algorithm to
  decrypt the message PCd mod N

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Example