diff --git a/tools/applypatch/xdelta3.c b/tools/applypatch/xdelta3.c
new file mode 100644
index 0000000..c9a0f73
--- /dev/null
+++ b/tools/applypatch/xdelta3.c
@@ -0,0 +1,121 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+
+#include "xdelta3.h"
+#include "mincrypt/sha.h"
+
+int ApplyXDelta3Patch(const unsigned char* old_data, ssize_t old_size,
+                      const char* patch_filename,
+                      FILE* output, SHA_CTX* ctx) {
+#define WINDOW_SIZE 32768
+
+  int ret;
+  xd3_stream stream;
+  xd3_config config;
+
+  xd3_init_config(&config, 0);
+  config.winsize = WINDOW_SIZE;
+  ret = xd3_config_stream(&stream, &config);
+  if (ret != 0) {
+    fprintf(stderr, "xd3_config_stream error: %s\n", xd3_strerror(ret));
+    return 1;
+  }
+
+  // In xdelta3 terms, the "input" is the patch file: it contains a
+  // sequence of instruction codes and data that will be executed to
+  // produce the output file.  The "source" is the original data file;
+  // it is a blob of data to which instructions in the input may refer
+  // (eg, an instruction may say "copy such-and-such range of bytes
+  // from the source to the output").
+
+  // For simplicity, we provide the entire source to xdelta as a
+  // single block.  This means it should never have to ask us to load
+  // blocks of the source file.
+  xd3_source source;
+  source.name = "old name";
+  source.size = old_size;
+  source.ioh = NULL;
+  source.blksize = old_size;
+  source.curblkno = 0;
+  source.curblk = old_data;
+  source.onblk = old_size;
+
+  ret = xd3_set_source(&stream, &source);
+  if (ret != 0) {
+    fprintf(stderr, "xd3_set_source error: %s\n", xd3_strerror(ret));
+    return 1;
+  }
+
+  unsigned char buffer[WINDOW_SIZE];
+  FILE* input = fopen(patch_filename, "rb");
+  if (input == NULL) {
+    fprintf(stderr, "failed to open patch file %s: %d (%s)\n",
+            patch_filename, errno, strerror(errno));
+    return 1;
+  }
+
+  size_t bytes_read;
+
+  do {
+    bytes_read = fread(buffer, 1, WINDOW_SIZE, input);
+    if (feof(input)) {
+      xd3_set_flags(&stream, XD3_FLUSH);
+    }
+    xd3_avail_input(&stream, buffer, bytes_read);
+ process:
+    ret = xd3_decode_input(&stream);
+    switch (ret) {
+      case XD3_INPUT:
+        continue;
+      case XD3_OUTPUT:
+        SHA_update(ctx, stream.next_out, stream.avail_out);
+        if (fwrite(stream.next_out, 1, stream.avail_out, output) !=
+            stream.avail_out) {
+          fprintf(stderr, "short write of output file: %d (%s)\n",
+                  errno, strerror(errno));
+          return 1;
+        }
+        xd3_consume_output(&stream);
+        goto process;
+      case XD3_GETSRCBLK:
+        // We provided the entire source file already; it should never
+        // have to ask us for a block.
+        fprintf(stderr, "xd3_decode_input: unexpected GETSRCBLK\n");
+        return 1;
+      case XD3_GOTHEADER:
+      case XD3_WINSTART:
+      case XD3_WINFINISH:
+        // These are informational events we don't care about.
+        goto process;
+      default:
+        fprintf(stderr, "xd3_decode_input: unknown error %s (%s)\n",
+                xd3_strerror(ret), stream.msg);
+        return 1;
+    }
+  } while (!feof(input));
+
+  fclose(input);
+  return 0;
+
+#undef WINDOW_SIZE
+}