Implement ISO 7816 APDU case helper function

This helper function can also identify extended APDU cases although EMV does not support them. The intention is that this helper function can be used by EMV TTL as well as future C-APDU decoder implementations.
openemv · Mar 31, 2024 · 4b3d31b · 4b3d31b
1 parent 112f313
commit 4b3d31b
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Showing 2 changed files with 66 additions and 23 deletions.
diff --git a/src/emv_ttl.c b/src/emv_ttl.c
@@ -58,10 +58,6 @@ int emv_ttl_trx(
 
 	emv_debug_capdu(c_apdu, c_apdu_len);
 
-	if (c_apdu_len < 4) {
-		return -1;
-	}
-
 	// Determine the APDU case
 	// See ISO 7816-3:2006, 12.1.3, table 13
 	// See EMV Contact Interface Specification v1.0, 9.3.1.1
@@ -73,27 +69,16 @@ int emv_ttl_trx(
 	// Case 3: CLA INS P1 P2 Lc [Data(Lc)]
 	// Case 4: CLA INS P1 P2 Lc [Data(Lc)] Le
 
-	if (c_apdu_len == 4) {
-		apdu_case = ISO7816_APDU_CASE_1;
-		emv_debug_apdu("APDU case 1");
-	} else if (c_apdu_len == 5) {
-		apdu_case = ISO7816_APDU_CASE_2S;
-		emv_debug_apdu("APDU case 2S");
-	} else {
-		// Extract byte C5 from header; See ISO 7816-3:2006, 12.1.3
-		unsigned int C5 = *(uint8_t*)(c_apdu + 4);
-
-		if (C5 != 0 && C5 + 5 == c_apdu_len) { // If C5 is Lc and Le is absent
-			apdu_case = ISO7816_APDU_CASE_3S;
-			emv_debug_apdu("APDU case 3S");
-		} else if (C5 != 0 && C5 + 6 == c_apdu_len) { // If C5 is Lc and Le is present
-			apdu_case = ISO7816_APDU_CASE_4S;
-			emv_debug_apdu("APDU case 4S");
-		} else {
-			// Unknown APDU case
+	apdu_case = iso7816_apdu_case(c_apdu, c_apdu_len);
+	switch (apdu_case) {
+		case ISO7816_APDU_CASE_1: emv_debug_apdu("APDU case 1"); break;
+		case ISO7816_APDU_CASE_2S: emv_debug_apdu("APDU case 2S"); break;
+		case ISO7816_APDU_CASE_3S: emv_debug_apdu("APDU case 3S"); break;
+		case ISO7816_APDU_CASE_4S: emv_debug_apdu("APDU case 4S"); break;
+
+		default:
 			emv_debug_error("Unknown APDU case");
 			return -2;
-		}
 	}
 
 	if (ctx->cardreader.mode == EMV_CARDREADER_MODE_APDU) {

diff --git a/src/iso7816_apdu.h b/src/iso7816_apdu.h
@@ -24,6 +24,7 @@
 #define ISO7816_APDU_H
 
 #include <sys/cdefs.h>
+#include <stddef.h>
 #include <stdint.h>
 #include <stdbool.h>
 
@@ -57,6 +58,63 @@ enum iso7816_apdu_case_t {
 	ISO7816_APDU_CASE_4E,                       ///< ISO 7816 C-APDU case 4 for long Lc/Le fields: CLA, INS, P1, P2, Lc(3), Data(Lc), Le(3)
 };
 
+/**
+ * Determine ISO 7816 Command Application Protocol Data Unit (C-APDU) case
+ * @remark See ISO 7816-3:2006, 12.1.3
+ * @param c_apdu Command Application Protocol Data Unit (C-APDU) buffer
+ * @param c_apdu_len Length of Application Protocol Data Unit (C-APDU) buffer in bytes
+ * @return Less than zero for error. Otherwise @ref iso7816_apdu_case_t
+ */
+static inline int iso7816_apdu_case(const void* c_apdu, size_t c_apdu_len)
+{
+	if (!c_apdu || c_apdu_len < 4) {
+		return -1;
+	}
+
+	if (c_apdu_len == 4) {
+		return ISO7816_APDU_CASE_1;
+	} else if (c_apdu_len == 5) {
+		return ISO7816_APDU_CASE_2S;
+	} else {
+		// Extract byte C5 from header
+		// See ISO 7816-3:2006, 12.1.3, table 13
+		unsigned int C5 = *(uint8_t*)(c_apdu + 4);
+
+		if (C5 != 0) { // If C5 is Lc
+			if (5 + C5 == c_apdu_len) { // Le is absent
+				return ISO7816_APDU_CASE_3S;
+			}
+			if (6 + C5 == c_apdu_len) { // Le is present
+				return ISO7816_APDU_CASE_4S;
+			}
+		} else { // If C5 is zero
+			if (c_apdu_len == 7) {
+				return ISO7816_APDU_CASE_2E;
+			}
+
+			if (c_apdu_len > 7) {
+				// Extract bytes C6C7 from header
+				// See ISO 7816-3:2006, 12.1.3, table 13
+				unsigned int C6 = *(uint8_t*)(c_apdu + 6);
+				unsigned int C7 = *(uint8_t*)(c_apdu + 7);
+				unsigned int C6C7 = (C6 << 8) + C7;
+
+				if (C6C7) {
+					if (7 + C6C7 == c_apdu_len) { // Le is absent
+						return ISO7816_APDU_CASE_3E;
+					}
+					if (9 + C6C7 == c_apdu_len) { // Le is present
+						return ISO7816_APDU_CASE_4E;
+					}
+				}
+			}
+		}
+	}
+
+	// Unknown C-APDU case
+	return -2;
+}
+
 /// ISO 7816 C-APDU case 1 structure
 struct iso7816_apdu_case_1_t {
 	uint8_t CLA;                                ///< Class byte: indicates the type of command (interindustry vs proprietary, command chaining, secure messaging, logical channel)