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"Times" 1 20 0 0 0 1 2 2 2 2 2 2 0 0 0 1 }} {SECT 0 {EXCHG {PARA 0 "" 0 "" {TEXT 211 22 "(p-1)-factoring method" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 145 "This method is designed to fa ctor composite numbers n such that for at least one of the prime facto rs p of n, (p-1) is a product of small primes." }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 178 "To illustrate the attck, let's first construct a pr ime p, such that p-1 is a product of only small primes. We start from \+ some even big number p0 that is a product of small primes" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 14 "p0:=2^5*13^50;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"enof\\B7:W\"Q[7'yvq84Q&[l\\m?LN^tLf\"" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 36 "p0+1 need not be a prime, of course:" }}} {EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 14 "isprime(p0+1);" }}{PARA 11 " " 1 "" {XPPMATH 20 "I&falseG%*protectedG" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 181 "We are looking for a prime number of the form k*p0+1 fo r small values of k. The distribution of prime numbers suggests that s uch k is usually not hard to find simply by brute force:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 23 "for k from 1 to 200 do\n" }{MPLTEXT 1 0 27 " if isprime(k*p0+1) then \n" }{MPLTEXT 1 0 16 " P:=k*p0+1 ; \n" }{MPLTEXT 1 0 12 " break; \n" }{MPLTEXT 1 0 10 " end if;\n" }{MPLTEXT 1 0 7 "end do;" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 31 "So o ur desired prime number is:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 2 "P;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"fn\\cXeMm&e>KPZO$y]+>R.i9t_') ['3r_<" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 11 "isprime(P);" }} {PARA 11 "" 1 "" {XPPMATH 20 "I%trueG%*protectedG" }}}{EXCHG {PARA 0 " " 0 "" {TEXT 207 39 "And the prime decomposition of (p-1) is" }{TEXT 207 0 "" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 13 "ifactor(P-1);" } }{PARA 11 "" 1 "" {XPPMATH 20 "*()-I!G6\"6#\"\"#\"\"&\"\"\"-F%6#\"#6F* )-F%6#\"#8\"#]F*" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 56 "Now we choos e another large prime number Q and form n=PQ" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 37 "Q:=nextprime(rand(10^100..10^120)());" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"cr8\"op@&3h(yJy)*>j>_ACT5bMu$QeH__OV\"zTz!zav3 -4,OZ$y0cHc;%[9\"ph%oO:F?" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 7 "n:=P*Q;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"]vP.sd5M`?G(y8E:P\\M\"3MIW !)z:okhcT9v:4vZR_vspO518w#4AT\"Q5$)Rw4o+aL(Q2 &fU,`N" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 24 "Note that maple comman d " }{TEXT 207 84 "ifactor(n); fails to factorize this n. We will now \+ factorize n using (p-1) algorithm" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 17 "Choose a base 'a'" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 5 "a:=2;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"#" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 13 "and a bound B" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 9 "B:=13*50;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"$]'" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 20 "Now compute b=a^(B!)" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 5 "b:=a;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"# " }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 21 "for i from 1 to B do\n" }{MPLTEXT 1 0 17 " b:=b&^i mod n;\n" }{MPLTEXT 1 0 7 "end do:" }}} {EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 2 "b;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"]v>rk([5a=Xj@&==6` #Q<2X5yl*>X#G%Rw$[`n;<))QtSwi?u*egLq27JLd<#" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 101 "According to (p-1) method, if p-1 divides b, then p di vides (b-1) and also n, so p divides their gcd:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 11 "gcd(b-1,n);" }}{PARA 11 "" 1 "" {XPPMATH 20 " \"fn\\cXeMm&e>KPZO$y]+>R.i9t_')['3r_<" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 40 "This is our factor that we started from." }}}{EXCHG {PARA 0 " " 0 "" {TEXT 207 122 "************************************************ ********************************************************************** ****" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 216 "The above attack shows \+ that in RSA, we have to choose primes p and q such that both p-1 and q -1 have large prime factors. We can achieve it similarly to the above \+ method. First, we start from a large prime factor p0:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 36 "p0:=nextprime(rand(10^50..10^60)()) ;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"gn>g[+=kji?Ijo\"HGKec/l]up9Y<()>! y&)" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 121 "Then we are looking for \+ a prime of the form (s+i)*p0+1, where s is a large number and i runs f rom 1 to some small number." }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 24 "s:=rand(10^70..10^80)();" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"[py7([ uGqD_>9pKM#4(" }}}{EXCHG {PARA 0 "> " 0 " " {MPLTEXT 1 0 23 "for i from 1 to 200 do\n" }{MPLTEXT 1 0 31 " if is prime((s+i)*p0+1) then \n" }{MPLTEXT 1 0 20 " P:=(s+i)*p0+1; \n" } {MPLTEXT 1 0 12 " break; \n" }{MPLTEXT 1 0 10 " end if;\n" } {MPLTEXT 1 0 7 "end do;" }{MPLTEXT 1 0 0 "" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 32 "So our \"secure\" prime number is:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 2 "P;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"gs,m&Hlt !=b/_*G;zCDrT:u5]bo=o9u$\\'4=\\fH=Xc.!o>8b:RJufx:A,&z\"Rc(QSik/ZOi4\"[ &\\<'" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 51 "A quick check that P-1 \+ has a large prime factor p0:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 11 "P-1 mod p0;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"!" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 25 "Similarly we construct Q:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 36 "q0:=nextprime(rand(10^50..10^60)()) ;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"gn`/Z))p'=I!po-11mV4%p.,#HOS[%piP My&" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 121 "Then we are looking for \+ a prime of the form (s+i)*p0+1, where s is a large number and i runs f rom 1 to some small number." }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 24 "s:=rand(10^60..10^68)();" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"_oFLqo xrn/:FNiMU`M**pru]F$3S%" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 23 "for i from 1 to 200 do\n" }{MPLTEXT 1 0 31 " if isprime((s+i )*q0+1) then \n" }{MPLTEXT 1 0 20 " Q:=(s+i)*q0+1; \n" }{MPLTEXT 1 0 12 " break; \n" }{MPLTEXT 1 0 10 " end if;\n" }{MPLTEXT 1 0 7 "e nd do;" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 32 "So our \"secure\" prim e number is:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 2 "Q;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"[sJZM0*o%p#oW#=\\#>ltL?#=i$f^A@z1#R#pF)4*Rt7 zWdel8GO+s\")=r6:p\\--i!G&=()p$>XD" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 51 "A quick check that P-1 has a large prime factor p0:" }}} {EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 11 "Q-1 mod q0;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"!" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 36 "Now o ur open key will include n=PQ:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 7 "n:=P*Q;" }}{PARA 11 "" 1 "" {XPPMATH 20 ",$I\"PG6\"\"[sJZM0*o%p #oW#=\\#>ltL?#=i$f^A@z1#R#pF)4*Rt7zWdel8GO+s\")=r6:p\\--i!G&=()p$>XD" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 66 "Out of curiosity, let's try to factorize this n with (p-1) method:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 5 "b:=2;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"#" }}} {EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 8 "B:=1500;" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"%+:" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 21 "for i from 1 to B do\n" }{MPLTEXT 1 0 17 " b:=b&^i mod n;\n" }{MPLTEXT 1 0 7 "end do:" }}}{EXCHG {PARA 0 "" 0 "" {TEXT 207 88 "Now when we comp ute gcd(b-1,n), we get a useless trivial factor, so our attack failed \+ :(" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 11 "gcd(b-1,n);" }}{PARA 11 "" 1 "" {XPPMATH 20 "\"\"\"" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 0 "" }}}} {MARK "0 0 0" 0 }{VIEWOPTS 1 1 0 1 1 1803 1 1 1 1 }{PAGENUMBERS 0 1 2 33 1 1 }