diff --git a/autogen.sh b/autogen.sh
index 32ea6f6..d8c4b5e 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -1,6 +1,6 @@
 #!/bin/bash
 
-ccan_modules="talloc read_write_all build_assert array_size"
+ccan_modules="talloc read_write_all build_assert array_size endian"
 
 # Add ccan upstream sources
 if [ ! -e lib/ccan.git/Makefile ]
diff --git a/src/image.c b/src/image.c
index 9fdeecd..6f6e8b0 100644
--- a/src/image.c
+++ b/src/image.c
@@ -38,6 +38,7 @@
 #include <unistd.h>
 #include <string.h>
 
+#include <ccan/endian/endian.h>
 #include <ccan/talloc/talloc.h>
 #include <ccan/read_write_all/read_write_all.h>
 #include <ccan/build_assert/build_assert.h>
@@ -134,6 +135,62 @@ static int align_up(int size, int align)
 	return (size + align - 1) & ~(align - 1);
 }
 
+static uint16_t csum_update_fold(uint16_t csum, uint16_t x)
+{
+	uint32_t new = csum + x;
+	new = (new >> 16) + (new & 0xffff);
+	return new;
+}
+
+static uint16_t csum_bytes(uint16_t checksum, void *buf, size_t len)
+{
+	unsigned int i;
+	uint16_t *p;
+
+	for (i = 0; i < len; i += sizeof(*p)) {
+		p = buf + i;
+		checksum = csum_update_fold(checksum, *p);
+	}
+
+	return checksum;
+}
+
+static void image_pecoff_update_checksum(struct image *image)
+{
+	bool is_signed = image->sigsize && image->sigbuf;
+	uint32_t checksum;
+	struct cert_table_header *cert_table = image->cert_table;
+
+	/* We carefully only include the signature data in the checksum (and
+	 * in the file length) if we're outputting the signature.  Otherwise,
+	 * in case of signature removal, the signature data is in the buffer
+	 * we read in (as indicated by image->size), but we do *not* want to
+	 * checksum it.
+	 *
+	 * We also skip the 32-bits of checksum data in the PE/COFF header.
+	 */
+	checksum = csum_bytes(0, image->buf,
+			(void *)image->checksum - (void *)image->buf);
+	checksum = csum_bytes(checksum,
+			image->checksum + 1,
+			(void *)(image->buf + image->data_size) -
+			(void *)(image->checksum + 1));
+
+	if (is_signed) {
+		checksum = csum_bytes(checksum,
+				cert_table, sizeof(*cert_table));
+
+		checksum = csum_bytes(checksum, image->sigbuf, image->sigsize);
+	}
+
+	checksum += image->data_size;
+
+	if (is_signed)
+		checksum += sizeof(*cert_table) + image->sigsize;
+
+	*(image->checksum) = cpu_to_le32(checksum);
+}
+
 static int image_pecoff_parse(struct image *image)
 {
 	struct cert_table_header *cert_table;
@@ -508,6 +565,8 @@ int image_add_signature(struct image *image, void *sig, int size)
 	if (aligned_size != tot_size)
 		memset(start + size, 0, aligned_size - tot_size);
 
+	image->cert_table = cth;
+
 	return 0;
 }
 
@@ -587,6 +646,8 @@ int image_write(struct image *image, const char *filename)
 		image->data_dir_sigtable->size = 0;
 	}
 
+	image_pecoff_update_checksum(image);
+
 	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
 	if (fd < 0) {
 		perror("open");