Cryptography Tool

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Cryptography Tool

• PGP

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Introduction

• Why PGP?
• History of PGP
• First version released by Philip Zimmermann in
  1991
• Politics issue (Senate Bill 266)
• Anti-crime bill enable government to read
  encrypted messages.
• Philip under criminal investigation
• Violation of export restrictions

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Introduction Cont.

• Patent issue with PGP
• Old version with RSA and IDEA(expires 2007)
• Another patent free, but incompatible version
  uses Triple-DES, with DSS/Diffie-Hellman.
• A typical commercial PGP, such as ViaCrypt PGP
  offers many features
• Encrypt, sign, encrypt and sign, verify
  signature, key management, and so on.

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How PGP works

• Compress data
• Create a session key randomly
• Encrypt message using a block cipher algorithm
• Use public key cryptography encrypt the session
  key
• Transmit the ciphertext and encrypted session key.

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PGP Encryption
source An Introduction to Cryptography
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How PGP Works Cont.

• Decryption works in the reverse order.
• If a PGP server receives a message, it decrypts
  the encrypted session key using your private key.
• Use this decrypted session key to decrypt the
  ciphertext.
• Then it decompresses the deciphered text to
  produce the original plaintext.

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PGP Decryption
source An Introduction to Cryptography
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PGP Keys

• The larger the key, the more secure the
  encryption
• You decide the size of the key
• Public keyring and private keyring
• Keyrings are files with a specific data structure
• Passphrase used to generate your private key
• Question How to choose a passphrase?
• Answer easy to remember, difficult to guess. (no
  famous quotes)

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Message Digest

• Fingerprint of your message or file
• MD5 in PGP
• 128-bit MD5 has serious weakness
• In 1996, Hans Dobbertin, a German cryptographer,
  partially broke MD5
• SHA-1 in PGP
• 160-bit SHA-1 developed by NSA
• Extremely well designed
• For compatibility reason, MD5 still used for RSA
  signatures

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Certificates

• Make sure the public key belongs to the person
  associated with that signature
• PGP certificates consist of
• Public key
• User id or name
• Email address or ICQ.
• One or more signatures
• validity period
• Preferred algorithm(CAST, AES, IDEA, Triple-DES,
  and Twofish )
• Pitfall Not attest the authenticity as a whole,
  only vouches the public key is bound to the
  appeared identity on the certificate.

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Certificate Revocation

• Validity period
• If a certificate expired, it becomes invalid
• Why a certificate need to be revoked?
• Private key is comprised
• Does not mean anything bad about the public key
  owner, only saying the users can no longer
  authenticate you by your public key.
• Communicating that a certificate has been revoked
• Post your revoked certificate on a server
• In PKI environment, Certificate Revocation List

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Protect Public key

• Problem
• If Trudy substituted his public key for Alices
  public key, she would be able to decrypt the
  messages that sent to Alice and encrypt it using
  Alices public key and send it to Alice again. No
  one suspects anything wrong.
• Forge the signature using her private key.

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Protect Public Key Cont.

• Solution
• Get Alices key in person
• Have your friend David signs Alices public key
  if he knows Alices public key and can be
  trusted.
• Any certificates from Certificate Authority can
  be trusted
• Store your own public keyring on your PC

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Validating Keys

• Check trust level, computer the result.
• Example two marginally trusted signatures is as
  credible as one fully trusted signature
• PGP trust level complete trust, marginal trust,
  no trust
• PGP let you decide who you can trust

• standard public key management schemes
• Internet Privacy Enhance Mail (PEM)
• Requires mandatory trust
• Relay on CA tells you who you can trust

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Vulnerabilities

• Passphrase attack
• Easy to remember, difficult to guess
• Public key tampering
• Make sure the public key is directly from that
  person or signed by a trusted third party.
• Access control of your own public and private
  keyrings
• Make a copy of both keyrings

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Vulnerabilities Cont.

• Not quite deleted files
• Marked that deleted location for reusability
• Overwrite that marked location on the disk
• PGPs Secure Wipe and Freespace features to clear
  any fragments left by your word processor
• Viruses or Trojan horses
• Attacks on passphrass, private key, and
  deciphered text. PGP offers no solution.
  Helpfully, you can be notified as soon as
  possible.
• Imitation of PGP has malfunctions not check
  signature.
• Solution directly download PGP from PGP
  Corporation

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Vulnerabilities Cont.

• Virtual memory or swap files
• Fairly low risk because PGP does not keep
  sensitive data long in memory
• Write sensitive data to the disk using LRU memory
  replacement algorithm. (data passphrase, private
  key, or deciphered text )
• Solution overwrite your swap files or simply
  turn off your virtual memory feature.
• Physical security breach
• Burglary, trash-picking, bribery, or blackmail

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Vulnerabilities Cont.

• Tempest attacks
• detect the electromagnetic signals emitted from
  video screen
• Secure Viewer feature (PGP 6.0 or later)
• using a special font that may reduce the levels
  of radio frequency emissions
• LCD has no such emissions
• Bogus timestamp
• Create a signature certificate of a signature
  certificate with a trustworthy timestamp.
• Multi-user system
• PGP is designed for a single user system
• Be aware of the risk on a multi-user system

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Vulnerabilities Cont.

• Traffic analysis
• Observing the size of the message, source and
  destination, the time it is sent.
• PGP alone has no solution for this problem
• Requires a special communication protocol that
  can reduce the traffic information
• Cryptanalysis
• PGP uses the best block cipher algorithms in the
  world
• No successfully attacks on public key
  cryptography since 1978

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References

• www.pgp.com
• http//web.mit.edu/network/pgp.html
• http//www.mit.edu/prz/EN/background/index.html

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The End

• PGP gives you Pretty Good Privacy