Thank you for the PR!

Regarding the test name: it's okay as is, but might also be called gmp_pow_crypto.phpt or so.

I don't care about the name, so if you really like the descriptive one, I'll simply rename it.
But if I do, the description inside the test should probably be generalized too?

@brainpower, I think we need to fix the actual issue first (very important wrt gmp_pow()); then we can figure out what to do with the test.

Hi, thanks you two for tackling this! I can help here add some unit cases of "real" crypto if this is still useful after the newest patch (and its tests) landed.

Common operations would include additions and multiplications on the curves. Using a more applied perspective, one could add examples of signatures, committments and zero-knowledge proofs.

@famoser, more test cases (especially if they are about real world usage) are certainly welcome! Not only to serve as regression tests, but also to explain/show-case some of the algorithms typically used for cryptographic purposes.

A proposal for a test, which I would place in ext/gmp/tests/gmp_cryptography.phpt:

--TEST--
Examples of the usage of gmp for cryptography.

This executes basic operations (addition, multiplication, inverse, exponentiation) as the "base operations".
Then, it performs a primality check, and finally diffie-hellman as the "application".
All operations are done in the 4096-bit MODP Group from RFC 3526: https://www.ietf.org/rfc/rfc3526.txt

Omitted are calculations on elliptic curves, which are also common, because of the complexity of these algorithms.
However, elliptic curves operate on smaller values, hence their use-case is implicitly covered here, too.

Further, omitted is explicit demonstration of (public-key) encryption, commitments, zero-knowledge proofs or similar common applications.
However, the operation used in the diffie-hellman is at the core of all these other applications, hence these use-cases are implicitly covered, too.

$a, $b, and $c generated with 
$random = gmp_random_range(0, $prime);
$randomHex = strtoupper(gmp_strval($random, 16));
echo chunk_split(chunk_split($randomHex, 8, " "), 54);
--EXTENSIONS--
gmp
--FILE--
<?php

$prime = gmp_init('
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF', 16);

$a = gmp_init('
BE774B3A 56642360 4B32CCF8 B721F519 E1FAD10F C8AB6109
D7B98E79 8A541A9B 5A747CC3 2927A1F9 AA8BFA3E 3F31858D
03DA94D5 B076FE11 35CBB577 70E8FF40 8B0F7E01 354C3436
1827ADF1 794E3C96 2BDBC8B9 6B894EF0 7CF67367 5F2B0B4F
0F6304FE A9F48EB9 D8D08C16 C00716F6 956AEEA5 9B7BB16A
7B29C225 1AF3988D 0F2381B2 6DDD130A 605BACD0 DE0104F9
71B1F8C9 43217768 D556A6BA AB2C5DED 69DC3CA3 79D6BBEA
8E9A8522 CCD6DD95 FA295909 C593D444 08A8832A A5429BFA', 16);

$b = gmp_init('
CA1CAE83 DBC72ACC 0BDA48CA 5AFF77D1 055F1CEA 0B4E3089
E2BC1661 F4878AF1 F28DE016 350F4182 ECF2DC26 691AAE10
BA6CB81B 375A1460 068CCB45 B948855B 3CE7FB9C 2754D50F
CE4B45F9 FF101B47 2F76A39B 707D1D0F F2EAA747 0E6AEB4B
37D582B3 2E724769 BB4D8088 FD2DB183 B67BB11F 3A61DF60
7C3029C9 33475CE6 9E3872EA 764AD8B4 CA42FDA0 04931B8C
046B2283 E06E291F 5CFE9369 7CC5A21B 13156554 59B11B21
CE206802 5738B90D EC4DB008 AA5B2BB3 1DCFE633 E05B91D9', 16);

$c = gmp_init('
8CA047D8 C5270CEF D43F181D 94901BF7 354BC803 AEFD1A1B
210B1500 C520C021 19CA1AE7 1422AD02 B326BBF3 19545111
FF5C284F 2A1083C5 31E9363E A98256F8 6F6B274C F44665F0
02FA45DD B3A1BF03 3A9BB662 EA5573AA 86BCD6A1 18E62597
A4B760A6 A393B406 265E4884 279B8C4E 209B3338 5A1A7D53
9D0F619A 18F967EF A0758D32 9E117136 F9ADDAE1 8AAF4718
837B4772 386C3B7D 988F8343 78F59991 81812F2D 93310E7E
9ED63DB7 5999C790 69D29570 08B7C39C 259BFFD2 74DC5E81', 16);

// check commutativity addition (a+b)+c = a+(b,c)
$d = gmp_mod(gmp_add($a, $b), $prime);
$left = gmp_mod(gmp_add($d, $c), $prime);
$d = gmp_mod(gmp_add($b, $c), $prime);
$right = gmp_mod(gmp_add($a, $d), $prime);
var_dump(gmp_cmp($left, $right) === 0);

// check distribution multiplication a(b+c) = ab + ac
$d = gmp_mod(gmp_add($b, $c), $prime);
$left = gmp_mod(gmp_mul($a, $d), $prime);
$d1 = gmp_mod(gmp_mul($a, $b), $prime);
$d2 = gmp_mod(gmp_mul($a, $c), $prime);
$right = gmp_mod(gmp_add($d1, $d2), $prime);
var_dump(gmp_cmp($left, $right) === 0);

// check inversion a * a^(-1) = 1
$inverse = gmp_invert($a, $prime);
$product = gmp_mod(gmp_mul($a, $inverse), $prime);
var_dump(gmp_cmp($product, 1) === 0);

// check exponentiation by group order a ^ (p-1) = 1
$groupOrder = gmp_sub($prime, 1);
$product = gmp_powm($a, $groupOrder, $prime);
var_dump(gmp_cmp($product, 1) === 0);

// check whether q is a safe prime (that is, p=(q-1)/2 is also prime)
$primeP = gmp_div($prime - 1, 2);
var_dump(gmp_prob_prime($primeP) > 0);

// diffie-hellman key exchange (g^a)^b = (g^b)^a
$generator = gmp_init(2);
$factorA = gmp_random_range(1, $primeP);
$factorB = gmp_random_range(1, $primeP);
$left = gmp_powm(gmp_powm($generator, $factorA, $primeP), $factorB, $primeP);
$right = gmp_powm(gmp_powm($generator, $factorB, $primeP), $factorA, $primeP);
var_dump(gmp_cmp($left, $right) === 0);

?>
--EXPECT--
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)
bool(true)

Can be tested by executing ./configure --with-gmp && env TESTS=ext/gmp/tests/gmp_cryptography.phpt make test, and passes on the master.

I've updated the PR to include the posted test, with the small change of splitting the long description into a --DESCRIPTION-- section.

And decided to keep my test, too, since it had still failed on the master I had before rebasing (c84b7ed),
where the gmp_pow checks had not been reverted/reworked, yet, while famoser's test passed.
Maybe gmp_powm did not have the same checks as gmp_pow?

Thank you for the cleanup with the DESCRIPTION section, I was not aware this exists.

And decided to keep my test, too, since it had still failed on the master (...) while famoser's test passed. Maybe gmp_powm did not have the same checks as gmp_pow?

Yes you are right, I do not actually use gmp_pow, I need to to use gmp_powm as my exponent is not an int, but also a GMP number. So I do not trigger the bug which however would be triggered in elliptic curve operations; e.g. when adding two points together.

It would therefore make sense to also either add elliptic curve operations, or try to test the different type of executed operations in such code (... which are not already tested in my testcase). For example, another function I never use is pow_div, which I however see in code implementing elliptic curve crypto. But in any case, it makes sense to keep the current testcase, as it operates on larger values that elliptic curve crypto would.

Another thing we might tackle is testing the overload operators: While gmp_pow is broken, its overload operator ** is not (so a fix at the moment is to replace gmp_pow with **). Should we also include testing the overload operators, and where would we find a complete list of these?

The operator overloads seem to be defined here:

So the overloaded operators should be: +, -, *, **, /, %, <<, >>, |, &, ^, ~ in the order of the switch cases in gmp.c...

To make some progress here, I propose:

• Rename gmp_cryptography.phpt to gmp_cryptography_ffc.phpt.
• Rename gmp_cryptography_ecc.phpt to the more generic gmp_cryptography.phpt, to be more in line with our current use as a "kitchensink" of operations that are used in cryptography, but without using them in a complete algorithm (i.e. concretely calculating something on the curve).
• Finalize the PR here, so master can already profit from this first step.

Then, I see two future PRs:

• (1) Add gmp_cryptography_ecc.phpt, with the same concept as gmp_cryptography_ffc.phpt, but on elliptic curves: Base operations to ensure the field properties emerge, and then a diffie-hellman as the core operation of all other applications.
• (2) Extend gmp_cryptography.phpt to add all operations used in crypto (or: all operations provided by gmp), as well as their overload operations. At least missing are the following functions I find used in an ECC library: gmp_sub, gmp_div, gmp_mul, gmp_and, gmp_xor, gmp_mod, gmp_invert, gmp_nextprime, gmp_jacobi, gmp_sign and gmp_abs.

Reasoning:

• (1) Needs some time, and should not block here, as the tests as currently proposed are already useful. On my side, I can tackle this probably in Jan/Feb.
• (2) Is easiest done after having the ffc and ecc unit tests both ready, as then it is clear what operations are clearly in use, and on what kind of numbers (what magnitude) they operate.

Makes sense.
I've applied the suggested changes, renamed the files and rebased on current master.

LGTM. So I guess you may now remove the draft status of the PR, so that @Girgias can take a look?

I think it would be good to add a similar test for the overloaded operators.

Would you do this now, and if so, for what operators? Or what do you think about the proposed approach here: #16896 (comment)

I think it would be good to add a similar test for the overloaded operators.

Would you do this now, and if so, for what operators? Or what do you think about the proposed approach here: #16896 (comment)

That approach makes sense, adding more tests that cover most operators is best.
Cannot have too little testing IMHO.

That approach makes sense, adding more tests that cover most operators is best.

So you would be OK with doing this in a future PR (by the reasoning of #16896 (comment)), and merge this PR in the state as it is now?

That approach makes sense, adding more tests that cover most operators is best.

So you would be OK with doing this in a future PR (by the reasoning of #16896 (comment)), and merge this PR in the state as it is now?

Sure, does not need to all be in the same one.

@brainpower if you also agree, could you please unset the draft / WIP state of the PR?

So GitHub is just being crap today, it somehow merged it as 47942be but does not close the PR....