Cryptography and Network Security Cryptography and Network Security | CS 549, Study notes of Cryptography and System Security

Download Cryptography and Network Security Cryptography and Network Security | CS 549 and more Study notes Cryptography and System Security in PDF only on Docsity! CS595-Cryptography and Network Security Cryptography and Network Security Key Management Xiang-Yang Li CS595-Cryptography and Network Security Key Exchange q Public key systems are much slower than private key system Ø Public key system is then often for short data § Signature, key distribution q Key distribution Ø One party chooses the key and transmits it to other user q Key agreement Ø Protocol such two parties jointly establish secret key over public communication channel Ø Key is the function of inputs of two users CS595-Cryptography and Network Security Blom Scheme q Scheme (when k=1) Ø Each user u has distinct element ru from Zp Ø TA choose a,b,c and defines § f(x,y)=a+b(x+y)+cxy mod p Ø For each u, TA computes § gu(x)=f(x, ru) mod p Ø TA transmits gu(x) to user u Ø Two users u and v compute the common key § f(ru, rv)= a+b(ru + rv)+c ru rv mod p § Here f(ru, rv)= gv(ru)= gu(rv) CS595-Cryptography and Network Security Security of Blom Scheme qLess than k users can not determine keys qHowever, more than k users can compute any keys Ø Solving equations to get a,b,c for k=1 qGenerally Ø Function f(x,y)=ai,jxiyi mod p ØHere ai,j=aj,i CS595-Cryptography and Network Security Diffie-Hellman Key Predist. q Computationally secure Ø if discrete logarithm is intractable q Scheme Ø Assume prime number p public and an integer c public Ø Each user u has secret component au Ø User u computes bu=c au mod p Ø TA certifies it by computing § (ID(u), bu, sigTA(ID(u), bu)) Ø The common key of two users u and v is § K=c au av mod p CS595-Cryptography and Network Security Authenticated Key Agreement CS595-Cryptography and Network Security q Public-key distribution of secret keys qDiffie-Hellman key exchange