diff --git a/tools/applypatch/applypatch.c b/tools/applypatch/applypatch.c
new file mode 100644
index 0000000..9954869
--- /dev/null
+++ b/tools/applypatch/applypatch.c
@@ -0,0 +1,457 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <errno.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <sys/statfs.h>
+#include <unistd.h>
+
+#include "mincrypt/sha.h"
+#include "applypatch.h"
+
+// Read a file into memory; store it and its associated metadata in
+// *file.  Return 0 on success.
+int LoadFileContents(const char* filename, FileContents* file) {
+  file->data = NULL;
+
+  if (stat(filename, &file->st) != 0) {
+    fprintf(stderr, "failed to stat \"%s\": %s\n", filename, strerror(errno));
+    return -1;
+  }
+
+  file->size = file->st.st_size;
+  file->data = malloc(file->size);
+
+  FILE* f = fopen(filename, "rb");
+  if (f == NULL) {
+    fprintf(stderr, "failed to open \"%s\": %s\n", filename, strerror(errno));
+    free(file->data);
+    return -1;
+  }
+
+  size_t bytes_read = fread(file->data, 1, file->size, f);
+  if (bytes_read != file->size) {
+    fprintf(stderr, "short read of \"%s\" (%d bytes of %d)\n",
+            filename, bytes_read, file->size);
+    free(file->data);
+    return -1;
+  }
+  fclose(f);
+
+  SHA(file->data, file->size, file->sha1);
+  return 0;
+}
+
+// Save the contents of the given FileContents object under the given
+// filename.  Return 0 on success.
+int SaveFileContents(const char* filename, FileContents file) {
+  FILE* f = fopen(filename, "wb");
+  if (f == NULL) {
+    fprintf(stderr, "failed to open \"%s\" for write: %s\n",
+            filename, strerror(errno));
+    return -1;
+  }
+
+  size_t bytes_written = fwrite(file.data, 1, file.size, f);
+  if (bytes_written != file.size) {
+    fprintf(stderr, "short write of \"%s\" (%d bytes of %d)\n",
+            filename, bytes_written, file.size);
+    return -1;
+  }
+  fflush(f);
+  fsync(fileno(f));
+  fclose(f);
+
+  if (chmod(filename, file.st.st_mode) != 0) {
+    fprintf(stderr, "chmod of \"%s\" failed: %s\n", filename, strerror(errno));
+    return -1;
+  }
+  if (chown(filename, file.st.st_uid, file.st.st_gid) != 0) {
+    fprintf(stderr, "chown of \"%s\" failed: %s\n", filename, strerror(errno));
+    return -1;
+  }
+
+  return 0;
+}
+
+
+// Take a string 'str' of 40 hex digits and parse it into the 20
+// byte array 'digest'.  'str' may contain only the digest or be of
+// the form "<digest>:<anything>".  Return 0 on success, -1 on any
+// error.
+int ParseSha1(const char* str, uint8_t* digest) {
+  int i;
+  const char* ps = str;
+  uint8_t* pd = digest;
+  for (i = 0; i < SHA_DIGEST_SIZE * 2; ++i, ++ps) {
+    int digit;
+    if (*ps >= '0' && *ps <= '9') {
+      digit = *ps - '0';
+    } else if (*ps >= 'a' && *ps <= 'f') {
+      digit = *ps - 'a' + 10;
+    } else if (*ps >= 'A' && *ps <= 'F') {
+      digit = *ps - 'A' + 10;
+    } else {
+      return -1;
+    }
+    if (i % 2 == 0) {
+      *pd = digit << 4;
+    } else {
+      *pd |= digit;
+      ++pd;
+    }
+  }
+  if (*ps != '\0' && *ps != ':') return -1;
+  return 0;
+}
+
+// Parse arguments (which should be of the form "<sha1>" or
+// "<sha1>:<filename>" into the array *patches, returning the number
+// of Patch objects in *num_patches.  Return 0 on success.
+int ParseShaArgs(int argc, char** argv, Patch** patches, int* num_patches) {
+  *num_patches = argc;
+  *patches = malloc(*num_patches * sizeof(Patch));
+
+  int i;
+  for (i = 0; i < *num_patches; ++i) {
+    if (ParseSha1(argv[i], (*patches)[i].sha1) != 0) {
+      fprintf(stderr, "failed to parse sha1 \"%s\"\n", argv[i]);
+      return -1;
+    }
+    if (argv[i][SHA_DIGEST_SIZE*2] == '\0') {
+      (*patches)[i].patch_filename = NULL;
+    } else if (argv[i][SHA_DIGEST_SIZE*2] == ':') {
+      (*patches)[i].patch_filename = argv[i] + (SHA_DIGEST_SIZE*2+1);
+    } else {
+      fprintf(stderr, "failed to parse filename \"%s\"\n", argv[i]);
+      return -1;
+    }
+  }
+
+  return 0;
+}
+
+// Search an array of Patch objects for one matching the given sha1.
+// Return the Patch object on success, or NULL if no match is found.
+const Patch* FindMatchingPatch(uint8_t* sha1, Patch* patches, int num_patches) {
+  int i;
+  for (i = 0; i < num_patches; ++i) {
+    if (memcmp(patches[i].sha1, sha1, SHA_DIGEST_SIZE) == 0) {
+      return patches+i;
+    }
+  }
+  return NULL;
+}
+
+// Returns 0 if the contents of the file (argv[2]) or the cached file
+// match any of the sha1's on the command line (argv[3:]).  Returns
+// nonzero otherwise.
+int CheckMode(int argc, char** argv) {
+  if (argc < 3) {
+    fprintf(stderr, "no filename given\n");
+    return 2;
+  }
+
+  int num_patches;
+  Patch* patches;
+  if (ParseShaArgs(argc-3, argv+3, &patches, &num_patches) != 0) { return 1; }
+
+  FileContents file;
+  file.data = NULL;
+
+  if (LoadFileContents(argv[2], &file) != 0 ||
+      FindMatchingPatch(file.sha1, patches, num_patches) == NULL) {
+    fprintf(stderr, "file \"%s\" doesn't have any of expected "
+            "sha1 sums; checking cache\n", argv[2]);
+
+    free(file.data);
+
+    // If the source file is missing or corrupted, it might be because
+    // we were killed in the middle of patching it.  A copy of it
+    // should have been made in CACHE_TEMP_SOURCE.  If that file
+    // exists and matches the sha1 we're looking for, the check still
+    // passes.
+
+    if (LoadFileContents(CACHE_TEMP_SOURCE, &file) != 0) {
+      fprintf(stderr, "failed to load cache file\n");
+      return 1;
+    }
+
+    if (FindMatchingPatch(file.sha1, patches, num_patches) == NULL) {
+      fprintf(stderr, "cache bits don't match any sha1 for \"%s\"\n",
+              argv[2]);
+      return 1;
+    }
+  }
+
+  free(file.data);
+  return 0;
+}
+
+int ShowLicenses() {
+  ShowBSDiffLicense();
+  return 0;
+}
+
+// Return the amount of free space (in bytes) on the filesystem
+// containing filename.  filename must exist.  Return -1 on error.
+size_t FreeSpaceForFile(const char* filename) {
+  struct statfs sf;
+  if (statfs(filename, &sf) != 0) {
+    fprintf(stderr, "failed to statfs %s: %s\n", filename, strerror(errno));
+    return -1;
+  }
+  return sf.f_bsize * sf.f_bfree;
+}
+
+// This program applies binary patches to files in a way that is safe
+// (the original file is not touched until we have the desired
+// replacement for it) and idempotent (it's okay to run this program
+// multiple times).
+//
+// - if the sha1 hash of <file> is <tgt-sha1>, does nothing and exits
+//   successfully.
+//
+// - otherwise, if the sha1 hash of <file> is <src-sha1>, applies the
+//   bsdiff <patch> to <file> to produce a new file (the type of patch
+//   is automatically detected from the file header).  If that new
+//   file has sha1 hash <tgt-sha1>, moves it to replace <file>, and
+//   exits successfully.
+//
+// - otherwise, or if any error is encountered, exits with non-zero
+//   status.
+
+int main(int argc, char** argv) {
+  if (argc < 2) {
+ usage:
+    fprintf(stderr, "usage: %s <file> <tgt-sha1> <tgt-size> [<src-sha1>:<patch> ...]\n"
+                    "   or  %s -c <file> [<sha1> ...]\n"
+                    "   or  %s -s <bytes>\n"
+                    "   or  %s -l\n",
+            argv[0], argv[0], argv[0], argv[0]);
+    return 1;
+  }
+
+  if (strncmp(argv[1], "-l", 3) == 0) {
+    return ShowLicenses();
+  }
+
+  if (strncmp(argv[1], "-c", 3) == 0) {
+    return CheckMode(argc, argv);
+  }
+
+  if (strncmp(argv[1], "-s", 3) == 0) {
+    if (argc != 3) {
+      goto usage;
+    }
+    size_t bytes = strtol(argv[2], NULL, 10);
+    if (MakeFreeSpaceOnCache(bytes) < 0) {
+      printf("unable to make %ld bytes available on /cache\n", (long)bytes);
+      return 1;
+    } else {
+      return 0;
+    }
+  }
+
+  uint8_t target_sha1[SHA_DIGEST_SIZE];
+
+  const char* source_filename = argv[1];
+
+  // assume that source_filename (eg "/system/app/Foo.apk") is located
+  // on the same filesystem as its top-level directory ("/system").
+  // We need something that exists for calling statfs().
+  char* source_fs = strdup(argv[1]);
+  char* slash = strchr(source_fs+1, '/');
+  if (slash != NULL) {
+    *slash = '\0';
+  }
+
+  if (ParseSha1(argv[2], target_sha1) != 0) {
+    fprintf(stderr, "failed to parse tgt-sha1 \"%s\"\n", argv[2]);
+    return 1;
+  }
+
+  unsigned long target_size = strtoul(argv[3], NULL, 0);
+
+  int num_patches;
+  Patch* patches;
+  if (ParseShaArgs(argc-4, argv+4, &patches, &num_patches) < 0) { return 1; }
+
+  FileContents copy_file;
+  FileContents source_file;
+  const char* source_patch_filename = NULL;
+  const char* copy_patch_filename = NULL;
+  int made_copy = 0;
+
+  if (LoadFileContents(source_filename, &source_file) == 0) {
+    if (memcmp(source_file.sha1, target_sha1, SHA_DIGEST_SIZE) == 0) {
+      // The early-exit case:  the patch was already applied, this file
+      // has the desired hash, nothing for us to do.
+      fprintf(stderr, "\"%s\" is already target; no patch needed\n",
+              source_filename);
+      return 0;
+    }
+
+    const Patch* to_use =
+        FindMatchingPatch(source_file.sha1, patches, num_patches);
+    if (to_use != NULL) {
+      source_patch_filename = to_use->patch_filename;
+    }
+  }
+
+  if (source_patch_filename == NULL) {
+    free(source_file.data);
+    fprintf(stderr, "source file is bad; trying copy\n");
+
+    if (LoadFileContents(CACHE_TEMP_SOURCE, &copy_file) < 0) {
+      // fail.
+      fprintf(stderr, "failed to read copy file\n");
+      return 1;
+    }
+
+    const Patch* to_use =
+        FindMatchingPatch(copy_file.sha1, patches, num_patches);
+    if (to_use != NULL) {
+      copy_patch_filename = to_use->patch_filename;
+    }
+
+    if (copy_patch_filename == NULL) {
+      // fail.
+      fprintf(stderr, "copy file doesn't match source SHA-1s either\n");
+      return 1;
+    }
+  }
+
+  // Is there enough room in the target filesystem to hold the patched file?
+  size_t free_space = FreeSpaceForFile(source_fs);
+  int enough_space = free_space > (target_size * 3 / 2);  // 50% margin of error
+  printf("target %ld bytes; free space %ld bytes; enough %d\n",
+         (long)target_size, (long)free_space, enough_space);
+
+  if (!enough_space && source_patch_filename != NULL) {
+    // Using the original source, but not enough free space.  First
+    // copy the source file to cache, then delete it from the original
+    // location.
+    if (MakeFreeSpaceOnCache(source_file.size) < 0) {
+      fprintf(stderr, "not enough free space on /cache\n");
+      return 1;
+    }
+
+    if (SaveFileContents(CACHE_TEMP_SOURCE, source_file) < 0) {
+      fprintf(stderr, "failed to back up source file\n");
+      return 1;
+    }
+    made_copy = 1;
+    unlink(source_filename);
+
+    size_t free_space = FreeSpaceForFile(source_fs);
+    printf("(now %ld bytes free for source)\n", (long)free_space);
+  }
+
+  FileContents* source_to_use;
+  const char* patch_filename;
+  if (source_patch_filename != NULL) {
+    source_to_use = &source_file;
+    patch_filename = source_patch_filename;
+  } else {
+    source_to_use = &copy_file;
+    patch_filename = copy_patch_filename;
+  }
+
+  // We write the decoded output to "<file>.patch".
+  char* outname = (char*)malloc(strlen(source_filename) + 10);
+  strcpy(outname, source_filename);
+  strcat(outname, ".patch");
+  FILE* output = fopen(outname, "wb");
+  if (output == NULL) {
+    fprintf(stderr, "failed to patch file %s: %s\n",
+            source_filename, strerror(errno));
+    return 1;
+  }
+
+#define MAX_HEADER_LENGTH 8
+  unsigned char header[MAX_HEADER_LENGTH];
+  FILE* patchf = fopen(patch_filename, "rb");
+  if (patchf == NULL) {
+    fprintf(stderr, "failed to open patch file %s: %s\n",
+            patch_filename, strerror(errno));
+    return 1;
+  }
+  int header_bytes_read = fread(header, 1, MAX_HEADER_LENGTH, patchf);
+  fclose(patchf);
+
+  SHA_CTX ctx;
+  SHA_init(&ctx);
+
+  if (header_bytes_read >= 4 &&
+      header[0] == 0xd6 && header[1] == 0xc3 &&
+      header[2] == 0xc4 && header[3] == 0) {
+    // xdelta3 patches begin "VCD" (with the high bits set) followed
+    // by a zero byte (the version number).
+    fprintf(stderr, "error:  xdelta3 patches no longer supported\n");
+    return 1;
+  } else if (header_bytes_read >= 8 &&
+             memcmp(header, "BSDIFF40", 8) == 0) {
+    int result = ApplyBSDiffPatch(source_to_use->data, source_to_use->size,
+                                  patch_filename, output, &ctx);
+    if (result != 0) {
+      fprintf(stderr, "ApplyBSDiffPatch failed\n");
+      return result;
+    }
+  } else {
+    fprintf(stderr, "Unknown patch file format");
+    return 1;
+  }
+
+  fflush(output);
+  fsync(fileno(output));
+  fclose(output);
+
+  const uint8_t* current_target_sha1 = SHA_final(&ctx);
+  if (memcmp(current_target_sha1, target_sha1, SHA_DIGEST_SIZE) != 0) {
+    fprintf(stderr, "patch did not produce expected sha1\n");
+    return 1;
+  }
+
+  // Give the .patch file the same owner, group, and mode of the
+  // original source file.
+  if (chmod(outname, source_to_use->st.st_mode) != 0) {
+    fprintf(stderr, "chmod of \"%s\" failed: %s\n", outname, strerror(errno));
+    return 1;
+  }
+  if (chown(outname, source_to_use->st.st_uid, source_to_use->st.st_gid) != 0) {
+    fprintf(stderr, "chown of \"%s\" failed: %s\n", outname, strerror(errno));
+    return 1;
+  }
+
+  // Finally, rename the .patch file to replace the original source file.
+  if (rename(outname, source_filename) != 0) {
+    fprintf(stderr, "rename of .patch to \"%s\" failed: %s\n",
+            source_filename, strerror(errno));
+    return 1;
+  }
+
+  // If this run of applypatch created the copy, and we're here, we
+  // can delete it.
+  if (made_copy) unlink(CACHE_TEMP_SOURCE);
+
+  // Success!
+  return 0;
+}