ENCRYPTION

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• ENCRYPTION
• Presented by
• Amit Choudhary

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• ENCRYPTION
• Data that can be read and understood without any
  special measures is called plaintext or
  cleartext.
• The method of disguising plaintext in such a way
  as to hide its substance is called encryption.
• Encrypting plaintext results in unreadable
  gibberish called ciphertext.
• The process of reverting ciphertext to its
  original plaintext is called decryption.

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• DEFINITIONS OF SECURITY
• Semantic Security
• An encryption scheme is semantically secure if
  it is infeasible to learn anything about the
  plaintext from the ciphertext.
• Defining more accurately, it means that whatever
  can be efficiently computed from the ciphertext,
  can be efficiently computed when given only the
  length of the plaintext.
• Indistinguishability of Encryptions
• It interprets security as the infeasibility of
  distinguishing between encryptions of a given
  pair of messages which are of same length.

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• ENCRYPTION ALGORITHMS
• Private Key Algorithms
• Single, Shared, Symmetric.
• Used for bulk data encryption, broadcast
  applications.
• Requires a secret key to be known by both
  parties.
• Examples are DES, RC4, Blowfish, IDEA.
• Public Key Algorithms
• Two, Asymmetric.
• Used for key agreement and distribution.
• Require a public and a private key pair for each
  party.
• Examples are RSA, Diffie-Hellman, ElGamal
• Hash Functions
• Used to compress data down to a fixed size for
  signing.
• Examples are MD5, SHA-1

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• DES Private Key Cryptosystem
• IBM developed Lucifer in 1974 which was adopted
  in 1977 as DES.
• DES encrypts and decrypts data in 64-bit blocks,
  using a 64-bit key (although the effective key
  strength is only 56 bits).
• Although the input key for DES is 64 bits long,
  the actual key used by DES is only 56 bits in
  length. Why??
• It takes a 64-bit block of plaintext as input
  and outputs a 64-bit block of ciphertext.
• DES has 16 rounds, i.e. the algorithm is
  repeated 16 times to produce the ciphertext.
• It has been found that the number of rounds is
  exponentially proportional to the amount of time
  required to find a key using a brute-force
  attack. So as the number of rounds increases, the
  security of the algorithm increases
  exponentially.

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• What After DES??
• DES is the workhorse of cryptography algorithms,
  and it's long past time to replace the
  19-year-old standard. The recent design of a 1M
  machine that could recover a DES key in 3.5 hours
  only confirmed what everybody knew DES's key
  size is far too small for today.
• The world only partly trusted DES because it
  survived the scrutiny of the NSA. Experts trusted
  DES because it was a published standard, and
  because it survived 20 years of intensive
  cryptanalysis by cryptographers around the world.
• Candidates for a replacement are emerging, but
  none has taken widespread hold.
• Triple-DES is the conservative approach.
• IDEA (used in PGP) is the most promising new
  algorithm.
• And there is a bevy of unpatented ones, RC4
  ,SAFER, and Blowfish.

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• Triple-DES Algorithm
• Triple DES is simply another mode of DES
  operation.
• It takes three 64-bit keys, for an overall key
  length of 192 bits.(168 bits)
• The procedure for encryption is exactly the same
  as regular DES, but it is repeated three times.
  Hence the name Triple DES.
• The data is encrypted with the first key,
  decrypted with the second key, and finally
  encrypted again with the third key.
• Consequently, Triple DES runs three times slower
  than standard DES, but is much more secure if
  used properly.

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• Triple Des Encryption - Problem
• Unfortunately, there are some weak keys that one
  should be aware of.
• If all three keys, the first and second keys, or
  the second and third keys are the same, then the
  encryption procedure is essentially the same as
  standard DES.
• Why is it a problem??
• This situation is to be avoided because it is
  the same as using a really slow version of
  regular DES.

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• Advantages of Public Key Cryptography
• Secure key exchange without prior exchange of
  secrets.
• Can provide a method for digital signatures and
  hence in authentication.
• Authentication of documents.
• Instead of encrypting information using someone
  else's public key, we encrypt it with our private
  key. If the information can be decrypted with our
  public key, then it must have originated by us.

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• RSA Public Key Cryptosystem
• Invented in 1977 by Ron Rivest, Adi Shamir and
  Leonard Adleman.
• Algorithm
• Take two large primes p and q and find their
  product n pq.

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• Security concern?
• It is difficult to obtain the private key d from
  public key pair (e,n).
• However not impossible. Lets see how?
• The Factoring Problem
• Factoring is the underlying foundation on which
  several Public Key Cryptosystems have been
  designed.
• Factoring an RSA modulus can help the attacker
  get a private key.
• Make factoring difficult by increasing the
  modulus n.
• Do hardware improvements make RSA less secure?
• No, since any improvement that allows an
  attacker to factor a number 2 digits longer than
  before will allow the RSA user to use a key
  dozens of digits longer than before.
• A lot of factoring algorithms have been designed
  in the recent years.
• Some of these methods and their running times
  are
• Pollard rho method O(sqrt(p)).
• Elliptic Curve Method O(exp(sqrt(2 ln p ln ln
  p))).
• Number Field Sieve O(exp(1.9(ln n)1/3(ln ln
  n)2/3)).

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• Security concern?
• The Discrete Log Problem
• The Log problem is to find the exponent x in the
  formula y gx mod p or
• to find the power that g must be raised in order
  to obtain y, modulo the prime number p.
• It has been the basis of several Public Key
  Cryposystems such as ElGamal System and DSS.
• The security of these systems rests on the
  assumption that discrete logs are difficult to
  compute.
• The best discrete log problems have expected
  running times similar to that of the best
  factoring algorithms.
• Which is easier to solve Factoring Problem or
  Discrete Log Problem?
• One paper suggests that the Discrete Log Problem
  is a little harder.

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• PGP
• PGP (short for Pretty Good Privacy) is a public
  key encryption program originally written by Phil
  Zimmermann in 1991. Over the past few years, PGP
  has become a de-facto standard for encryption of
  email on the Internet.
• PGP first compresses the plaintext.
• PGP then creates a session key, which is a
  one-time-only secret key.
• This session key is used to encrypt the
  plaintext the result is ciphertext.
• Once the data is encrypted, the session key is
  encrypted to the recipient's public key.
• This public key-encrypted session key is
  transmitted along with the ciphertext to the
  recipient.

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• PGP
• Decryption works in the reverse.
• The recipient's copy of PGP uses his or her
  private key to recover the temporary session key.
• PGP then uses this to decrypt the
  conventionally-encrypted ciphertext.

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• Hash functions
• A typical one way hash function takes a variable
  length message and produces a fixed length hash.
  Given the hash, it is computationally impossible
  to find a message with that hash.
• MD2, MD4, and MD5 are the different
  message-digest algorithms developed by Rivest.
• They are meant for digital signature
  applications where a large message has to be
  compressed'' in a secure manner before being
  signed with the private key.

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• MD2 was optimized for 8-bit machines whereas MD4
  and MD5 were aimed at 32-bit machines.
• MD2
• MD2 was developed by Rivest in 1989.
• The message is first padded so its length in
  bytes is divisible by 16.
• A 16-byte checksum is then appended to the
  message, and the hash value is computed on the
  resulting message.
• MD4
• MD4 was developed by Rivest in 1990.
• The message is padded to ensure that its length
  in bits plus 64 is divisible by 512.
• A 64-bit binary representation of the original
  length of the message is then concatenated to the
  message. The message is processed in 512-bit
  blocks and each block is processed in three
  distinct rounds.
• MD5
• MD5 was developed by Rivest in 1991.
• It is MD4 with safety-belts and while it is
  slightly slower than MD4, it is more secure.
• The algorithm consists of four distinct rounds,
  which has a slightly different design from that
  of MD4. Message-digest size, as well as padding
  requirements, remain the same.

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