Pretty Good Privacy

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Pretty Good Privacy
by Philip Zimmerman
presented by Chris Ward
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What is PGP?

• a non-proprietary protocol for the secure
  exchange of information between individuals
• a free, open-source program implementing this
  protocol

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Why do you need PGP?

• privacy for protecting personal information
• secrecy for hiding illegal, unethical, or
  immoral activity
• you dont need it you have nothing to protect or
  hide

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Why should you know about PGP?

• It uses tools that are essential to all forms of
  secure communication.
• public-key cryptography
• private-key cryptography
• message digests
• digital signatures

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Why should you know about PGP?

• It uses tools to provide all aspects of secure
  communication.
• privacy (private-key cryptography)
• integrity (message digests)
• authentication (digital signature)
• non-repudiation (digital signature message
  digest)

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In this presentation I will

• introduce the tools used by PGP
• describe how each tool can be used to provide
  some aspect(s) of secure communication
• show how PGP combines all of these tools to
  facilitate the secure exchange of information
  between individuals

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Tool 1 Private-Key Cryptography

• two parties share a single secret key
• an encryption scheme encrypts/decrypts
  information according to the value of the key
• good encryption algorithm most efficient attack
  is not significantly better than brute force
  (guessing each possible key)
• Examples DES, Triple DES, AES

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Privacy with Private-Key Cryptography

• Alice and Bob share a secret key Ks
• Alice composes a message m in plaintext
• She encrypts it with the secret key Ksm
• She sends Ksm to Bob
• How did Alice and Bob agree on Ks ?

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Tool 2 Public-Key Cryptography

• Each party maintains a key pair (Kpublic,
  Kprivate).
• If a message is encrypted with Kpublic, it can
  only be decrypted with Kprivate.
• If a message is encrypted with Kprivate, it can
  only be decrypted with Kpublic.
• good encryption algorithm private key cannot be
  feasibly computed from its corresponding public
  key.
• Examples RSA, ElGamal

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Convenient Key Agreement with Public-Key
Cryptography

• provides one answer to How did Alice and Bob
  agree on Ks ?
• Alice generates a random key and encrypts it
  using Bobs public key KBOB-PUBLICKs
• Only Bob has KBOB-PRIVATE, so only Bob can
  decrypt the secret key.
• Alice and Bob no share a secret. Or do they?
• How does Bob know that the message really came
  from Alice?.

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Authentication and Integrity with Public-Key
Cryptography

• Before, Alice sent KBOB-PUBLICKs
• She could send KBOB-PUBLICKs, KALICE-PRIVATEKs
  
• As before, only Bob can decrypt the information
• Now, Bob can use KALICE-PUBLIC to decrypt
  KALICE-PRIVATEKs
• If the result is Ks then Bob knows that the key
  was sent by Alice and that the key has not been
  altered in transmission.
• This is just an example to illustrate a digital
  signature.
• Is there a more efficient way to prove message
  integrity than by copying the whole thing?

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Tool 3 Message Digests

• For a message m, apply a hash function H which
  yields a fixed-length message digest H(m) (also
  called a digital fingerprint).
• good hash function easy to compute and
  infeasible to compute m given H(m)
• Examples SHA-1, MD5

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Tool 4 Digital Signature (using message digest)

• Alice composes a message m to Bob.
• She computes H(m) and encrypts it with her
  private key.
• She sends (m, KALICE-PRIVATE H(m)) to Bob.
• Bob, or anyone, computes H(m), and compares it to
  the result of decrypting KALICE-PRIVATE H(m)
  with Alices public key.
• If they are the same, authentication and
  integrity have been proven.

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Sending secure information with PGP

• Alice composes message m for Bob
• generates a random session key Ks
• encrypts m Ksm ( AESENCRYPT(Ks, m))
• encrypts Ks with Bobs public key
  KBOB-PUBLICKs
• computes the message digest H(m)
• encrypts H(m) with her private key
  KALICE-PRIVATEH(m)
• sends this (Ksm, KBOB-PUBLICKs,
  KALICE-PRIVATEH(m))

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Receiving Secure Information with PGP

• Bob receives this (Ksm, KBOB-PUBLICKs,
  KALICE-PRIVATEH(m))
• decrypts KBOB-PUBLICKs using his private key,
  yielding Ks
• decrypts Ksm using Ks, yielding m
• computes H(m) using message digest algorithm H
• decrypts KALICE-PRIVATEH(m) using KALICE-PUBLIC
  yielding H(m)
• compares H(m) to H(m)

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Summary

• PGP combines the use of several tools
• public-key cryptography, private-key
  cryptography,
• message digests
• to provide all of the elements necessary for
  secure communication
• privacy, integrity, authentication,
  non-repudiation

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Other Stuff

• Can you trust that the public key you are using
  really belongs to the intended recipient?
• Fake public keys
• Authentication by trusted third party