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Pretty Good Privacy

by Philip Zimmerman. Pretty Good Privacy. presented by: Chris Ward. What is PGP?. a non-proprietary protocol for the secure exchange of information between individuals a free, open-source program implementing this protocol. Why do you need PGP?. privacy: for protecting personal information

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Pretty Good Privacy

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  1. by Philip Zimmerman Pretty Good Privacy presented by: Chris Ward

  2. What is PGP? • a non-proprietary protocol for the secure exchange of information between individuals • a free, open-source program implementing this protocol

  3. Why do you need PGP? • privacy: for protecting personal information • secrecy: for hiding illegal, unethical, or immoral activity • you don’t need it: you have nothing to protect or hide

  4. 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

  5. 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)

  6. 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

  7. 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

  8. 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: Ks{m} • She sends Ks{m} to Bob • How did Alice and Bob agree on Ks ?

  9. 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

  10. 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 Bob’s public key: KBOB-PUBLIC{Ks} • 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?.”

  11. Authentication and Integrity with Public-Key Cryptography • Before, Alice sent KBOB-PUBLIC{Ks} • She could send KBOB-PUBLIC{Ks, KALICE-PRIVATE{Ks}} • As before, only Bob can decrypt the information • Now, Bob can use KALICE-PUBLICto decrypt KALICE-PRIVATE{Ks} • 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?”

  12. 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

  13. 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 Alice’s public key. • If they are the same, authentication and integrity have been proven.

  14. Sending secure information with PGP • Alice: composes message m for Bob • generates a random session key Ks • encrypts m: Ks{m} ( = AESENCRYPT(Ks,m)) • encrypts Kswith Bob’s public key: KBOB-PUBLIC{Ks} • computes the message digest H(m) • encrypts H(m) with her private key: KALICE-PRIVATE{H(m)} • sends this: (Ks{m}, KBOB-PUBLIC{Ks}, KALICE-PRIVATE{H(m)})

  15. Receiving Secure Information with PGP • Bob receives this: (Ks{m}, KBOB-PUBLIC{Ks}, KALICE-PRIVATE{H(m)}) • decrypts KBOB-PUBLIC{Ks}using his private key, yielding Ks • decrypts Ks{m} using Ks, yielding m • computes H(m)’ using message digest algorithm H • decrypts KALICE-PRIVATE{H(m)} using KALICE-PUBLIC yielding H(m) • compares H(m)’ to H(m)

  16. 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

  17. 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

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