mini-gmp mpz_powm incorrect result

[Guido Vranken]

Thank you, I have confirmed that your patch resolves the issue.

On Mon, Aug 29, 2022 at 10:29 AM Paul Zimmermann <Paul.Zimmermann at inria.fr>
wrote:

> thank you, I confirm the bug, here is a potential fix:
>
> $ diff -u mini-gmp.c.orig mini-gmp.c
> --- mini-gmp.c.orig     2022-08-29 10:28:20.700995412 +0200
> +++ mini-gmp.c  2022-08-29 10:27:36.112191428 +0200
> @@ -3060,6 +3060,7 @@
>    if (en == 0)
>      {
>        mpz_set_ui (r, 1);
> +      mpz_tdiv_r (r, r, m);
>        return;
>      }
>
> Paul
>
> > From: Guido Vranken <guidovranken at gmail.com>
> > Date: Sun, 28 Aug 2022 16:22:40 +0200
> >
> > The following program computes 1^0 % 1:
> >
> > //#include <gmp.h>
> > #include "mini-gmp.c"
> > #include <stdio.h>
> >
> > #define CF_CHECK_EQ(expr, res) if ( (expr) != (res) ) { goto end; }
> >
> > int main(void)
> > {
> >     mpz_t a, b, c, res;
> >     char* s = NULL;
> >     /* noret */ mpz_init(a);
> >     /* noret */ mpz_init(b);
> >     /* noret */ mpz_init(c);
> >     /* noret */ mpz_init(res);
> >
> >     CF_CHECK_EQ(mpz_set_str(a, "1", 10), 0);
> >     CF_CHECK_EQ(mpz_set_str(b, "0", 10), 0);
> >     CF_CHECK_EQ(mpz_set_str(c, "1", 10), 0);
> >     CF_CHECK_EQ(mpz_set_str(res, "0", 10), 0);
> >
> >     /* noret */ mpz_powm(res, a, b, c);
> >
> >     printf("%s\n", mpz_get_str(NULL, 10, res));
> > end:
> >     return 0;
> > }
> >
> > The result should be 0, which is the case with regular libgmp, but with
> > mini-gmp the result is 1, and this is incorrect.
> >
> > Tested on version 6.2.1 and the latest repository checkout, with recent
> > clang and gcc, on x64 Linux.
> > _______________________________________________
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> > gmp-bugs at gmplib.org
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>