diff --git a/payload_generator/delta_diff_utils_unittest.cc b/payload_generator/delta_diff_utils_unittest.cc
index 76d7624..436e265 100644
--- a/payload_generator/delta_diff_utils_unittest.cc
+++ b/payload_generator/delta_diff_utils_unittest.cc
@@ -24,6 +24,7 @@
 #include <base/files/scoped_file.h>
 #include <base/format_macros.h>
 #include <base/strings/stringprintf.h>
+#include <bsdiff/patch_writer.h>
 #include <gtest/gtest.h>
 
 #include "update_engine/common/test_utils.h"
@@ -594,4 +595,115 @@
   ASSERT_EQ(diff_utils::GetOldFile(old_files_map, "a").name, "filename");
 }
 
+TEST_F(DeltaDiffUtilsTest, XorOpsSourceNotAligned) {
+  ScopedTempFile patch_file;
+  bsdiff::BsdiffPatchWriter writer{patch_file.path()};
+  ASSERT_TRUE(writer.Init(kBlockSize * 10));
+  ASSERT_TRUE(writer.AddControlEntry(ControlEntry(0, 0, 123 + kBlockSize)));
+  ASSERT_TRUE(writer.AddControlEntry(ControlEntry(kBlockSize, 0, 0)));
+  ASSERT_TRUE(writer.Close());
+
+  std::string patch_data;
+  utils::ReadFile(patch_file.path(), &patch_data);
+
+  AnnotatedOperation aop;
+  *aop.op.add_src_extents() = ExtentForRange(50, 10);
+  *aop.op.add_dst_extents() = ExtentForRange(500, 10);
+
+  diff_utils::PopulateXorOps(
+      &aop,
+      reinterpret_cast<const uint8_t*>(patch_data.data()),
+      patch_data.size());
+  ASSERT_EQ(aop.xor_ops.size(), 1UL) << "Only 1 block can possibly be XORed";
+  ASSERT_EQ(aop.xor_ops[0].src_extent().num_blocks(), 1UL);
+  ASSERT_EQ(aop.xor_ops[0].src_extent().start_block(), 51UL);
+  ASSERT_EQ(aop.xor_ops[0].src_offset(), 123UL);
+
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().num_blocks(), 1UL);
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().start_block(), 500UL);
+}
+
+TEST_F(DeltaDiffUtilsTest, XorOpsTargetNotAligned) {
+  ScopedTempFile patch_file;
+  bsdiff::BsdiffPatchWriter writer{patch_file.path()};
+  ASSERT_TRUE(writer.Init(kBlockSize * 10));
+  ASSERT_TRUE(writer.AddControlEntry(
+      ControlEntry(0, kBlockSize - 456, 123 + kBlockSize)));
+  ASSERT_TRUE(writer.AddControlEntry(ControlEntry(kBlockSize + 456, 0, 0)));
+  ASSERT_TRUE(writer.Close());
+
+  std::string patch_data;
+  utils::ReadFile(patch_file.path(), &patch_data);
+
+  AnnotatedOperation aop;
+  *aop.op.add_src_extents() = ExtentForRange(50, 10);
+  *aop.op.add_dst_extents() = ExtentForRange(500, 10);
+
+  diff_utils::PopulateXorOps(
+      &aop,
+      reinterpret_cast<const uint8_t*>(patch_data.data()),
+      patch_data.size());
+  ASSERT_EQ(aop.xor_ops.size(), 1UL) << "Only 1 block can possibly be XORed";
+  ASSERT_EQ(aop.xor_ops[0].src_extent().num_blocks(), 1UL);
+  ASSERT_EQ(aop.xor_ops[0].src_extent().start_block(), 51UL);
+  ASSERT_EQ(aop.xor_ops[0].src_offset(), 123UL + 456UL);
+
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().num_blocks(), 1UL);
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().start_block(), 501UL);
+}
+
+TEST_F(DeltaDiffUtilsTest, XorOpsStrided) {
+  ScopedTempFile patch_file;
+  bsdiff::BsdiffPatchWriter writer{patch_file.path()};
+  ASSERT_TRUE(writer.Init(kBlockSize * 10));
+  ASSERT_TRUE(writer.AddControlEntry(ControlEntry(0, kBlockSize - 456, 123)));
+  ASSERT_TRUE(
+      writer.AddControlEntry(ControlEntry(kBlockSize * 10 + 456, 0, 0)));
+  ASSERT_TRUE(writer.Close());
+
+  std::string patch_data;
+  utils::ReadFile(patch_file.path(), &patch_data);
+
+  AnnotatedOperation aop;
+  *aop.op.add_src_extents() = ExtentForRange(50, 5);
+  *aop.op.add_src_extents() = ExtentForRange(60, 5);
+
+  *aop.op.add_dst_extents() = ExtentForRange(500, 2);
+  *aop.op.add_dst_extents() = ExtentForRange(600, 2);
+  *aop.op.add_dst_extents() = ExtentForRange(700, 7);
+
+  diff_utils::PopulateXorOps(
+      &aop,
+      reinterpret_cast<const uint8_t*>(patch_data.data()),
+      patch_data.size());
+  ASSERT_EQ(aop.xor_ops.size(), 4UL);
+  for (const auto& op : aop.xor_ops) {
+    ASSERT_EQ(op.src_offset(), 123UL + 456UL);
+    LOG(INFO) << op.src_extent() << ", " << op.dst_extent();
+  }
+  ASSERT_EQ(aop.xor_ops[0].src_extent().num_blocks(), 2UL);
+  ASSERT_EQ(aop.xor_ops[0].src_extent().start_block(), 50UL);
+
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().num_blocks(), 1UL);
+  ASSERT_EQ(aop.xor_ops[0].dst_extent().start_block(), 501UL);
+
+  ASSERT_EQ(aop.xor_ops[1].src_extent().num_blocks(), 3UL);
+  ASSERT_EQ(aop.xor_ops[1].src_extent().start_block(), 51UL);
+
+  ASSERT_EQ(aop.xor_ops[1].dst_extent().num_blocks(), 2UL);
+  ASSERT_EQ(aop.xor_ops[1].dst_extent().start_block(), 600UL);
+
+  ASSERT_EQ(aop.xor_ops[2].src_extent().num_blocks(), 3UL);
+  ASSERT_EQ(aop.xor_ops[2].src_extent().start_block(), 53UL);
+
+  ASSERT_EQ(aop.xor_ops[2].dst_extent().num_blocks(), 2UL);
+  ASSERT_EQ(aop.xor_ops[2].dst_extent().start_block(), 700UL);
+
+  ASSERT_EQ(aop.xor_ops[3].src_extent().num_blocks(), 6UL);
+  ASSERT_EQ(aop.xor_ops[3].src_extent().start_block(), 60UL);
+
+  ASSERT_EQ(aop.xor_ops[3].dst_extent().num_blocks(), 5UL);
+  ASSERT_EQ(aop.xor_ops[3].dst_extent().start_block(), 702UL);
+}
+
 }  // namespace chromeos_update_engine
diff --git a/payload_generator/extent_utils.h b/payload_generator/extent_utils.h
index 1cf7c77..2bd6626 100644
--- a/payload_generator/extent_utils.h
+++ b/payload_generator/extent_utils.h
@@ -131,7 +131,7 @@
 size_t GetNthBlock(const Container& extents, const size_t n) {
   size_t cur_block_count = 0;
   for (const auto& extent : extents) {
-    if (cur_block_count + extent.num_blocks() >= n) {
+    if (n - cur_block_count < extent.num_blocks()) {
       return extent.start_block() + (n - cur_block_count);
     }
     cur_block_count += extent.num_blocks();