Pretty Good Privacy PGP

1
Pretty Good Privacy (PGP)

• Freeware for email by Phil Zimmermann since 1991
• He was the target of a three-year criminal
  investigation because of so-call violation of US
  export law.
• Although we honest people dont really think we
  need to encrypt our emailwere not hiding
  anything we should all start encrypting our
  email so that in case someone needs privacy, the
  poor soul wont arouse suspicion by being the
  only one encrypting email.
• if privacy is outlawed, only outlaws will have
  privacy

2
PGP overview

• Not just for email, it performs encryption and
  integrity protection on files
• Your email is treated as a file
• Encrypt the file
• Send the encrypted file by regular e-mailer.
• The receiver saves the email to a file and then,
  decrypt the file by PGP
• Directly embedded in email for convenience.

3
PGP overview mechanism

• Anybody creates his/her RSA public key and
  private key (512, 768, or 1024 bits)
  (automatically generated by PGP)
• Anybody (e.g., Alice) can send encrypted (as well
  as signed) email to anybody else (e.g., Bob).
• Generate a one-time random key to encrypt the
  email using a secret key system (e.g., IDEA)
• Encrypt the random key with Bobs public key
• May sign the email with her own private key
• May compress the email before encryption
• Bob can use his private key to decrypt the
  encrypted email.
• Moreover, pass phrase is required for
  decryption
• The pass phrase is typed by Bob when PGP
  generates RSA keys for him

4
PGP overviewkey distribution

• Public key system (RSA), key distribution
• PEM rigid hierarchy of CAs.
• S/MIME (being agnostic), assume that a number of
  parallel independent hierarchies.
• PGP anarchy, each user decides which keys to
  trust.
• You contact Alice in person to get Alices public
  key, and trust it
• You find the public key of Alice on her web page
  or from email, you can copy it to your PGP system
  to trust it if you want.
• Public key server (e.g., http//math-www.uni-pader
  born.de/pgp/).

5
PGP--certificates

• Certificates are an optional in PGP
• anyone can issue a certificate to anyone else
• If you trust Alice and get Carols public key
  certificate signed by Alice, you will trust
  Carols public key
• If you get Carols two public key certificates,
  one signed by Alice, and the other signed by Bob,
  both Alice and Bob are trusted by you, then you
  can trust both Carols certificates.
• Therefore PGP is very flexible and easy to use

6
PGP trust levels

• Problem with PGP anarchistic structure
• Alice was bribed to issue certificate for Carol
• Alice was sloppy about checking (key,identity)
  and sign the certificate.
• Suppose Ted is honest and never sloppy, could you
  trust Teds signature for Carols public key,
  from whom he had a bitter divorce?
• Solutions
• All are determined by yourself
• Give a trust level for a public key
• Given a trust level for the certificates signed
  by the key

7
Certificate and key revocation

• You can revoke (delete) any public key anytime
• A public key of a person can be revoked by the
  corresponding private key
• The issuer of a certificate can revoke the
  certificate
• Does not mean that the holder of revoked
  certificate is a bad person, but the issuers does
  not want to vouch for its authenticity.
• Validity period of a key and a certificate

8
PGPkey ring

• A data structure containing key materials
• pubring.pgp containing your public keys, other
  peoples public keys, information about people,
  and certificates.
• secring.pgp containing your private keys.
• Three trust levels currently in PGP none,
  partial, complete.
• A trust level of a person may determine the trust
  level of the certificates signed by the person.

9
PGP--fingerprint

• A short, fixed-seize string intended to be a
  unique representation of a string of arbitrary
  length and obtained by a one-way hash function.
• For PGP, the fingerprint is a 256 bit string for
  a public key (which may be 1024 bits) by MD5
• Purpose of a fingerprint for a public key is for
  people to easy to remember and easy to verify the
  public key