miniscript: the 'd:' wrapper must not be 'u'

The value it leaves on the stack depends on the last element on the
stack. However, we can't make sure this element is OP_1 (which would
give us the 'u' property) without the MINIMALIF rule.
MINIMALIF is only policy for P2WSH, therefore giving 'd:' the 'u'
property breaks consensus soundness: it makes it possible (by consensus
but not policy) for instance to satisfy a thresh() without satisfying
at least k of its subs.

This bug was found and reported by Andrew Poelstra.

2 files changed, 12 insertions, 2 deletions

diff --git a/src/script/miniscript.cpp b/src/script/miniscript.cpp
index d0bb937885..019f02f159 100644
--- a/src/script/miniscript.cpp
+++ b/src/script/miniscript.cpp
@@ -116,7 +116,8 @@ Type ComputeType(Fragment nodetype, Type x, Type y, Type z, const std::vector<Ty
             "e"_mst.If(x << "f"_mst) | // e=f_x
             (x & "ghijk"_mst) | // g=g_x, h=h_x, i=i_x, j=j_x, k=k_x
             (x & "ms"_mst) | // m=m_x, s=s_x
-            "nudx"_mst; // n, u, d, x
+            // NOTE: 'd:' is not 'u' under P2WSH as MINIMALIF is only a policy rule there.
+            "ndx"_mst; // n, d, x
         case Fragment::WRAP_V: return
             "V"_mst.If(x << "B"_mst) | // V=B_x
             (x & "ghijk"_mst) | // g=g_x, h=h_x, i=i_x, j=j_x, k=k_x
diff --git a/src/test/miniscript_tests.cpp b/src/test/miniscript_tests.cpp
index 949d30dfd5..930582ea24 100644
--- a/src/test/miniscript_tests.cpp
+++ b/src/test/miniscript_tests.cpp
@@ -224,7 +224,7 @@ BOOST_AUTO_TEST_CASE(fixed_tests)
     Test("and_b(hash256(32ba476771d01e37807990ead8719f08af494723de1d228f2c2c07cc0aa40bac),a:and_b(hash256(131772552c01444cd81360818376a040b7c3b2b7b0a53550ee3edde216cec61b),a:older(1)))", "82012088aa2032ba476771d01e37807990ead8719f08af494723de1d228f2c2c07cc0aa40bac876b82012088aa20131772552c01444cd81360818376a040b7c3b2b7b0a53550ee3edde216cec61b876b51b26c9a6c9a", TESTMODE_VALID | TESTMODE_NONMAL, 15, 3);
     Test("thresh(2,multi(2,03a0434d9e47f3c86235477c7b1ae6ae5d3442d49b1943c2b752a68e2a47e247c7,036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a00),a:multi(1,036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a00),ac:pk_k(022f01e5e15cca351daff3843fb70f3c2f0a1bdd05e5af888a67784ef3e10a2a01))", "522103a0434d9e47f3c86235477c7b1ae6ae5d3442d49b1943c2b752a68e2a47e247c721036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a0052ae6b5121036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a0051ae6c936b21022f01e5e15cca351daff3843fb70f3c2f0a1bdd05e5af888a67784ef3e10a2a01ac6c935287", TESTMODE_VALID | TESTMODE_NONMAL | TESTMODE_NEEDSIG, 13, 7);
     Test("and_n(sha256(d1ec675902ef1633427ca360b290b0b3045a0d9058ddb5e648b4c3c3224c5c68),t:or_i(v:older(4252898),v:older(144)))", "82012088a820d1ec675902ef1633427ca360b290b0b3045a0d9058ddb5e648b4c3c3224c5c68876400676303e2e440b26967029000b269685168", TESTMODE_VALID, 14, 3);
-    Test("or_d(d:and_v(v:older(4252898),v:older(4252898)),sha256(38df1c1f64a24a77b23393bca50dff872e31edc4f3b5aa3b90ad0b82f4f089b6))", "766303e2e440b26903e2e440b26968736482012088a82038df1c1f64a24a77b23393bca50dff872e31edc4f3b5aa3b90ad0b82f4f089b68768", TESTMODE_VALID, 14, 3);
+    Test("or_d(nd:and_v(v:older(4252898),v:older(4252898)),sha256(38df1c1f64a24a77b23393bca50dff872e31edc4f3b5aa3b90ad0b82f4f089b6))", "766303e2e440b26903e2e440b2696892736482012088a82038df1c1f64a24a77b23393bca50dff872e31edc4f3b5aa3b90ad0b82f4f089b68768", TESTMODE_VALID, 15, 3);
     Test("c:and_v(or_c(sha256(9267d3dbed802941483f1afa2a6bc68de5f653128aca9bf1461c5d0a3ad36ed2),v:multi(1,02c44d12c7065d812e8acf28d7cbb19f9011ecd9e9fdf281b0e6a3b5e87d22e7db)),pk_k(03acd484e2f0c7f65309ad178a9f559abde09796974c57e714c35f110dfc27ccbe))", "82012088a8209267d3dbed802941483f1afa2a6bc68de5f653128aca9bf1461c5d0a3ad36ed28764512102c44d12c7065d812e8acf28d7cbb19f9011ecd9e9fdf281b0e6a3b5e87d22e7db51af682103acd484e2f0c7f65309ad178a9f559abde09796974c57e714c35f110dfc27ccbeac", TESTMODE_VALID | TESTMODE_NEEDSIG, 8, 3);
     Test("c:and_v(or_c(multi(2,036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a00,02352bbf4a4cdd12564f93fa332ce333301d9ad40271f8107181340aef25be59d5),v:ripemd160(1b0f3c404d12075c68c938f9f60ebea4f74941a0)),pk_k(03fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556))", "5221036d2b085e9e382ed10b69fc311a03f8641ccfff21574de0927513a49d9a688a002102352bbf4a4cdd12564f93fa332ce333301d9ad40271f8107181340aef25be59d552ae6482012088a6141b0f3c404d12075c68c938f9f60ebea4f74941a088682103fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556ac", TESTMODE_VALID | TESTMODE_NONMAL | TESTMODE_NEEDSIG, 10, 6);
     Test("and_v(andor(hash256(8a35d9ca92a48eaade6f53a64985e9e2afeb74dcf8acb4c3721e0dc7e4294b25),v:hash256(939894f70e6c3a25da75da0cc2071b4076d9b006563cf635986ada2e93c0d735),v:older(50000)),after(499999999))", "82012088aa208a35d9ca92a48eaade6f53a64985e9e2afeb74dcf8acb4c3721e0dc7e4294b2587640350c300b2696782012088aa20939894f70e6c3a25da75da0cc2071b4076d9b006563cf635986ada2e93c0d735886804ff64cd1db1", TESTMODE_VALID, 14, 3);
@@ -263,6 +263,15 @@ BOOST_AUTO_TEST_CASE(fixed_tests)
     BOOST_CHECK(ms_multi);
     BOOST_CHECK_EQUAL(ms_multi->GetOps(), 4); // 3 pubkeys + CMS
     BOOST_CHECK_EQUAL(ms_multi->GetStackSize(), 3); // 1 sig + dummy elem + script push
+    // The 'd:' wrapper leaves on the stack what was DUP'ed at the beginning of its execution.
+    // Since it contains an OP_IF just after on the same element, we can make sure that the element
+    // in question must be OP_1 if OP_IF enforces that its argument must only be OP_1 or the empty
+    // vector (since otherwise the execution would immediately fail). This is the MINIMALIF rule.
+    // Unfortunately, this rule is consensus for Taproot but only policy for P2WSH. Therefore we can't
+    // (for now) have 'd:' be 'u'. This tests we can't use a 'd:' wrapper for a thresh, which requires
+    // its subs to all be 'u' (taken from https://github.com/rust-bitcoin/rust-miniscript/discussions/341).
+    const auto ms_minimalif = miniscript::FromString("thresh(3,c:pk_k(03d30199d74fb5a22d47b6e054e2f378cedacffcb89904a61d75d0dbd407143e65),sc:pk_k(03fff97bd5755eeea420453a14355235d382f6472f8568a18b2f057a1460297556),sc:pk_k(0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798),sdv:older(32))", CONVERTER);
+    BOOST_CHECK(!ms_minimalif);
 
     // Timelock tests
     Test("after(100)", "?", TESTMODE_VALID | TESTMODE_NONMAL); // only heightlock