index calculus vs pollard rho

[Pascal Molin <molin@math.univ-paris-diderot.fr>]

The following znlog uses index calculus on a 46 bits subgroup, but p itself is large,

this is slow (and memory-demanding)

gp > p=nextprime(2^120); znlog(Mod(3,p),Mod(2,p),p-1)

time = 51,617 ms.

%21 = 391862826185609110238504885400229618

while the same is easier to compute with pollard

gp > p=nextprime(2^120); znlog(Mod(3,5*p)^4,Mod(2,5*p)^4,p-1)

time = 5,615 ms.

%20 = 391862826185609110238504885400229618

I do not know how the choice of algorithm is made, maybe there is an issue here.

-- 

Pascal