🐞 Fix Base58 decodeWord sanitizer ordering

src/utils/Base58.sol

@@ -189,10 +189,17 @@ library Base58 {
 
             for { let j := 0 } 1 {} {
                 let c := sub(byte(0, mload(add(s, j))), 49)
+                // Check if the input character is valid before `mload(c)`.
+                // Otherwise an out-of-bounds `c` expands memory and reverts
+                // with out-of-gas instead of `Base58DecodingError`.
+                if iszero(and(shl(c, 1), 0x3fff7ff03ffbeff01ff)) {
+                    mstore(0x00, 0xe8fad793) // `Base58DecodingError()`.
+                    revert(0x1c, 0x04)
+                }
                 let p := mul(result, 58)
                 let acc := add(byte(0, mload(c)), p)
-                // Check if the input character is valid.
-                if iszero(and(0x3fff7ff03ffbeff01ff, shl(c, lt(lt(acc, p), lt(result, t))))) {
+                // Check for multiplication or addition overflow.
+                if iszero(lt(lt(acc, p), lt(result, t))) {
                     mstore(0x00, 0xe8fad793) // `Base58DecodingError()`.
                     revert(0x1c, 0x04)
                 }

test/Base58.t.sol

@@ -259,6 +259,21 @@ contract Base58Test is SoladyTest {
         this.decodeWord("JEKNVnkbo3jma5nREBBJCDoXFVeKkD56V3xKrvRmWxFH@");
     }
 
+    function testDecodeWordLowCharacterReverts() public {
+        // Characters below '1' (0x31) underflow the lookup index. The
+        // sanitizer must run before `mload(c)`, otherwise the out-of-bounds
+        // load expands memory and reverts with out-of-gas instead of a clean
+        // `Base58DecodingError`. See https://github.com/Vectorized/solady/issues/1543.
+        vm.expectRevert(Base58.Base58DecodingError.selector);
+        this.decodeWord("0");
+        vm.expectRevert(Base58.Base58DecodingError.selector);
+        this.decodeWord("\x00");
+        // Also cover an underflowing byte after a valid character, where
+        // `result` is already nonzero (loop position > 0).
+        vm.expectRevert(Base58.Base58DecodingError.selector);
+        this.decodeWord("z0");
+    }
+
     function decodeWord(string memory encoded) public pure returns (bytes32) {
         return Base58.decodeWord(encoded);
     }