From 9368c265c1ff52214b7ab690512974dffd90292a Mon Sep 17 00:00:00 2001
From: Frank Denis <github@pureftpd.org>
Date: Tue, 30 Dec 2025 12:00:08 +0100
Subject: [PATCH] core_ed25519_is_valid_point: check Y==Z in addition to X==0

---
 src/libsodium/crypto_core/ed25519/ref10/ed25519_ref10.c | 5 ++++-
 test/default/core_ed25519.c                             | 3 ++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/libsodium/crypto_core/ed25519/ref10/ed25519_ref10.c b/src/libsodium/crypto_core/ed25519/ref10/ed25519_ref10.c
index 8129d8be..6feecc2e 100644
--- a/src/libsodium/crypto_core/ed25519/ref10/ed25519_ref10.c
+++ b/src/libsodium/crypto_core/ed25519/ref10/ed25519_ref10.c
@@ -1029,10 +1029,13 @@ int
 ge25519_is_on_main_subgroup(const ge25519_p3 *p)
 {
     ge25519_p3 pl;
+    fe25519    t;
 
     ge25519_mul_l(&pl, p);
 
-    return fe25519_iszero(pl.X);
+    fe25519_sub(t, pl.Y, pl.Z);
+
+    return fe25519_iszero(pl.X) & fe25519_iszero(t);
 }
 
 int
diff --git a/test/default/core_ed25519.c b/test/default/core_ed25519.c
index b2461260..f0a0398d 100644
--- a/test/default/core_ed25519.c
+++ b/test/default/core_ed25519.c
@@ -142,11 +142,12 @@ main(void)
     assert(crypto_core_ed25519_is_valid_point(p) == 0);
 
     p[0] = 9;
-    assert(crypto_core_ed25519_is_valid_point(p) == 1);
+    assert(crypto_core_ed25519_is_valid_point(p) == 0);
 
     assert(crypto_core_ed25519_is_valid_point(max_canonical_p) == 1);
     assert(crypto_core_ed25519_is_valid_point(non_canonical_invalid_p) == 0);
     assert(crypto_core_ed25519_is_valid_point(non_canonical_p) == 0);
+    assert(crypto_core_ed25519_is_valid_point(not_main_subgroup_p) == 0);
 
     memcpy(p2, p, crypto_core_ed25519_BYTES);
     add_P(p2);