Do your own research!
Try out searching for "lost bitcoins", "stole my money" or "scammers" together with the wallet's name, even if you think the wallet is generally trustworthy. For all the bigger wallets you will find accusations. Make sure you understand why they were made and if you are comfortable with the provider's reaction.
The Analysis ¶
Update: After feedback from the developer we removed the mention of
disable 'TrulyRandom', "GoogleAppIndexingWarning", added more files to the list of worry-some binaries and hope for feedback regarding App Bundles as this appears to be the first Bitcoin Wallet we came across that uses this new format promoted by Google.
This app is an open source paper wallet generator but can we reproduce the build
3.2.0 from Google Play?
$ git clone https://github.com/ValleZ/Paper-Wallet $ cd Paper-Wallet/ $ git tag | grep 3.... 3.0.0
not good but
$ git log -n 1 commit 06b3e50e798bb2219b327dc4bba636343ca9f678 (HEAD -> master, origin/master, origin/HEAD) Author: Valentin Konovalov <firstname.lastname@example.org> Date: Wed Mar 25 20:46:53 2020 -0400 3.2.0
So the last commit had the label
3.2.0 which looks like the author just did not tag it. Let’s see …
As there are no build instructions, we took the freedom and looked around a bit. Normally we do not provide code reviews and our findings are so blatant that they might actually be irrelevant upon deeper inspection but just in case this app turns out to be reproducible/reproducible, the following two items require a deeper look:
- This binary blob is not accounted for. Where does it come from? Can it be reproduced?
- The developer pointed us to more binary blobs in these folders, too.
Anyway … back to reproducibility:
$ docker run -it --volume $PWD:/mnt --workdir /mnt --rm beevelop/cordova bash root@971258b281bb:/mnt# yes | /opt/android/tools/bin/sdkmanager "build-tools;29.0.3" root@971258b281bb:/mnt# ./gradlew :app:assemble BUILD SUCCESSFUL in 6m 18s 50 actionable tasks: 50 executed root@971258b281bb:/mnt# exit $ apktool d -o fromBuild app/build/outputs/apk/release/app-release-unsigned.apk $ apktool d -o fromGoogle "/path/to/Paper Wallet 3.2.0 (ru.valle.btc).apk" $ diff --recursive --brief from* Files fromBuild/AndroidManifest.xml and fromGoogle/AndroidManifest.xml differ Files fromBuild/apktool.yml and fromGoogle/apktool.yml differ Only in fromBuild: lib Files fromBuild/original/AndroidManifest.xml and fromGoogle/original/AndroidManifest.xml differ Only in fromGoogle/original/META-INF: CERT.RSA Only in fromGoogle/original/META-INF: CERT.SF Files fromBuild/original/META-INF/MANIFEST.MF and fromGoogle/original/META-INF/MANIFEST.MF differ Only in fromBuild/res: drawable-hdpi Only in fromBuild/res: drawable-ldpi Only in fromBuild/res: drawable-mdpi Only in fromBuild/res: drawable-night-hdpi Only in fromBuild/res: drawable-night-ldpi Only in fromBuild/res: drawable-night-mdpi Only in fromBuild/res: drawable-night-xhdpi Only in fromBuild/res: drawable-night-xxhdpi Only in fromBuild/res: drawable-night-xxxhdpi Only in fromBuild/res: drawable-xhdpi Only in fromBuild/res: drawable-xxhdpi Only in fromBuild/res: drawable-xxxhdpi Files fromBuild/res/layout/main.xml and fromGoogle/res/layout/main.xml differ Files fromBuild/res/layout-v22/main.xml and fromGoogle/res/layout-v22/main.xml differ Files fromBuild/res/menu/main.xml and fromGoogle/res/menu/main.xml differ Files fromBuild/res/menu-v11/main.xml and fromGoogle/res/menu-v11/main.xml differ Files fromBuild/res/values/public.xml and fromGoogle/res/values/public.xml differ Only in fromGoogle/res: values-anydpi-v21 Only in fromBuild/res: values-ja Only in fromBuild/res: values-pt Only in fromBuild/res: values-ru Only in fromGoogle/res/xml: splits0.xml
That is a big diff but on a closer look after the developer told us this was the result of an App Bundle it actually does not look that bad:
Extra “stuff” is mainly in what we built, not in the app we got from Google Play. Let’s remove that and the other known benign files from the list:
$ diff --recursive --brief from* | grep -v "Only in fromBuild" | grep -v "META-INF" | grep -v apktool Files fromBuild/AndroidManifest.xml and fromGoogle/AndroidManifest.xml differ Files fromBuild/original/AndroidManifest.xml and fromGoogle/original/AndroidManifest.xml differ Files fromBuild/res/layout/main.xml and fromGoogle/res/layout/main.xml differ Files fromBuild/res/layout-v22/main.xml and fromGoogle/res/layout-v22/main.xml differ Files fromBuild/res/menu/main.xml and fromGoogle/res/menu/main.xml differ Files fromBuild/res/menu-v11/main.xml and fromGoogle/res/menu-v11/main.xml differ Files fromBuild/res/values/public.xml and fromGoogle/res/values/public.xml differ Only in fromGoogle/res: values-anydpi-v21 Only in fromGoogle/res/xml: splits0.xml
Now looking into some of those diffs they look harmless. UI components with different labels. An extra
res/xml/splits0.xml for example:
$ cat fromGoogle/res/values-anydpi-v21/drawables.xml <?xml version="1.0" encoding="utf-8"?> <resources> <item type="drawable" name="APKTOOL_DUMMY_0">false</item> </resources> $ cat fromGoogle/res/xml/splits0.xml $ cat fromGoogle/res/xml/splits0.xml <?xml version="1.0" encoding="utf-8"?> <splits> <module name=""> <language> <entry key="ja" split="config.ja" /> <entry key="pt" split="config.pt" /> <entry key="ru" split="config.ru" /> </language> </module> </splits>
All that looks like it is following some automatism by Google but until we have a deterministic way of reproducing this, can we not reproduce this build and don’t feel comfortable dismissing the possibility of missing something. After all,
Binary files fromBuild/original/AndroidManifest.xml and fromGoogle/original/AndroidManifest.xml differ
does look a bit scary.
In summary, this app is not verifiable.
For later reference, the app from Google Play had the sha256sum
ce90b2c62cae520a0f643a34b5a2a2a6b6961d5d194d06b07c21f2dd22748dea and the use of App Bundle can be detected in
$ cat fromGoogle/original/META-INF/MANIFEST.MF | grep Built-By Built-By: BundleTool
We could not verify that the provided code matches the binary!
As part of our Methodology, we ask:Is the published binary matching the published source code? If not, we tag it
Published code doesn’t help much if it is not what the published app was built from. That is why we try to reproduce the binary. We
- obtain the binary from the provider
- compile the published source code using the published build instructions into a binary
- compare the two binaries
- we might spend some time working around issues that are easy to work around
If this fails, we might search if other revisions match or if we can deduct the source of the mismatch but generally consider it on the provider to provide the correct source code and build instructions to reproduce the build, so we usually open a ticket in their code repository.
In any case, the result is a discrepancy between the app we can create and the app we can find on the app store and any discrepancy might leak your backup to the server on purpose or by accident.
As we cannot verify that the source provided is the source the app was compiled from, this category is only slightly better than closed source but for now we have hope projects come around and fix verifiability issues.
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