diff --git a/lib/xdelta3/xdelta3-internal.h b/lib/xdelta3/xdelta3-internal.h
index 0c6a1bb..f2cf6e2 100644
--- a/lib/xdelta3/xdelta3-internal.h
+++ b/lib/xdelta3/xdelta3-internal.h
@@ -37,7 +37,7 @@ int main_file_read (main_file  *ifile,
 		    size_t     size,
 		    size_t    *nread,
 		    const char *msg);
-int main_file_write (main_file *ofile, uint8_t *buf, 
+int main_file_write (main_file *ofile, uint8_t *buf,
 		     usize_t size, const char *msg);
 void* main_malloc (size_t size);
 void main_free (void *ptr);
diff --git a/src/errorAndExit.c b/src/errorAndExit.c
index f0493a3..1af7287 100644
--- a/src/errorAndExit.c
+++ b/src/errorAndExit.c
@@ -5,7 +5,7 @@
 
 void errorAndExit() {
   fprintf(stderr, "\nUsage:\n");
-  fprintf(stderr, "  ./patcher-installer --disk main.z01 --offset 456z --size 1000 [--keys 123,123,123] [--prev oldfile.tor]\n");
+  fprintf(stderr, "  ./patcher-installer --disk main.z01 --offset 456z --size 1000 [--keys 123,456,789] [--previous oldfile.tor]\n");
   fprintf(stderr, "Options:\n");
   fprintf(stderr, "  -d, --disk: The path to the disk file that contains the file we want to extract.\n");
   fprintf(stderr, "  -o, --offset: The offset into the disk, given in bytes, where the local file header of the file we want to extract starts.\n");
diff --git a/src/main.c b/src/main.c
index 7ab3627..eff7cf8 100644
--- a/src/main.c
+++ b/src/main.c
@@ -5,12 +5,13 @@
 #include <unistd.h>
 
 //Import our code
+#include "utils/min.h"
 #include "decrypt.h"
 #include "errorAndExit.h"
 #include "fileReader.h"
 #include "inflate.h"
 #include "parseArguments.h"
-#include "utils/min.h"
+#include "xdelta3.h"
 
 //The size of the buffers where the compressed and uncompressed data is stored
 #define BUFFER_SIZE 512UL * 1024UL //512 KiB
@@ -75,6 +76,11 @@ int main(int argc, char *argv[]) {
     decrypt(compressedChunk, ENCRYPTION_HEADER_LENGTH);
   }
 
+  //Initialize xdelta3
+  if (state.prevFile) {
+    xdelta3Init(state.prevFile);
+  }
+
   //-------------------------------------------------
 
   struct InflateOutput inflateResult;
@@ -106,19 +112,17 @@ int main(int argc, char *argv[]) {
 
     //important: we must not modify uncompressedChunk since miniz may use it as dictionary and read from it during the next invocation of inflateInflate()
 
-    //Write to stdout
-    write(1, uncompressedChunk + uncompressedPosition, inflateResult.numBytesWrittenToOutput);
+    //Optionally perform xdelta3
+    if (state.prevFile) {
+      xdelta3AddInput(uncompressedChunk + uncompressedPosition, inflateResult.numBytesWrittenToOutput, remainingBytes == 0 && hasReachedEnd);
+    } else { //otherwise write to stdout
+      write(1, uncompressedChunk + uncompressedPosition, inflateResult.numBytesWrittenToOutput);
+    }
+
     uncompressedPosition += inflateResult.numBytesWrittenToOutput;
     while (uncompressedPosition >= BUFFER_SIZE) {
       uncompressedPosition -= BUFFER_SIZE;
     }
-
-    //Optionally perform xdelta3
-    if (state.prevFile) {
-      //TODO
-      fprintf(stderr, "xdelta3 is not yet implemented.\n");
-      errorAndExit();
-    }
   }
 
   //release memory
diff --git a/src/xdelta3.c b/src/xdelta3.c
index 9a8a2f7..b66cde8 100644
--- a/src/xdelta3.c
+++ b/src/xdelta3.c
@@ -2,7 +2,107 @@
 #define SIZEOF_SIZE_T 4
 #define static_assert(e,m) /* do nothing */
 #define XD3_ENCODER 0
+#define XD3_USE_LARGESIZET 0
 typedef unsigned int usize_t;
 typedef unsigned long long xoff_t;
+//Include xdelta3
+#include "../lib/xdelta3/xdelta3.c"
 
-#include "../lib/xdelta3/xdelta3.h"
+//Include other libraries
+#include <stdbool.h>
+#include <unistd.h>
+
+//Include our own files
+#include "errorAndExit.h"
+#include "xdelta3.h"
+
+//Various variables for Xdelta3
+xd3_stream stream;
+xd3_config config;
+xd3_source source;
+//The previous version of the file we want to extract
+FILE* srcFile;
+//Total number of bytes written to output
+unsigned long totalOut = 0UL;
+
+/** Initializes xdelta3 with the given previous file */
+void xdelta3Init(char* prevFile) {
+  srcFile = fopen(prevFile, "rb");
+  if (srcFile == NULL) {
+    fprintf(stderr, "Could not open previous file %s for reading during xdelta3.\n", prevFile);
+    errorAndExit();
+  }
+
+  xd3_init_config(&config, XD3_ADLER32);
+  config.winsize = XD3_ALLOCSIZE;
+  xd3_config_stream(&stream, &config);
+  //Set source (= old file from previous release)
+  source.blksize = XD3_ALLOCSIZE;
+  source.curblk = malloc(source.blksize);
+  if (source.curblk == NULL) {
+    fprintf(stderr, "ERROR: Could not allocate source.blksize in VCDIFF.\n");
+    errorAndExit();
+  }
+
+  fseek(srcFile, 0, SEEK_SET);
+  source.onblk = fread((void*)source.curblk, 1, source.blksize, srcFile);
+  source.curblkno = 0;
+  xd3_set_source(&stream, &source);
+  //Set input (= xdelta file from current release)
+  xd3_set_flags(&stream, XD3_FLUSH | stream.flags);
+}
+
+
+/** Supplies the given input buffer to xdelta3 and undiffs the data. */
+void xdelta3AddInput(unsigned char* inputBuffer, unsigned int inputLength, bool isLastPart) {
+  xd3_avail_input(&stream, inputBuffer, inputLength);
+
+  int returnValue;
+
+process:
+  returnValue = xd3_decode_input(&stream);
+  switch (returnValue) {
+    case XD3_INPUT: {
+      //fprintf(stderr, "XD3_INPUT\n");
+      break;
+    }
+    case XD3_OUTPUT: {
+      //fprintf(stderr, "XD3_OUTPUT\n");
+      totalOut += stream.avail_out;
+      write(1, stream.next_out, stream.avail_out);
+      xd3_consume_output(&stream);
+      goto process;
+    }
+    case XD3_GETSRCBLK: {
+      //fprintf(stderr, "XD3_GETSRCBLK\n");
+      fseek(srcFile, source.blksize * source.getblkno, SEEK_SET);
+      source.onblk = fread((void*)source.curblk, 1, source.blksize, srcFile);
+      source.curblkno = source.getblkno;
+      goto process;
+    }
+    case XD3_GOTHEADER: {
+      //fprintf(stderr, "XD3_GOTHEADER\n");
+      goto process;
+    }
+    case XD3_WINSTART: {
+      //fprintf(stderr, "XD3_WINSTART\n");
+      goto process;
+    }
+    case XD3_WINFINISH: {
+      //fprintf(stderr, "XD3_WINFINISH\n");
+      goto process;
+    }
+    default: {
+      fprintf(stderr, "ERROR: !!! INVALID %s %d !!!\n", stream.msg, returnValue);
+      errorAndExit();
+    }
+  }
+
+  //If we are done, free up memory
+  if (isLastPart) {
+    free((void*)source.curblk);
+    xd3_close_stream(&stream);
+    xd3_free_stream(&stream);
+    fclose(srcFile);
+  }
+}
diff --git a/src/xdelta3.h b/src/xdelta3.h
new file mode 100644
index 0000000..20e7014
--- /dev/null
+++ b/src/xdelta3.h
@@ -0,0 +1,7 @@
+#pragma once
+
+/** Initializes xdelta3 with the given previous file */
+void xdelta3Init(char* prevFile);
+
+/** Supplies the given input buffer to xdelta3 and undiffs the data. */
+void xdelta3AddInput(unsigned char* inputBuffer, unsigned int inputLength, bool isLastPart);