utils: add util.c and functions is_potential_green_*_path()

diff --git a/main/amalgamated.c b/main/amalgamated.c
index 78d6fec..99d3dd0 100644
--- a/main/amalgamated.c
+++ b/main/amalgamated.c
@@ -180,6 +180,7 @@ void __wrap_abort(void);
 #include "./utils/temporary_stack.c"
 #endif // CONFIG_LIBJADE
 #include "./utils/urldecode.c"
+#include "./utils/util.c"
 #include "./utils/wally_ext.c"
 #include "./versioninfo.c"
 #include "./wallet.c"
diff --git a/main/utils/util.c b/main/utils/util.c
new file mode 100644
index 0000000..6386078
--- /dev/null
+++ b/main/utils/util.c
@@ -0,0 +1,69 @@
+#ifndef AMALGAMATED_BUILD
+#include "util.h"
+#include "../wallet.h"
+
+// Green user path:
+// Main account:
+//     m/1/pointer
+// Subaccounts:
+//     m/3'/subaccount'/1/pointer
+bool is_potential_green_user_path(const uint32_t* path, const size_t path_len, uint32_t* subaccount_out)
+{
+    if (path_len != GA_USER_PATH_MAX_LEN) {
+        // Main account path looks just like a recovery path, so check that
+        if (is_potential_green_recovery_path(path, path_len)) {
+            *subaccount_out = 0; // Main account
+            return true;
+        }
+        return false;
+    }
+    if (path[0] == harden(3) && ishardened(path[1]) && path[2] == 1 && path[3] && !ishardened(path[3])) {
+        *subaccount_out = unharden(path[1]); // Subaccount
+        return true;
+    }
+    return false;
+}
+
+// Green (user) recovery path:
+// Subaccounts:
+//     m/1/pointer
+// NOTE: The main account (subaccount 0) cannot be a recovery path,
+//       so there is no main account path formulation.
+bool is_potential_green_recovery_path(const uint32_t* path, const size_t path_len)
+{
+    if (path_len != GA_RECOVERY_PATH_LEN) {
+        return false;
+    }
+    return path[0] == 1 && !ishardened(path[1]);
+}
+
+// Green server path
+// Main account
+//     m/1/gait_path/pointer
+// Subaccounts:
+//     m/3/gait_path/subaccount/pointer
+bool is_potential_green_server_path(const uint32_t* path, const size_t path_len, uint32_t* subaccount_out)
+{
+    if (path_len == MAX_GASERVICE_PATH_LEN - 1) {
+        if (path[0] != 1) {
+            return false; // Main account indicator not present
+        }
+    } else if (path_len == MAX_GASERVICE_PATH_LEN) {
+        if (path[0] != 3) {
+            return false; // Subaccount indicator not present
+        }
+    } else {
+        return false;
+    }
+    const size_t tail_len = path_len == MAX_GASERVICE_PATH_LEN ? 2 : 1;
+    for (size_t i = 1; i < path_len; ++i) {
+        if (i < path_len - tail_len && path[i] > 0xffff) {
+            return false; // Not a Green server path element
+        } else if (ishardened(path[i])) {
+            return false; // Hardened subaccount or pointer not allowed
+        }
+    }
+    *subaccount_out = path_len == MAX_GASERVICE_PATH_LEN ? path[path_len - 2] : 0;
+    return true;
+}
+#endif // AMALGAMATED_BUILD
diff --git a/main/utils/util.h b/main/utils/util.h
index 80159ea..bb370a9 100644
--- a/main/utils/util.h
+++ b/main/utils/util.h
@@ -104,4 +104,8 @@ static inline size_t path_get_unhardened_tail_index(const uint32_t* path, const
     return path_tail_start;
 }
 
+bool is_potential_green_user_path(const uint32_t* path, size_t path_len, uint32_t* subaccount_out);
+bool is_potential_green_recovery_path(const uint32_t* path, size_t path_len);
+bool is_potential_green_server_path(const uint32_t* path, size_t path_len, uint32_t* subaccount_out);
+
 #endif /* UTIL_H_ */
diff --git a/main/wallet.h b/main/wallet.h
index 36cc018..48c3683 100644
--- a/main/wallet.h
+++ b/main/wallet.h
@@ -43,6 +43,8 @@ typedef struct {
 #define GASERVICE_ROOT_PATH_LEN (1 + 32)
 #define MAX_GASERVICE_PATH_TAIL_LEN (1 + 1)
 #define MAX_GASERVICE_PATH_LEN (GASERVICE_ROOT_PATH_LEN + MAX_GASERVICE_PATH_TAIL_LEN)
+#define GA_USER_PATH_MAX_LEN 4
+#define GA_RECOVERY_PATH_LEN 2
 
 // 'm' + ( ('/' + <10 digit number>[+ ']) * n) + '\0'
 #define MAX_PATH_STR_LEN(max_path_elems) (1 + ((1 + 10 + 1) * max_path_elems) + 1)