android reproducibility: add Signal's "apkdiff.py" tool, and instructions

contrib/android/Readme.md

@@ -56,6 +56,9 @@ folder.
         ./contrib/android/make_apk
     ```
     
+    Note: this builds a debug apk. `make_apk` takes an optional parameter
+    which can be either `release` or `release-unsigned`.
+    
     This mounts the project dir inside the container,
     and so the modifications will affect it, e.g. `.buildozer` folder
     will be created.
@@ -63,6 +66,25 @@ folder.
 5. The generated binary is in `./bin`.
 
 
+## Verifying reproducibility and comparing against official binary
+
+Every user can verify that the official binary was created from the source code in this 
+repository.
+
+1. Build your own binary as described above.
+   Make sure you don't build in `debug` mode (which is the default!),
+   instead use either of `release` or `release-unsigned`.
+   If you build in `release` mode, the apk will be signed, which requires a keystore
+   that you need to create manually (see source of `make_apk` for an example).
+2. Note that the binaries are not going to be byte-for-byte identical, as the official
+   release is signed by a keystore that only the project maintainers have.
+   You can use the `apkdiff.py` python script (written by the Signal developers) to compare
+   the two binaries.
+    ```
+    $ python3 contrib/android/apkdiff.py Electrum_apk_that_you_built.apk Electrum_apk_official_release.apk
+    ```
+   This should output `APKs match!`.
+
 
 ## FAQ
 

contrib/android/apkdiff.py

@@ -0,0 +1,78 @@
+#! /usr/bin/env python3
+# from https://github.com/signalapp/Signal-Android/blob/2029ea378f249a70983c1fc3d55b9a63588bc06c/reproducible-builds/apkdiff/apkdiff.py
+
+import sys
+from zipfile import ZipFile
+
+class ApkDiff:
+    IGNORE_FILES = ["META-INF/MANIFEST.MF", "META-INF/CERT.RSA", "META-INF/CERT.SF"]
+
+    def compare(self, sourceApk, destinationApk):
+        sourceZip      = ZipFile(sourceApk, 'r')
+        destinationZip = ZipFile(destinationApk, 'r')
+
+        if self.compareManifests(sourceZip, destinationZip) and self.compareEntries(sourceZip, destinationZip) == True:
+            print("APKs match!")
+        else:
+            print("APKs don't match!")
+
+    def compareManifests(self, sourceZip, destinationZip):
+        sourceEntrySortedList      = sorted(sourceZip.namelist())
+        destinationEntrySortedList = sorted(destinationZip.namelist())
+
+        for ignoreFile in self.IGNORE_FILES:
+            while ignoreFile in sourceEntrySortedList: sourceEntrySortedList.remove(ignoreFile)
+            while ignoreFile in destinationEntrySortedList: destinationEntrySortedList.remove(ignoreFile)
+
+        if len(sourceEntrySortedList) != len(destinationEntrySortedList):
+            print("Manifest lengths differ!")
+
+        for (sourceEntryName, destinationEntryName) in zip(sourceEntrySortedList, destinationEntrySortedList):
+            if sourceEntryName != destinationEntryName:
+                print("Sorted manifests don't match, %s vs %s" % (sourceEntryName, destinationEntryName))
+                return False
+
+        return True
+
+    def compareEntries(self, sourceZip, destinationZip):
+        sourceInfoList      = list(filter(lambda sourceInfo: sourceInfo.filename not in self.IGNORE_FILES, sourceZip.infolist()))
+        destinationInfoList = list(filter(lambda destinationInfo: destinationInfo.filename not in self.IGNORE_FILES, destinationZip.infolist()))
+
+        if len(sourceInfoList) != len(destinationInfoList):
+            print("APK info lists of different length!")
+            return False
+
+        for sourceEntryInfo in sourceInfoList:
+            for destinationEntryInfo in list(destinationInfoList):
+                if sourceEntryInfo.filename == destinationEntryInfo.filename:
+                    sourceEntry      = sourceZip.open(sourceEntryInfo, 'r')
+                    destinationEntry = destinationZip.open(destinationEntryInfo, 'r')
+
+                    if self.compareFiles(sourceEntry, destinationEntry) != True:
+                        print("APK entry %s does not match %s!" % (sourceEntryInfo.filename, destinationEntryInfo.filename))
+                        return False
+
+                    destinationInfoList.remove(destinationEntryInfo)
+                    break
+
+        return True
+
+    def compareFiles(self, sourceFile, destinationFile):
+        sourceChunk      = sourceFile.read(1024)
+        destinationChunk = destinationFile.read(1024)
+
+        while sourceChunk != b"" or destinationChunk != b"":
+            if sourceChunk != destinationChunk:
+                return False
+
+            sourceChunk      = sourceFile.read(1024)
+            destinationChunk = destinationFile.read(1024)
+
+        return True
+
+if __name__ == '__main__':
+    if len(sys.argv) != 3:
+        print("Usage: apkdiff <pathToFirstApk> <pathToSecondApk>")
+        sys.exit(1)
+
+    ApkDiff().compare(sys.argv[1], sys.argv[2])