COMP 424 Lecture 04 Advanced Encryption Techniques (DES, AES, RSA)

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Presentation transcript:

COMP 424 Lecture 04 Advanced Encryption Techniques (DES, AES, RSA)

Secret Key Systems ● A message M, encrypted with key K is denoted as [M] K. ● Decryption is done with the same key and denoted: [[M] K ] K = M. ● The basic disadvantage to these systems is a problem of combinatorics.

Public Key Systems ● A message M, from A to B is encrypted with B's public key and decrypted with B's private key. – Anyone and everyone can know B's public key. ● The encrypted message is denoted: [M] KBpub. ● The decryption is denoted: [[M] KBpub ] KBpriv = M. ● What the advantages and disadvantages?

Can Provide Authentication ● Reverse the use of the keys (encrypt with private and decrypt with public) and you can now authenticate who sent the message. ● [[M] KBpriv ] KBpub = M – If B is the only person who holds the key Kbpriv then only B could have been the person to have encrypted M.

Increasing Confiedentiality ● A message from A to B could be encrypted as: [[M] KApriv ] KBpub = M ● Decryption: [[M] KBpriv ] KApub = M ● Where: [[[[M] KApriv ] KBpub ] KBpriv ] KApub = M ● Wouldn't such a system be neat... ● (Not all asymmetric key cryptosystems have the propery: [[M] KBpriv ] KBpub = M

Rivest-Shamir-Adelman (MIT 1977) ● Based on mathematical number theory. ● Relies on the underlying Number Theory of difficulty of determining multiplicative factors (prime numbers). ● Large numbers are extremely difficult to factor. ● Will this always be true? (probably not) – Moore's law (possibly) – Quantum / non-deterministic computing (probably)

RSA Basic property ● Given two (related) keys, e and d the basics of the RSA cryptosystem is: – P = E(e, D(d, P)) = D(d, E(e, P)) ● That is to say that if we encrypt with e we can decrypt with d and if we encrypt with d we can decrypt with e. ● Math: and ● Because the exponentiation of P is performed mod n it is difficult to factor to recover plaintext

RSA details – Select two large prime numbers p and q about 100 digits in length – Compute n = pq and psi=(p-1)(q-1) – Choose an integer E between 3 and psi which has no common factors with psi – Select an integer D, such that DE differs from 1 by a multiple of psi – Make E and n public but keep p, q, D and psi private.

Some Problems ● Compromised keys can be used for masquerading and disclosure attacks ● Key distribution can be complex ● Not scalable in large systems ● Many secret (symmetric) key systems are weak compared to PKI (Public Key Infrastructure) approaches.

DES Overview – Early 1970s (1972 died, 1974, 1976 adopted) – Targeted for public cryptography – Efficient and unbreakable – Easy to understand – Publishable – Availability – Economical – Exportable

DES (IBM's lucifer) ● Combination of – Substitution – Transposition ● 16 cycles of each ● Processes plain text in blocks of 64 bits (56data bits plus 8 checking bits) ● Uses only standard arithmetic and logic – (RSA uses 500 and 1000 bit arithmetic operations.)

Unbreakable?? ● Since 1976 (biven Moore's law) computers have increased in speed 2 18 fold. (262,144 times) ● DES 56-bit is no longer considered secure. ● Solutions? – Use keys longer than 56bits? – Well... no. The DES algorithm is fixed at 56 bits. The nature of the algorithm requires it.

Double DES ● Use two keys k1 and k2 and perform the encryption E(k 2,E(k 1,P)) ● Should be doubly strong just as two locks are harder to pick than one right? ● Wrong. Because of the nature of the DES algorithm the work required to crack the encryption increases from 2 56 to just ● Not worth the effort.

Triple DES (3DES) ● A simple trick solves the problem... ● C = E(k 1,D(k 2,E(k 1,P)))

Differential Cryptanalysis ● Biham and Shamir, 1990 ● A method that investigates the change in cryptographic strength when a change is made to the encryption algorithm – Almost any change made to the DES algorithm weakens it. – Is DES optimal then? Possibly but the point is moot...

Breaking DES ● 1997: 3,500 machines where used to infer a DES key. It required four months to do so. ● 1998: special hardware (DES cracker) was construced at a cost of $100,000 and could find a DES key in just four days. ● So its still rather secure because the resources to break it are still rather expensive. But it is certainly on the verge of being obsolete. ● 3DES is still pretty good bits effectively.

Future... ● AES: Rijndael