Cryptographic Attacks

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Cryptographic Attacks

• ITN 220

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Definitions

• Cleartext or plaintext the original unencrypted
  text
• Ciphertext resulting encrypted output

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• Attacks on Cryptographic Algorithms

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Assumptions for these attacks

• The encryption algorithm is known
• The same key is always used (not necessarily the
  same key for both encryption decryption)
• The strength of the encryption is based on the
  secrecy of the key
• The encryption algorithm is properly implemented
  (otherwise these attacks can also be used to
  discover weaknesses)
• One or more of the above may or may not be true,
  which makes the attackers job harder (or easier)
  (Kryptos, WEP, etc.)

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Attacks on Cryptosystems

• Ciphertext-only attack
• Known-plaintext attack
• Chosen-plaintext attack
• Adaptive chosen-plaintext attack
• Chosen-ciphertext attack
• Adaptive chosen-ciphertext attack
• Related-key attack

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Ciphertext-only Attack

• Where the attacker only has access to the
  ciphertext
• Goal Determine the key
• Example Most brute force attacks, early (slow)
  attacks on WEP, Zip archives

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Known-plaintext Attack

• Where The attacker has access to server
  ciphertexts and their corresponding plaintexts
  (assuming that all ciphertexts were produced with
  the same key)
• Goal Try to decrypt one or more ciphertexts for
  which the attacker does not have the plaintexts
  (i.e., discover the key)
• Example Rainbow table attacks

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Chosen-plaintext Attack

• Where The attacker does not know the key but the
  attacker is able to submit one or more plaintexts
  to an entity (e.g., program) which performs the
  encryption for them. The attacker then analyzes
  the plaintext and corresponding ciphertext.
• Goal To try to decrypt one or more ciphertexts
  for which the attacker does not have the
  plaintexts (i.e., discover the key)

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Adaptive Chosen-plaintext Attack

• A subset of the chosen-plaintext attacks
• Where Similar to chosen-plaintext attack except
  the attacker submits plaintext and chooses
  subsequent plaintext to submit based on the
  resulting ciphertext.
• Goal Again, to try to decrypt one or more
  ciphertexts for which the attacker does not have
  plaintexts (i.e., discover the key)

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Chosen-ciphertext Attack

• The reverse of the chosen-plaintext attack
• Where The attacker is able to submit one or more
  ciphertexts to a decryption entity (e.g., a
  program) which performs the decryption for them.
  The attacker then analyzes the ciphertext and
  corresponding plaintext.
• Goal Again, to try to decrypt one or more
  ciphertexts for which the attacker does not have
  plaintext (i.e., discover the key)
• Example Very early versions of SSL were
  susceptible to this

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Adaptive Chosen-ciphertext Attack

• The reverse of the adaptive chosen-plaintext
  attack
• Where The attacker is able to submit ciphertext
  to a decryption entity and then analyzes the
  subsequent plaintext to determine the next set of
  ciphertexts to submit.
• Goal discovery the key
• Used to break SSL in 1998 (see Daniel
  Bleichenbacher)

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Related-key attack

• Where the attacker observes encryption involving
  several different keys whose values are initially
  unknown, but have some sort of mathematical
  relationship (a weakness) which is known to the
  attacker
• Goal
• Example Later attacks on WEP

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• Attacks on Signature Algorithms

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Attacks on Signature Algorithms

• Key-only attack
• Known-message attack
• Chosen-message attack
• Adaptive chosen-message attack

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Key-only Attack

• Where The attack only has access to the public
  key
• Goal To be able to produce a valid signature on
  any message (i.e., determine the private key)

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Known-Message Attack

• Where The attacker has access to several
  messages and their corresponding signatures for
  analysis
• Goal To be able to produce a valid signature on
  any message (i.e., determine the private key)

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Chosen-Message Attack

• Where The attacker is able to submit messages to
  an entity (e.g., program) for signing (where it's
  assumed that the entity always uses the same key)
• Goal To be able to produce a valid signature on
  any message (i.e., determine the private key)

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Adaptive Chosen-Message Attack

• Where The attacker is able to submit messages to
  a signing entity and then analyzes the signatures
  to determine the next set of messages to submit
• Goal To be able to produce a valid signature on
  any message (i.e., determine the private key)

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Remember

• Encryption is only a delaying tactic. The trick
  is to make sure the delay amounts to not in this
  lifetime.
• Given enough time and resources, any encryption
  can be broken
• Time and resources are inversely relative to each
  other (i.e., more resources less time)

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Additional Reading

• Search on
• Bletchley Park
• Birthday attack
• Donald Davies
• John W. Herivel
• Freidrich Kasiski
• William F. Friedman
• Lambros Demetrious Callimahos