Public key cryptography

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Public key cryptography

• Prof. ?????????? Eris
  Asllani

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Public Key Cryptography introduced by Whitfield
Diffie Martin Hellman 1976, (Asymmetric Keys
Cryptography)

• Two mathematically related keys, 1024 or 2048
  bits, a public and a private one, impossible to
  calculate one from the other.

• Private key (secret) used for encryption, Public
  key (distributed) used for decryption.

• Public key algorithms RSA , ElGamal , Elliptic
  Curves.

• Made possible the use of digital/electronic
  signatures.

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• The digital signature scheme involves RSA
  public and private keys, MD5 hash functions and
  PKCS or BASE 64 algorithms to encrypt the
  envelope.

• RSA algorithm includes 3 steps
• Key generation, encryption, decryption
• Keys are interdependent and generated using two
  large prime numbers
• John encrypts the message using his private key,
  and Volin can decrypt and read the message using
  Johns pubic key. Volin knows the message is
  generated by John because hic public key can
  decrypt only msgs encrypted with his private
  key. (keys are assymetric)

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• The originality of the message
• After Volin is assured that the msg was sent from
  John, he still has no proff that the message was
  not intercepted and changed by a third party.
• For reliability of the infrastructure, HASH
  functions are introduced.
• HASH footprint takes a block of data and
  returns a fixed size string of bits, (same string
  size for any msg size) Also called msg digest.
• Easy to compute the HASH but impossible to
  compute the msg from the hash
• HASH functions SHA, RIPEMD 160, MD5

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• John computes the HASH digest of the message,
  together with the original msg forms the first
  envelope that is encrypted with his private key.
  This data is again encrypted with a password with
  a (eg BASE 64 algorithm.
• Volin decrypts the envelope with the password
  given by John. Decrypts the second one with
  Johns public key (assured it was sent from
  John). He than has the original msg and the msg
  digest. From the msg he recalculates the HASH and
  confronts in with the one sent by John.
• If the digests are equal than the msg was not
  changed and is original.

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• Goal To create a simple system that encrypts and
  decrypts a message
• Difficulties
• Assembling the different functions/algorithms is
  not easy.
• Key length has to be short (computational power
  required to generate/encrypt/decrypt)
• Message length will be short. (eg not a large
  size word document)
• Pluses
• Algorithms are public as well as theoretical
  pseudo codes