Implement R-APDU print helper

* iso7816_sw1sw2_get_string() already provides a one-line description of
  the R-APDU status bytes.
* print_rapdu() will print the R-APDU buffer followed by the one-line
  description provided by iso7816_sw1sw2_get_string() in parentheses.
  The decoded response data should be printed separately using other
  helper functions.
* emv-tool does not use iso7816 directly and therefore should not link
  it. Instead, print_helpers already links iso7816,

src/iso7816_strings.c

@@ -65,6 +65,11 @@ const char* iso7816_sw1sw2_get_string(uint8_t SW1, uint8_t SW2, char* str, size_
 	int r;
 	char* str_ptr = str;
 
+	if (!str || !str_len) {
+		// Invalid parameters
+		return NULL;
+	}
+
 	// Normal processing (see ISO 7816-4:2005, 5.1.3)
 	if (SW1 == 0x90 && SW2 == 0x00) {
 		snprintf(str, str_len, "Normal");

tools/CMakeLists.txt

@@ -81,7 +81,7 @@ if(BUILD_EMV_TOOL)
 	endif()
 
 	add_executable(emv-tool emv-tool.c ../src/pcsc.c)
-	target_link_libraries(emv-tool PRIVATE print_helpers iso7816 emv emv_strings)
+	target_link_libraries(emv-tool PRIVATE print_helpers emv emv_strings)
 	if(TARGET libargp::argp)
 		target_link_libraries(emv-tool PRIVATE libargp::argp)
 	endif()

tools/print_helpers.c

@@ -246,6 +246,42 @@ void print_atr_historical_bytes(const struct iso7816_atr_info_t* atr_info)
 	}
 }
 
+void print_rapdu(const void* r_apdu, size_t r_apdu_len)
+{
+	const uint8_t* ptr = r_apdu;
+	char str[1024];
+	const char* s;
+
+	if (!r_apdu || !r_apdu_len) {
+		printf("(null)\n");
+		return;
+	}
+
+	for (size_t i = 0; i < r_apdu_len; i++) {
+		printf("%02X", ptr[i]);
+	}
+
+	if (r_apdu_len < 2) {
+		// No status
+		printf("\n");
+		return;
+	}
+
+	s = iso7816_sw1sw2_get_string(
+		ptr[r_apdu_len - 2],
+		ptr[r_apdu_len - 1],
+		str,
+		sizeof(str)
+	);
+	if (!s || !s[0]) {
+		// No string or empty string
+		printf("\n");
+		return;
+	}
+
+	printf(" (%s)\n", s);
+}
+
 void print_sw1sw2(uint8_t SW1, uint8_t SW2)
 {
 	char str[1024];
@@ -524,6 +560,11 @@ static void print_emv_debug_internal(
 				print_atr(buf);
 				return;
 
+			case EMV_DEBUG_TYPE_RAPDU:
+				printf("%s: ", str);
+				print_rapdu(buf, buf_len);
+				return;
+
 			default:
 				print_buf(str, buf, buf_len);
 				return;

tools/print_helpers.h

@@ -2,7 +2,7 @@
  * @file print_helpers.h
  * @brief Helper functions for command line output
  *
- * Copyright (c) 2021, 2022 Leon Lynch
+ * Copyright (c) 2021-2022, 2024 Leon Lynch
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
@@ -71,6 +71,13 @@ void print_atr(const struct iso7816_atr_info_t* atr_info);
  */
 void print_atr_historical_bytes(const struct iso7816_atr_info_t* atr_info);
 
+/**
+ * Print R-APDU
+ * @param r_apdu Response Application Protocol Data Unit (C-APDU)
+ * @param r_apdu_len Length of Response Application Protocol Data Unit (C-APDU). Must be at least 4 bytes.
+ */
+void print_rapdu(const void* r_apdu, size_t r_apdu_len);
+
 /**
  * Print status bytes SW1-SW2
  * @param SW1 Status byte 1