Read regkey username before using GetUserName fallback

If the user has changed their username after the authorization key has been created, then GetUserName will return the new username which can't decrypt the key. The username at the time of authorization should be stored at `HKEY_CURRENT_USER\Software\\Adobe\\Adept\\Device`, which we first attempt to read before falling back if it doesn't exist.

calibre-plugin/keyextract/main.c

@@ -50,10 +50,16 @@ int main() {
 	TCHAR user[USERBUFSIZE];
 	memset(&user, 0, sizeof(user));  // GetUserName only sets bytes as needed for length of username, but we need null bytes to fill the rest
 	DWORD bufsize = USERBUFSIZE	;
-	if (GetUserName(user, &bufsize) == 0) {
-		DWORD err = GetLastError();
-		fprintf(stderr, "Error with GetUserName: %ld\n", err);
-		return err;
+	LSTATUS user_retval = RegGetValue(HKEY_CURRENT_USER, "Software\\Adobe\\Adept\\Device", "username", RRF_RT_REG_SZ, NULL, &user, &bufsize);
+	if (user_retval != ERROR_SUCCESS) {
+		fprintf(stderr, "Error with RegGetValue: %ld\n", user_retval);
+		fprintf(stderr, "bufsize: %ld\n", bufsize);
+		fprintf(stderr, "Falling back to GetUserName");
+		if (GetUserName(user, &bufsize) == 0) {
+			DWORD err = GetLastError();
+			fprintf(stderr, "Error with GetUserName: %ld\n", err);
+			return err;
+		}
 	}
 	fprintf(stderr, "Username: %s\n", user);
 
@@ -62,11 +68,11 @@ int main() {
 	#define KEYBUFSIZE 180  // As measured
 	BYTE key[KEYBUFSIZE];
 	DWORD regkeysize = KEYBUFSIZE;
-	LSTATUS retval = RegGetValue(HKEY_CURRENT_USER, "Software\\Adobe\\Adept\\Device", "key", RRF_RT_REG_BINARY, NULL, &key, &regkeysize);
-	if (retval != ERROR_SUCCESS) {
-		fprintf(stderr, "Error with RegGetValue: %ld\n", retval);
+	LSTATUS key_retval = RegGetValue(HKEY_CURRENT_USER, "Software\\Adobe\\Adept\\Device", "key", RRF_RT_REG_BINARY, NULL, &key, &regkeysize);
+	if (key_retval != ERROR_SUCCESS) {
+		fprintf(stderr, "Error with RegGetValue: %ld\n", key_retval);
 		fprintf(stderr, "regkeysize: %ld\n", regkeysize);
-		return retval;
+		return key_retval;
 	}
 	fprintf(stderr, "Encrypted key (hex): ");
 	for (size_t i = 0; i < KEYBUFSIZE; i++ )