NFC: Improve URI decoding in NDEF parser (#267)

* add decode_char

* Skip double malloc(), can just tell printf() how long the string is

* Decode URI within the same buffer

* Only decode if next 2 chars are valid hex

* Update changelog

* Clearer code for hex decode

Co-authored-by: jaylikesbunda <jaylikesbunda@users.noreply.github.com>

---------

Co-authored-by: Willy-JL <49810075+Willy-JL@users.noreply.github.com>
Co-authored-by: jaylikesbunda <jaylikesbunda@users.noreply.github.com>

CHANGELOG.md

@@ -112,6 +112,7 @@
 - NFC:
   - Added 6 new Mifare Classic keys from Bulgaria Hotel (#216 by @z3r0l1nk)
   - NDEF parser supports NTAG I2C Plus 1k and 2k chips too (by @RocketGod-git)
+  - NDEF parser decodes URL-encoded URI characters (#267 by @jaylikesbunda)
   - UL: Add iq aparts hotel key (by @xMasterX)
   - OFW/UL: Rename 'Detect Reader' to 'Extract MFC Keys' (by @bettse & @xMasterX)
   - OFW: Plantain parser improvements (by @assasinfil)

applications/main/nfc/plugins/supported_cards/ndef.c

@@ -27,11 +27,7 @@ static void
     print_data(FuriString* str, const char* prefix, const uint8_t* buf, size_t len, bool force_hex) {
     if(prefix) furi_string_cat_printf(str, "%s: ", prefix);
     if(!force_hex && is_text(buf, len)) {
-        char* tmp = malloc(len + 1);
+        furi_string_cat_printf(str, "%.*s", len, buf);
-        memcpy(tmp, buf, len);
-        tmp[len] = '\0';
-        furi_string_cat_printf(str, "%s", tmp);
-        free(tmp);
     } else {
         for(uint8_t i = 0; i < len; i++) {
             furi_string_cat_printf(str, "%02X ", buf[i]);
@@ -40,6 +36,17 @@ static void
     furi_string_cat(str, "\n");
 }
 
+static inline uint8_t hex_to_int(char c) {
+    if(c >= '0' && c <= '9') return c - '0';
+    if(c >= 'A' && c <= 'F') return c - 'A' + 10;
+    if(c >= 'a' && c <= 'f') return c - 'a' + 10;
+    return 0;
+}
+
+static char decode_char(const char* str) {
+    return (hex_to_int(str[1]) << 4) | hex_to_int(str[2]);
+}
+
 static void parse_ndef_uri(FuriString* str, const uint8_t* payload, uint32_t payload_len) {
     // https://learn.adafruit.com/adafruit-pn532-rfid-nfc/ndef#uri-records-0x55-slash-u-607763
     const char* prepends[] = {
@@ -88,26 +95,44 @@ static void parse_ndef_uri(FuriString* str, const uint8_t* payload, uint32_t pay
     size_t prepend_len = strlen(prepend);
 
     size_t uri_len = prepend_len + (payload_len - 1);
-    char* const uri_buf = malloc(uri_len);
+    char* const uri_buf = malloc(uri_len); // const to keep the original pointer to free later
     memcpy(uri_buf, prepend, prepend_len);
     memcpy(uri_buf + prepend_len, payload + 1, payload_len - 1);
-    char* uri = uri_buf;
+    char* uri = uri_buf; // cursor we can iterate and shift freely
+
+    // Encoded chars take 3 bytes (%AB), decoded chars take 1 byte
+    // We can decode by iterating and overwriting the same buffer
+    size_t decoded_len = 0;
+    for(size_t encoded_idx = 0; encoded_idx < uri_len; encoded_idx++) {
+        if(uri[encoded_idx] == '%' && encoded_idx + 2 < uri_len) {
+            char hi = toupper(uri[encoded_idx + 1]);
+            char lo = toupper(uri[encoded_idx + 2]);
+            if(((hi >= 'A' && hi <= 'F') || (hi >= '0' && hi <= '9')) &&
+               ((lo >= 'A' && lo <= 'F') || (lo >= '0' && lo <= '9'))) {
+                uri[decoded_len++] = decode_char(&uri[encoded_idx]);
+                encoded_idx += 2;
+                continue;
+            }
+        }
+        uri[decoded_len++] = uri[encoded_idx];
+    }
 
     const char* type = "URI";
-    if(strncmp(uri, "http", strlen("http")) == 0) {
+    if(strncmp(uri, "http", 4) == 0) {
         type = "URL";
-    } else if(strncmp(uri, "tel:", strlen("tel:")) == 0) {
+    } else if(strncmp(uri, "tel:", 4) == 0) {
         type = "Phone";
-        uri += strlen("tel:");
+        uri += 4;
-        uri_len -= strlen("tel:");
+        decoded_len -= 4;
-    } else if(strncmp(uri, "mailto:", strlen("mailto:")) == 0) {
+    } else if(strncmp(uri, "mailto:", 7) == 0) {
         type = "Mail";
-        uri += strlen("mailto:");
+        uri += 7;
-        uri_len -= strlen("mailto:");
+        decoded_len -= 7;
     }
 
     furi_string_cat_printf(str, "%s\n", type);
-    print_data(str, NULL, (uint8_t*)uri, uri_len, false);
+    print_data(str, NULL, (uint8_t*)uri, decoded_len, false);
+
     free(uri_buf);
 }