PGP - PowerPoint PPT Presentation


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Then purchased by McAfee Software (maker of anti-virus software) ... As a matter of policy, PGP makes their source code available for download at ... – PowerPoint PPT presentation

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Title: PGP

  • An example of Public Key Encryption software

  • Was a personal project of Phil Zimmerman
  • Then purchased by McAfee Software (maker of
    anti-virus software)
  • The repurchased with a leveraged buyout
  • It is now private

  • As a matter of policy, PGP makes their source
    code available for download at
  • http//
    The software supplied here is the source code
  • the PGP Desktop product and
  • includes the sources for the PGP Software
    Development Kit (SDK)

PGP (Pretty Good Privacy)
  • What is PGP?
  • Pretty Good Privacy (PGP) is strong encryption
    software that enables you to protect your email
    and files by scrambling them so others cannot
    read them.
  • It also allows you to digitally "sign" your
    messages in a way that allows others to verify
    that a message was actually sent by you. PGP is
    available in freeware and commercial versions all
    over the world.
  • PGP was first released in 1991 as a DOS program
    that earned a reputation for being difficult.
  • Current version is 9.5 and is idiot proof
  • Available at http//

How Does PGP Work?
  • When you install PGP, you will generate a pair of
    keys for yourself a "paublic key" and a "private
  • The private key is like a regular key. You will
    use it to unlock your messages.
  • The public key is like a set of keyed-alike
  • You publish your public key (your lock) by
    sending it to a PGP key server on the Internet
    (PGP does this automatically)
  • People who wish to send you private email use a
    copy of your lock to lock the message.
  • You keep the (private) key to yourself, so that
    only you can open and read the messages.

Applying the Keys
Digital Signatures
  • PGP also allows you to sign a message or a file,
    with or without locking (encrypting) it.
  • Each digital signature is uniquely generated by
    PGP based on the contents of the message and the
    signer's private key.
  • The signature can be checked by anyone using the
    signer's public key.
  • Since the signature is based partly on the
    contents of the message, if even one character of
    the message is changed, PGP will report that the
    signature is invalid.
  • The signature is also based on the signer's
    private key, and the private key is held only by
    the signer, so recipients can be sure of exactly
    who signed the message.
  • while handwritten signatures are supposedly
    unique per signer, digital signatures are unique
    per document and signer.
  • Written signatures can be photocopied from
    document to document and still appear valid.
  • Digital signatures fail verification when applied
    to another document.

PGP Flowchart
Various flavors of PGP
  • Determined by the particular communication path
  • PGP Email Email sent from client to client
  • PGP Disk Encryption Screen to Disk
  • PGP Shredder File to Delete
  • PGP Zip Readable to compressed files
  • Etc.

Key Rings
  • Just as you would carry a set of keys to various
    assets (safe, car, house, etc.)
  • You carry keys to various data assets on your PGP
    Key ring
  • PGP manages pairs, so that the private is kept
    secret, and the public is widely circulated

Key Generation and Passphrase
  • You enter a long Passphrase that you can remember
  • Which creates your unique keys
  • The longer the more secure
  • You passphrase is required for all public key
    server updates
  • This way your public key is always under your
  • Since people can find your private key on your
    key ring,
  • but they can never see your passphrase

Key Services
  • Fingerprint
  • When you get a public key from another source,
    you compare it to the fingerprint that they send
    you in the message
  • The fingerprint may either be a Hexadecimal
    sequence, or a biometric sequence of
    recognizable words

Key Services
  • Subkeys
  • You can have systems in PGP Keys
  • Similar to Master and individual keys for manual
    locks in an organization
  • Normally master keys are for signatures
  • And subkeys are for encryption
  • Legally binding documents will demand this
    arrangement in some regions
  • You also have an organization-wide Additional
    Decryption Key (ADK)
  • Where the security administrator gives you a
    particular additional set of keys (like classes
    of master keys in the real world)

Key Services
  • Revocation
  • You can grant various other people the right to
    revoke your key
  • E.g., your employers PGP administrator
  • This is used in conjunction with the larger Key
    and Security administration policy

Who can revoke a key?
  • Obviously, a malicious (or erroneously)
    revocation of some (or all!) of the keys in the
    system will most likely be a system-wide failure
  • It is impossible to arrange things so that this
    can not happen (if keys can be revoked at all)
  • Because the principal having authority to revoke
    keys is very powerful,
  • the mechanisms used to control it should involve
    as many participants as possible to guard against
    malicious attacks,
  • while at the same time as few as possible to
    ensure that a key can be revoked without delay

How to distribute a new key
  • After a key has been revoked, a new key must be
    distributed in some pre-determined manner.
  • Assume that Carol's key has been revoked.
  • Until a new key has been disseminated, Carol is
    effectively silenced.
  • No one will be able to send her data without
    violating system security, and data coming from
    her will be discarded for the same reason.
  • Or, in other words, the part of the system
    controlled by Carol is disconnected and so
  • The need for security was deemed higher than the
    need for availability in this design.
  • One could lump together the authority to create
    new keys (and certify them) with the authority to
    revoke keys,
  • but there is no need to do so.
  • In fact, for reasons of security, this likely a
    bad idea.

How to spread the revocation
  • The notification that a key has been revoked and
    should not be used again must be spread to all
    those that potentially hold the key, and as
    rapidly as possible.
  • There are two means of spreading information
    (e.g., a key revocation here) in a distributed
  • either the information is pushed to users from a
    central point(s),
  • or it is pulled from a central point(s) to end
  • Pushing the information is the simplest solution
    in that a message is sent to all participants.
    However, there is no way of knowing that all
    participants actually receive the message, and,
    pushing is not very securable nor very reliable.
  • The alternative to pushing is pulling. In this
    setup, all keys are included within a certificate
    that requires the one using them to verify that
    the key is valid.

Recovery from a leaked key
  • If loss of secrecy and/or authenticity is a
    system-wide failure, a strategy for recovery must
    be in place.
  • This strategy will determine who has authority to
    revoke the key,
  • how to spread the revocation,
  • also how to deal with all messages encrypted with
    the key since the leak is recognized
  • This recovery procedure can be extremely
    complicated, and while it is in progress the
    system might be very vulnerable to Denial of
    Service attacks