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RSA Encryption. William Lu. RSA Background. Basic technique first discovered in 1973 by Clifford Cocks of CESG (part of British GCHQ) Invented in 1977 by Ron Rivest, Adi Shamir and Len Adleman. RSA Uses. Public key encryption Digital signatures. RSA Algorithm.

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rsa encryption

RSA Encryption

William Lu

rsa background
RSA Background
  • Basic technique first discovered in 1973 by Clifford Cocks of CESG (part of British GCHQ)
  • Invented in 1977 by Ron Rivest, Adi Shamir and Len Adleman
rsa uses
RSA Uses
  • Public key encryption
  • Digital signatures
rsa algorithm
RSA Algorithm
  • Generate two large random primes, p and q, of approximately the same size
    • e.g. for 1024 bit encryption, p and q should be about 512 bits each
  • Compute n = pq and φ = (p-1)(q-1)
  • Choose e where 1
  • Compute d where 1
rsa algorithm1
RSA Algorithm
  • Public key = (e,n)
  • Private key = (d,n)
generate primes
Generate Primes
  • Get a pseudo random number
  • Use Fermat’s Little Theorem to test for prime
    • For prime n and any a, an mod n = a
    • For composite n and any a, an mod n ≠ a
  • BUT
  • If an mod n = a, n could be a composite
generate primes1
Generate Primes
  • Does Fermat’s Little Theorem guarantee primes?
    • NO!
  • What is it for?
    • With enough rounds, n is probably prime
    • Much faster then testing with all primes < n
generate exponents e and d
Generate Exponents e and d
  • For public exponent, e, pick any prime
    • Common choices are 3, 17 and 65537 (216 + 1)
  • For secret exponent, d, compute the modular inverse of e mod φ
    • Use Extended Euclidean Algorithm
extended euclidean algorithm
Extended Euclidean Algorithm
  • To find inverse of e mod n:
    • Find quotient and remainder of n/e at each step
    • Also carry an auxiliary number ui = ui-2 – ui-1qi-2 mod n
    • Initialize u0 = 0 and u1 = 1
    • For each step use the previous e as the current n and the previous remainder as the current e
    • Repeat until e = 0 and the auxiliary number is the inverse of e mod n
extended euclidean algorithm1
Extended Euclidean Algorithm

Inverse of 5 mod 72

encryption decryption
Encryption/Decryption
  • To encrypt message m
    • Public key = (e,n)
    • c = me mod n
  • To decrypt cipher c
    • Private key = (d,n)
    • m = cd mod n
encryption decryption1
Encryption/Decryption
  • Public key = (5,91)
  • Private key = (29,91)
  • To encrypt message 17
    • c = 175 mod 91
    • c = 75
  • To decrypt cipher 75
    • m = 7529 mod 91
    • m = 17
signature
Signature
  • To sign message m
    • Private key = (d,n)
    • [m] = md mod n
  • To verify signature
    • Public key = (e,n)
    • {m} = me mod n
references
References
  • RSA Algorithm
    • DI Management Services
  • Fermat’s Little Theorem
    • Mathworld
  • Extended Euclidean Algorithm
    • Wolfgang Stöcher at Profactor Research
    • Bill Cherowitzo’s references at the University of Colorado at Denver
      • Ph. D (1983) in mathematics at Columbia University
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