test: Test that key expression indexes match key count

diff --git a/src/script/descriptor.cpp b/src/script/descriptor.cpp
index 8558c8e2..2be3301b 100644
--- a/src/script/descriptor.cpp
+++ b/src/script/descriptor.cpp
@@ -161,12 +161,11 @@ typedef std::vector<uint32_t> KeyPath;
 /** Interface for public key objects in descriptors. */
 struct PubkeyProvider
 {
-protected:
+public:
     //! Index of this key expression in the descriptor
     //! E.g. If this PubkeyProvider is key1 in multi(2, key1, key2, key3), then m_expr_index = 0
-    uint32_t m_expr_index;
+    const uint32_t m_expr_index;
 
-public:
     explicit PubkeyProvider(uint32_t exp_index) : m_expr_index(exp_index) {}
 
     virtual ~PubkeyProvider() = default;
@@ -229,6 +228,9 @@ public:
 
     /** Whether this PubkeyProvider is a BIP 32 extended key that can be derived from */
     virtual bool IsBIP32() const = 0;
+
+    /** Get the count of keys known by this PubkeyProvider. Usually one, but may be more for key aggregation schemes */
+    virtual size_t GetKeyCount() const { return 1; }
 };
 
 class OriginPubkeyProvider final : public PubkeyProvider
@@ -788,6 +790,10 @@ public:
         // musig() can only be a BIP 32 key if all participants are bip32 too
         return std::all_of(m_participants.begin(), m_participants.end(), [](const auto& pubkey) { return pubkey->IsBIP32(); });
     }
+    size_t GetKeyCount() const override
+    {
+        return 1 + m_participants.size();
+    }
 };
 
 /** Base class for all Descriptor implementations. */
@@ -1041,6 +1047,40 @@ public:
         }
         return all;
     }
+
+    uint32_t GetMaxKeyExpr() const final
+    {
+        uint32_t max_key_expr{0};
+        std::vector<const DescriptorImpl*> todo = {this};
+        while (!todo.empty()) {
+            const DescriptorImpl* desc = todo.back();
+            todo.pop_back();
+            for (const auto& p : desc->m_pubkey_args) {
+                max_key_expr = std::max(max_key_expr, p->m_expr_index);
+            }
+            for (const auto& s : desc->m_subdescriptor_args) {
+                todo.push_back(s.get());
+            }
+        }
+        return max_key_expr;
+    }
+
+    size_t GetKeyCount() const final
+    {
+        size_t count{0};
+        std::vector<const DescriptorImpl*> todo = {this};
+        while (!todo.empty()) {
+            const DescriptorImpl* desc = todo.back();
+            todo.pop_back();
+            for (const auto& p : desc->m_pubkey_args) {
+                count += p->GetKeyCount();
+            }
+            for (const auto& s : desc->m_subdescriptor_args) {
+                todo.push_back(s.get());
+            }
+        }
+        return count;
+    }
 };
 
 /** A parsed addr(A) descriptor. */
diff --git a/src/script/descriptor.h b/src/script/descriptor.h
index fe33476d..3ac748a1 100644
--- a/src/script/descriptor.h
+++ b/src/script/descriptor.h
@@ -181,6 +181,12 @@ struct Descriptor {
 
     /** Semantic/safety warnings (includes subdescriptors). */
     virtual std::vector<std::string> Warnings() const = 0;
+
+    /** Get the maximum key expression index. Used only for tests */
+    virtual uint32_t GetMaxKeyExpr() const = 0;
+
+    /** Get the number of key expressions in this descriptor. Used only for tests */
+    virtual size_t GetKeyCount() const = 0;
 };
 
 /** Parse a `descriptor` string. Included private keys are put in `out`.
diff --git a/src/test/descriptor_tests.cpp b/src/test/descriptor_tests.cpp
index 8ff44d13..00782c37 100644
--- a/src/test/descriptor_tests.cpp
+++ b/src/test/descriptor_tests.cpp
@@ -283,6 +283,14 @@ void DoCheck(std::string prv, std::string pub, const std::string& norm_pub, int
     BOOST_CHECK_EQUAL(parse_pub->IsRange(), (flags & RANGE) != 0);
     BOOST_CHECK_EQUAL(parse_priv->IsRange(), (flags & RANGE) != 0);
 
+    // Check that the highest key expression index matches the number of keys in the descriptor
+    BOOST_TEST_INFO("Pub desc: " + pub);
+    uint32_t key_exprs = parse_pub->GetMaxKeyExpr();
+    BOOST_CHECK_EQUAL(key_exprs + 1, parse_pub->GetKeyCount());
+    BOOST_TEST_INFO("Priv desc: " + prv);
+    BOOST_CHECK_EQUAL(key_exprs, parse_priv->GetMaxKeyExpr());
+    BOOST_CHECK_EQUAL(key_exprs + 1, parse_priv->GetKeyCount());
+
     // * For ranged descriptors,  the `scripts` parameter is a list of expected result outputs, for subsequent
     //   positions to evaluate the descriptors on (so the first element of `scripts` is for evaluating the
     //   descriptor at 0; the second at 1; and so on). To verify this, we evaluate the descriptors once for
diff --git a/src/test/fuzz/descriptor_parse.cpp b/src/test/fuzz/descriptor_parse.cpp
index eeba5f13..6b3084e2 100644
--- a/src/test/fuzz/descriptor_parse.cpp
+++ b/src/test/fuzz/descriptor_parse.cpp
@@ -61,6 +61,10 @@ static void TestDescriptor(const Descriptor& desc, FlatSigningProvider& sig_prov
     const bool is_nontop_or_nonsolvable{!*is_solvable || !desc.GetOutputType()};
     const bool is_input_size_info_set{max_sat_maxsig && max_sat_nonmaxsig && max_elems};
     assert(is_input_size_info_set || is_nontop_or_nonsolvable);
+
+    auto max_key_expr = desc.GetMaxKeyExpr();
+    auto key_count = desc.GetKeyCount();
+    assert((max_key_expr == 0 && key_count == 0) || max_key_expr + 1 == key_count);
 }
 
 void initialize_descriptor_parse()
diff --git a/src/wallet/test/walletload_tests.cpp b/src/wallet/test/walletload_tests.cpp
index ec31cda3..a397ffc0 100644
--- a/src/wallet/test/walletload_tests.cpp
+++ b/src/wallet/test/walletload_tests.cpp
@@ -37,6 +37,8 @@ public:
     std::optional<int64_t> MaxSatisfactionElems() const override { return {}; }
     void GetPubKeys(std::set<CPubKey>& pubkeys, std::set<CExtPubKey>& ext_pubs) const override {}
     std::vector<std::string> Warnings() const override { return {}; }
+    uint32_t GetMaxKeyExpr() const override { return 0; }
+    size_t GetKeyCount() const override { return 0; }
 };
 
 BOOST_FIXTURE_TEST_CASE(wallet_load_descriptors, TestingSetup)