Pretty Good Privacy (PGP)

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Pretty Good Privacy (PGP)
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How PGP works

• PGP uses both public-key cryptography and
  symmetric key cryptography, and includes a system
  which binds the public keys to user identities.
• The first version of this system is generally
  known as a web of trust as opposed to developed
  later X.509 system with top-down approach based
  on certificate authority. Later versions of PGP
  have included something more akin to a public key
  infrastructure (PKI) that includes certificate
  authority.

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PGP Contd.

• PGP uses asymmetric key encryption algorithms. In
  these, the recipient must have previously
  generated a linked key pair, a public key, and a
  private key. The sender uses the recipient's
  public key to encrypt a shared key (aka a secret
  key or conventional key) for a symmetric cipher
  algorithm. That key is used, finally, to encrypt
  the plaintext of a message. Many PGP users'
  public keys are available to all from the many
  PGP key servers around the world which act as
  mirror sites for each other.

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PGP Cont.

• The recipient of a PGP-protected message decrypts
  it using the session key for a symmetric
  algorithm. That session key was, of course,
  included in the message in encrypted form and was
  itself decrypted using the recipient's private
  key. Use of two ciphers in this way is sensible
  because of the very considerable difference in
  operating speed between asymmetric key and
  symmetric key ciphers (the differences are often
  1000 times).

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PGP Cont.

• A similar strategy is (by default) used to detect
  whether a message has been altered since it was
  completed, or (also by default) whether it was
  actually sent by the person/entity claimed to be
  the sender.

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PGP Cont.

• To do both at once, the sender uses PGP to 'sign'
  the message with either the RSA or DSA signature
  algorithms.
• To do so, PGP computes a hash (also called a
  message digest) from the plaintext, and then
  creates the digital signature from that hash
  using the sender's private key.
• The message recipient computes a message digest
  over the recovered plaintext, and then uses the
  sender's public key and the signed message digest
  value with the signature algorithm.
• If the signature matches the received plaintext's
  message digest, it must be presumed (to a very
  high degree of confidence) that the message
  received has not been tampered with, either
  deliberately or accidentally, since it was
  properly signed.
• http//en.wikipedia.org/wiki/OpenPGP

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PGP Cont.

• It is critical that the public key one uses to
  send messages to some person or entity actually
  does 'belong' to the intended recipient.
• Users must also verify by some means that the
  public key in a certificate actually does belong
  to the person/entity claiming it. From its first
  release, PGP has used a web of trust. A given
  public key (or more specifically, information
  binding a person to a key) may be digitally
  signed by a third party to attest the association
  between the person and the key. There are several
  levels of confidence that can be expressed in
  this signature.

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PGP Cont.

• In the (more recent) OpenPGP specification, trust
  signatures can be used to support creation of
  certificate authorities. A trust signature
  indicates both that the key belongs to its
  claimed owner and that the owner of the key is
  trustworthy to sign other keys at one level below
  their own.
• A level 0 signature is comparable to a web of
  trust signature, since only the validity of the
  key is certified.
• A level 1 signature is similar to the trust one
  has in a certificate authority because a key
  signed to level 1 is able to issue an unlimited
  number of level 0 signatures.
• A level 2 signature is highly analogous to the
  trust assumption users must rely on whenever they
  use the default certificate authority list in
  Internet Explorer it allows the owner of the key
  to make other keys certificate authorities.

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PGP Cont.

• PGP has also included a way to 'revoke' identity
  certificates which may have become invalid. More
  recent PGP versions have also supported
  certificate expiration dates.
• The problem of correctly identifying a public key
  as belonging to some other user is not unique to
  PGP. All public key and private key cryptosystems
  have the same problem.

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Downloading PGP etc.

• PGP 6.5.8 Tutorial
• http//www.pitt.edu/poole/PGP.htmwaiting
• E-mail privacy and PGP Tutorial
• http//www.emailprivacy.info/privacy_pgp