Add a single, central base64 decoder

Similar to the hex blob decoder, we need this once for tar and
once for the filemap xattr parser.

Simply add a single, central implementation to libutil, with a
simple unit test, and then use it in both libtar and gensquashfs.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

2 files changed, 104 insertions, 0 deletions

diff --git a/lib/util/Makemodule.am b/lib/util/Makemodule.am
index 4f1f0da..ec38b7a 100644
--- a/lib/util/Makemodule.am
+++ b/lib/util/Makemodule.am
@@ -15,6 +15,7 @@ libutil_a_SOURCES += lib/util/filename_sane.c
 libutil_a_SOURCES += lib/util/source_date_epoch.c
 libutil_a_SOURCES += lib/util/file_cmp.c
 libutil_a_SOURCES += lib/util/hex_decode.c
+libutil_a_SOURCES += lib/util/base64_decode.c
 libutil_a_CFLAGS = $(AM_CFLAGS)
 libutil_a_CPPFLAGS = $(AM_CPPFLAGS)
 
diff --git a/lib/util/base64_decode.c b/lib/util/base64_decode.c
new file mode 100644
index 0000000..b1cf5b6
--- /dev/null
+++ b/lib/util/base64_decode.c
@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * base64_decode.c
+ *
+ * Copyright (C) 2022 David Oberhollenzer <goliath@infraroot.at>
+ */
+#include "config.h"
+#include "util/util.h"
+#include "util/test.h"
+
+#include <ctype.h>
+
+static int base64_digit(int c)
+{
+	if (isupper(c))
+		return c - 'A';
+	if (islower(c))
+		return c - 'a' + 26;
+	if (isdigit(c))
+		return c - '0' + 52;
+	if (c == '+')
+		return 62;
+	if (c == '/' || c == '-')
+		return 63;
+	return -1;
+}
+
+int base64_decode(const char *in, size_t in_len, sqfs_u8 *out, size_t *out_len)
+{
+	int i1, i2, i3, i4;
+	size_t count = 0;
+
+	while (in_len >= 4) {
+		i1 = base64_digit(*(in++));
+		i2 = base64_digit(*(in++));
+		i3 = *(in++);
+		i4 = *(in++);
+		in_len -= 4;
+
+		if (i1 < 0 || i2 < 0 || count >= *out_len)
+			goto fail;
+
+		out[count++] = (i1 << 2) | (i2 >> 4);
+
+		if (i3 == '=' || i3 == '_') {
+			if ((i4 != '=' && i4 != '_') || in_len > 0)
+				goto fail;
+			break;
+		}
+
+		i3 = base64_digit(i3);
+		if (i3 < 0 || count >= *out_len)
+			goto fail;
+
+		out[count++] = ((i2 & 0x0F) << 4) | (i3 >> 2);
+
+		if (i4 == '=' || i4 == '_') {
+			if (in_len > 0)
+				goto fail;
+			break;
+		}
+
+		i4 = base64_digit(i4);
+		if (i4 < 0 || count >= *out_len)
+			goto fail;
+
+		out[count++] = ((i3 & 0x3) << 6) | i4;
+	}
+
+	/* libarchive has this bizarre bastardization of truncated base64 */
+	if (in_len > 0) {
+		if (in_len == 1)
+			goto fail;
+
+		i1 = base64_digit(*(in++));
+		i2 = base64_digit(*(in++));
+		in_len -= 2;
+
+		if (i1 < 0 || i2 < 0 || count >= *out_len)
+			goto fail;
+
+		out[count++] = (i1 << 2) | (i2 >> 4);
+
+		if (in_len > 0) {
+			i3 = *(in++);
+			--in_len;
+
+			if (i3 != '=' && i3 != '_') {
+				i3 = base64_digit(i3);
+				if (i3 < 0 || count >= *out_len)
+					goto fail;
+
+				out[count++] = ((i2 & 0x0F) << 4) | (i3 >> 2);
+			}
+		}
+	}
+
+	*out_len = count;
+	return 0;
+fail:
+	*out_len = 0;
+	return -1;
+}