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1
Cryptography
CS177
2011
Cryptography
The science and study of secret writings
Cipher
– Is a secret method of writing that transforms
plaintext
into
ciphertext
The transformation is determined by a
key
Cryptographic systems
–
One key
–
Two key
–
Public key
–
Digital signatures
2
Cryptography
CS177
2011
Cryptography
• Comes in two flavors: Symmetric and
Asymmetric
• Best for protection of “online”
communications
• Good for archival data
• Soso for electronic mail
• Not good for active databases
3
Cryptography
CS177
2011
Communication Security
Secure communication should provide:
• Privacy
• Authentication
• Integrity
• Nonrepudiation
Anna
Bruno
Carlo
4
Cryptography
CS177
2011
Terminology
• To lock (encipher): transforms into
unintelligible form based on independent
data element called a
key
• To unlock (decipher): transforms back into
intelligible form, again using a key
• Locked data is called
ciphertext
or
black
• Unlocked data is called
plaintext
,
cleartext
,
or
red
• Keys are themselves data and can be locked
and unlocked
5
Cryptography
CS177
2011
Cryptography
Enciphering
Deciphering
Clear
Text
Cipher
Text
6
Cryptography
CS177
2011
General Observations
• Cryptography never solves a problem; it
transforms a security problem into a key
management problem
• It takes a secret to keep a secret
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7
Cryptography
CS177
2011
Cryptographic System
(Cryptosystem)
• A plaintext message space M
• A ciphertext message space C
• A key space K
• A family of enciphering transformations
Ek: M
→
C
• A family of deciphering transformations
Dk: C
→
M
8
Cryptography
CS177
2011
Crypto Systems Should
Guarantee Both
– Secrecy
– Authenticity
Secrecy requirements
1.
Should be computationally infeasible to systematically
determine Dk from c, even if corresponding m is known
2.
Should be computationally infeasible to determine m from
intercepted c
9
Cryptography
CS177
2011
Crypto Systems Should
Guarantee Both
– Secrecy
– Authenticity
Authenticity requirements
1.
Should be computationally infeasible to systematically
determine Ek from c, even if corresponding m is known
2.
Should be computationally infeasible to find c' such that
Dk(c') is valid plaintext in the set M
10
Cryptography
CS177
2011
Desirable Properties of
Crypto Systems
• Enciphering and deciphering must be
efficient for all keys
• System must be easy to use
• The security of the system should depend on
the secrecy of the keys and not on the
secrecy of the algorithms E or D
11
Cryptography
CS177
2011
Cryptanalysis
• Cryptanalysis attempts to discover the key or the
plaintext of an encrypted message
– Assume analyst knows the algorithm but not the key
• Types of attack:
– Ciphertext only
• Given: C1 = Ek(M1), C2 = Ek(M2), .
.., Ci = Ek(Mi)
• Obtain: either M1,M2, .
.., Mi
or k
– Known plaintext
• Given: M1, C1 = Ek(M1), M2, C2 = Ek(M2), .
.., Mi,
Ci = Ek(Mi)
• Obtain: either k or an algorithm to obtain Mi+1, from
Ci+1 = Ek(Mi+1)
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 Fall '09
 Kemm

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