From 09c664791bb64a81df1392bcaebceef71e81c896 Mon Sep 17 00:00:00 2001
From: Per Allansson <pallansson@gmail.com>
Date: Thu, 7 Mar 2013 07:26:56 +0100
Subject: [PATCH] Calculate correct PE checksum instead of setting it to 0.

Patch provided by Roland Schwingel.
---
 TODO           |  1 -
 osslsigncode.c | 29 ++++++++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/TODO b/TODO
index ccbf0cb..8daf424 100644
--- a/TODO
+++ b/TODO
@@ -4,7 +4,6 @@
 - verify signatures
 - remove mmap usage to increase portability
 - tests
-- calculate correct PE checksum instead of setting it to 0
 - fix other stuff marked 'XXX'
 - free memory properly :)
 
diff --git a/osslsigncode.c b/osslsigncode.c
index 89040f2..635e0e5 100644
--- a/osslsigncode.c
+++ b/osslsigncode.c
@@ -534,6 +534,32 @@ static void get_indirect_data_blob(u_char **blob, int *len, const EVP_MD *md, in
     i2d_SpcIndirectDataContent(idc, &p);
 }
 
+static void recalc_pe_checksum(BIO *outdata, unsigned int peheader)
+{
+	unsigned int checkSum = 0;
+	unsigned short  val;
+	unsigned int size = 0;
+
+	/* First set current checksum to 0. */
+	(void)BIO_seek(outdata, peheader + 88);
+	BIO_write(outdata, &checkSum, 4);
+
+	/* recalc checksum. */
+	(void)BIO_seek(outdata, 0);
+	while (BIO_read(outdata, &val, 2) == 2) {
+		checkSum += val;
+		checkSum = 0xffff & (checkSum + (checkSum >> 0x10));
+		size += 2;
+	}
+
+	checkSum = 0xffff & (checkSum + (checkSum >> 0x10));
+	checkSum += size;
+
+	/* write back checksum. */
+	(void)BIO_seek(outdata, peheader + 88);
+	BIO_write(outdata, &checkSum, 4);
+}
+
 int main(int argc, char **argv)
 {
     BIO *btmp, *sigdata, *hash, *outdata;
@@ -751,7 +777,7 @@ int main(int argc, char **argv)
     }
 
     /* Create outdata file */
-    outdata = BIO_new_file(outfile, "wb");
+    outdata = BIO_new_file(outfile, "w+b");
     if (outdata == NULL)
         DO_EXIT_1("Failed to create file: %s\n", outfile);
 
@@ -991,6 +1017,7 @@ int main(int argc, char **argv)
         BIO_write(outdata, buf, 4);
         PUT_UINT32_LE(len+8+padlen, buf);
         BIO_write(outdata, buf, 4);
+        recalc_pe_checksum(outdata, peheader);
     } else {
         (void)BIO_seek(outdata, 0x30);
         PUT_UINT32_LE(len+padlen, buf);