diff --git libtransmission/crypto-utils.cc libtransmission/crypto-utils.cc
index a77e2c9..aaea1f1 100644
--- libtransmission/crypto-utils.cc
+++ libtransmission/crypto-utils.cc
@@ -171,6 +171,11 @@ constexpr void tr_binary_to_hex(void const* vinput, void* voutput, size_t byte_l
     }
 }
 
+// Custom constexpr function to convert a character to lowercase
+constexpr char toLower(char c) {
+    return (c >= 'A' && c <= 'Z') ? (c - 'A' + 'a') : c;
+}
+
 constexpr void tr_hex_to_binary(char const* input, void* voutput, size_t byte_length)
 {
     auto constexpr Hex = "0123456789abcdef"sv;
@@ -179,9 +184,9 @@ constexpr void tr_hex_to_binary(char const* input, void* voutput, size_t byte_le
 
     for (size_t i = 0; i < byte_length; ++i)
     {
-        auto const upper_nibble = Hex.find(std::tolower(*input++));
-        auto const lower_nibble = Hex.find(std::tolower(*input++));
-        *output++ = (uint8_t)((upper_nibble << 4) | lower_nibble);
+        auto const upper_nibble = Hex.find(toLower(*input++));
+        auto const lower_nibble = Hex.find(toLower(*input++));
+        *output++ = static_cast<uint8_t>((upper_nibble << 4) | lower_nibble);
     }
 }