Commit notes from Killer Queen CTF 2021

Signed-off-by: Malfurious <m@lfurio.us>

1 files changed, 55 insertions, 0 deletions

diff --git a/templates/image_manip/png_crc.c b/templates/image_manip/png_crc.c
new file mode 100644
index 0000000..dfe9d44
--- /dev/null
+++ b/templates/image_manip/png_crc.c
@@ -0,0 +1,55 @@
+/* PNG Chunk layout:
+   4-byte: length (of data) // Excluded from CRC
+   4-byte: type (ASCII alphabetic)
+   ?-byte: chunk data
+   4-byte: crc (see below...)
+
+   References:
+   http://www.libpng.org/pub/png/spec/1.2/PNG-Structure.html
+   http://www.libpng.org/pub/png/spec/1.2/PNG-CRCAppendix.html */
+
+
+/* Table of CRCs of all 8-bit messages. */
+unsigned long crc_table[256];
+int crc_table_computed = 0;
+
+/* Make the table for a fast CRC. */
+void make_crc_table(void)
+{
+  unsigned long c;
+  int n, k;
+
+  for (n = 0; n < 256; n++) {
+    c = (unsigned long) n;
+    for (k = 0; k < 8; k++) {
+      if (c & 1)
+        c = 0xedb88320L ^ (c >> 1);
+      else
+        c = c >> 1;
+    }
+    crc_table[n] = c;
+  }
+  crc_table_computed = 1;
+}
+
+/* Update a running CRC with the bytes buf[0..len-1]--the CRC should be
+   initialized to all 1's, and the transmitted value is the 1's complement of
+   the final running CRC (see the crc() routine below). */
+unsigned long update_crc(unsigned long crc, const unsigned char *buf, int len)
+{
+  unsigned long c = crc;
+  int n;
+
+  if (!crc_table_computed)
+    make_crc_table();
+  for (n = 0; n < len; n++) {
+    c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
+  }
+  return c;
+}
+
+/* Return the CRC of the bytes buf[0..len-1]. */
+unsigned long crc(const unsigned char *buf, int len)
+{
+  return update_crc(0xffffffffL, buf, len) ^ 0xffffffffL;
+}