From 4dbfc77ebac265903980a471e04546af85e21a67 Mon Sep 17 00:00:00 2001
From: Peter Marschall <peter@adpm.de>
Date: Thu, 21 May 2020 10:50:53 +0200
Subject: [PATCH] opensc-explorer: extend do_asn1() to accept offsets

Have do_asn1() accept an optional parameter indicating an offset.
If this is given then start ASN.1-decoding the file/record at this offset.
---
 doc/tools/opensc-explorer.1.xml |  7 +++++++
 src/tools/opensc-explorer.c     | 32 +++++++++++++++++++++-----------
 2 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/doc/tools/opensc-explorer.1.xml b/doc/tools/opensc-explorer.1.xml
index ddc78f26..30aed112 100644
--- a/doc/tools/opensc-explorer.1.xml
+++ b/doc/tools/opensc-explorer.1.xml
@@ -167,6 +167,7 @@
 						<command>asn1</command>
 						<replaceable>file-id</replaceable>
 						<arg choice="opt"><replaceable>rec-no</replaceable></arg>
+						<arg choice="opt"><replaceable>offs</replaceable></arg>
 					</term>
 					<listitem>
 						<para>
@@ -176,6 +177,12 @@
 							<replaceable>rec-no</replaceable> is given and the file is
 							a record-oriented EF, parse and print only the record
 							indicated by this parameter.
+							If the optional parameter
+							<replaceable>offs</replaceable> is given, start parsing
+							and printing the file or record at the offset indicated
+							by the value given.
+							If this parameter is not given, the default offset is
+							<literal>0</literal>.
 						</para>
 					</listitem>
 				</varlistentry>
diff --git a/src/tools/opensc-explorer.c b/src/tools/opensc-explorer.c
index b801e241..224df68f 100644
--- a/src/tools/opensc-explorer.c
+++ b/src/tools/opensc-explorer.c
@@ -206,7 +206,7 @@ static struct command	cmds[] = {
 		"apdu",	"<data> ...",
 		"send a custom apdu command"		},
 	{ do_asn1,
-		"asn1",	"[<file-id> [<rec-no>]]",
+		"asn1",	"[<file-id> [<rec-no>] [<offs>]]",
 		"decode an ASN.1 file or record"	},
 	{ do_sm,
 		"sm",	"{open|close}",
@@ -2055,19 +2055,19 @@ static int do_apdu(int argc, char **argv)
 		fprintf(stderr, "Failure: %s\n", sc_strerror(r));
 	else
 		printf("Success!\n");
-
 	return 0;
 }
 
 static int do_asn1(int argc, char **argv)
 {
 	int r, err = 1;
+	unsigned int offs = 0;
 	sc_path_t path;
 	sc_file_t *file = NULL;
 	int not_current = 1;
 	u8 buf[SC_MAX_EXT_APDU_DATA_SIZE];
 
-	if (argc > 2)
+	if (argc > 3)
 		return usage(do_asn1);
 
 	/* select file */
@@ -2098,11 +2098,14 @@ static int do_asn1(int argc, char **argv)
 	if (file->ef_structure == SC_FILE_EF_TRANSPARENT) {
 		size_t size = (file->size > 0) ? file->size : sizeof(buf);
 
-		if (argc > 1) {
-			fprintf(stderr, "Transparent EFs do not support records\n");
+		if (argc > 2) {
+			fprintf(stderr, "Transparent EFs do not have records\n");
 			goto err;
 		}
 
+		if (argc > 1)
+			offs = (unsigned int) strtoul(argv[1], NULL, 10);
+
 		r = sc_lock(card);
 		if (r == SC_SUCCESS)
 			r = sc_read_binary(card, 0, buf, MIN(size, sizeof(buf)), 0);
@@ -2114,9 +2117,9 @@ static int do_asn1(int argc, char **argv)
 		if ((size_t) r != file->size) {
 			fprintf(stderr, "WARNING: expecting %"SC_FORMAT_LEN_SIZE_T"u, got %d bytes.\n",
 				 file->size, r);
-			/* some cards return a bogus value for file length. As
-			 * long as the actual length is not higher than the expected
-			 * length, continue */
+			/* some cards return a bogus value for file length.
+			 * As long as the actual length is not higher
+			 * than the expected length, continue */
 			if ((size_t) r > file->size)
 				goto err;
 		}
@@ -2130,6 +2133,9 @@ static int do_asn1(int argc, char **argv)
 		}
 
 		rec = (unsigned int) strtoul(argv[1], NULL, 10);
+		if (argc > 2)
+			offs = (unsigned int) strtoul(argv[2], NULL, 10);
+
 		if (rec < 1 || rec > file->record_count) {
 			fprintf(stderr, "Invalid record number %u.\n", rec);
 			goto err;
@@ -2147,10 +2153,14 @@ static int do_asn1(int argc, char **argv)
 		}
 	}
 
-	/* asn1 dump */
-	sc_asn1_print_tags(buf, r);
+	/* if offset does not exceed the length read from file/record, ... */
+	if (offs <= (unsigned int) r) {
+		/* ... perform the ASN.1 dump */
+		sc_asn1_print_tags(buf + offs, (unsigned int) r - offs);
+
+		err = 0;
+	}
 
-	err = 0;
 err:
 	if (not_current) {
 		sc_file_free(file);