1. Cryptography

2. Insecure communications
Sender
Snooper
Recipient
Insecure channel
Confidential
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3. Encryption and Decryption
Ciphertext
Plaintext
Plaintext
Requirements:
Confidentiality
Authentication
Between communicating parties
Third-party authentication
Non-repudiation
Integrity verification
Key distribution
Secret key (secure distribution)
Public key (reliable distribution)
CSCE Farkas

4. How can cryptography support these objectives?
Security Objectives
Confidentiality
Integrity
Availability
Authenticity
Non-repudiation
How can cryptography support these objectives?
CSCE Farkas

5. Security Objectives Confidentiality: Hiding message/file content
Secret key, public key encryption
Integrity: Detecting modification
Hash function
Availability: Not much – hiding existence of data
Authenticity: Verify origin
Public key encryption
Non-repudiation: Verify activity
CSCE Farkas
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6. What makes a good cryptosystem?
A good cryptosystem is one whose security does not depend upon the secrecy of the algorithm.
From Bruce Schneier:
“Good cryptographers rely on peer review to separate the good algorithms from the bad.''
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7. Cryptanalysis Cryptanalyst’s goal: Break message Break key
Break algorithm
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8. Breakable versus Practically breakable
Unconditionally secure: impossible to decrypt. No amount of ciphertext will enable a cryptanalyst to obtain the plaintext
Computationally secure: an algorithm that is not breakable in practice based on worst case scenario
Breakable: all algorithms (except one-time pad) are theoretically breakable
CSCE Farkas

9. Secret Key Cryptosystem
Plaintext
Ciphertext
Plaintext
Encryption
Decryption
Sender
Recipient K
C=E(K,M)
M=D(K,C)
K needs
secure channel
CSCE Farkas

10. Secret Key Cryptosystem Vulnerabilities (1
Passive Attacker (Eavesdropper)
Obtain and/or guess key and cryptosystem use these to decrypt messages
Capture text in transit and try a ciphertext-only attack to obtain plaintext.
CSCE Farkas

11. Secret Key Cryptosystem Vulnerabilities
Active Attacker
Break communication channel (denial of service)
Obtain and/or guess key and cryptosystem and use these to send fake messages
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12. Inherent Weaknesses of Symmetric Cryptography
Key distribution must be done secretly (difficult when parties are geographically distant, or don't know each other)
Need a key for each pair of users
n users need n*(n-1)/2 keys
If the secret key (and cryptosystem) is compromised, the adversary will be able to decrypt all traffic and produce fake messages
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13. Compare DES and AES DES AES Date 1976 1999 Block size 64 bits 128 bits
Key length
56 bits
128, 192, 256 bits
Encryption
Substitution, permutation
Substitution, shift, mixing
Cryptography
Confusion, diffusion
Design
Open
Design rationale
Closed
Selection process
Secret
Secret with public comments
Source
IBM, enhanced by NSA
Independent Dutch cryptographers
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14. Public-Key Encryption
Needed for security:
One of the keys must be kept secret
Impossible (at least impractical) to decipher message if no other information is available
Knowledge of algorithm, one of the keys, and samples of ciphertext must be insufficient to determine the other key
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15. Confidentiality B A Insecure channel B’s private key B’s public key
Ciphertext
Plaintext
Plaintext
Encryption
Alg.
Decryption
Alg. B
Recipient A
Sender
B’s private key
B’s public key
(need reliable channel)
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16. Signature and EncryptionB A
Encrypted
Signed
Plaintext
Signed
Plaintext
Signed
Plaintext
Plaintext
Plaintext D E D E
B’s public key
A’s public key
B’s private key
A’s private key
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17. Non-repudiation Requires notarized signature, involving a third party
Large system: hierarchies of notarization
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18. RSA Med mod n = M mod n Both sender and receiver know n Sender knows e
Only receiver knows d
Modulus: Remainder after division, i.e., if a mod n=b then a=c*n+b
Need:
Find values e,d,n such that
Easy to calculate Me, Cd for all M < n
Infeasible to determine d give e
Med mod n = M mod n
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19. Next Class
Review for Final
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