Re: Cracking decrypted file when knowing partial contents

From: Bill Unruh (unruh_at_string.physics.ubc.ca)
Date: 04/21/04 

Date: Tue, 20 Apr 2004 22:09:20 +0000 (UTC)

"Douglas A. Gwyn" <DAGwyn@null.net> writes:

]Bill Unruh wrote:
]> It depends on the encryption used. Most modern good encryption is
]> designed to resist known plaintext attacks-- ie even if you know the
]> plaintext and how it encrypts, you still cannot determine the key except
]> by exhaustive search of the key space.
]> Known plaintext attacks are so well known (That was how individual
]> enigma messages were cracked) that any competent cryptographer spends
]> most of his time designing the encryption scheme to be resistant,
]> although even compentent ones can screw up.

]That's misleading. "Known plaintext" does not describe a
]*method* of attack, but a general scenario that is assumed
]to exist, which potentially provides more information that
]*might* be useful in mounting an actual attack than for
]some other scenarios (such as ciphertext-only). In fact

??
]most encryption systems cannot correctly guarantee that
]knowing the plaintext does not facilitate some method of
]attack against them. At best, they are designed to not

No encryption can guarentee anything. They are however designed to be
resistant to known plaintext attacks (ie attacks which make use of
knowing both the plaintext and the encrypted text). Any crypto system
which is weak if plaintext is known, is weak period.

]run afoul of a handful of *known* methods of attack that
]may require a known-plaintext scenario. But it is the
]*unknown* methods that really need to be guarded against,
]and most systems don't take sufficient precautions in that
]regard.

Most of life is like that. The unknown is hard to guard against.
Do you have a way of defending against the unkown? How would you advise
systems to take sufficient precautions against the unknown?

Relevant Pages

  • Re: Encrypted Communications and Predictable Communications?
    ... If the attack you describe (the known plaintext, ... | on one machine and a database server on another. ... | Should the encryption system take steps to ensure that the encrypted data ...
    (SecProg)
  • Re: Cracking decrypted file when knowing partial contents
    ... ]> It depends on the encryption used. ... ]> plaintext and how it encrypts, you still cannot determine the key except ... ]> most of his time designing the encryption scheme to be resistant, ... ]*method* of attack, but a general scenario that is assumed ...
    (sci.crypt)
  • Re: Bad news for Block Ciphers?
    ... >Any ciphertext only attack should use some knowledge about plaintexts ... If you cannot distinguish possible plaintext from random ... No in most modern encryption sytems that use compression and padding ...
    (sci.crypt)
  • Re: Cracking decrypted file when knowing partial contents
    ... > It depends on the encryption used. ... > plaintext and how it encrypts, you still cannot determine the key except ... *method* of attack, but a general scenario that is assumed ... may require a known-plaintext scenario. ...
    (comp.security.misc)
  • Re: Cracking decrypted file when knowing partial contents
    ... > It depends on the encryption used. ... > plaintext and how it encrypts, you still cannot determine the key except ... *method* of attack, but a general scenario that is assumed ... may require a known-plaintext scenario. ...
    (sci.crypt)