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Nunchuk Bitcoin Wallet

Google Play
Latest Release: 1.9.62 7th February 2025
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Our wallet review process

We examine wallets starting at the code level and continue all the way up to the finished app that lives on your device. Provided below is an outline of each of these steps along with security tips for you and general test results.

Developer

Nunchuk Inc

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Custody

Self-custodial: The user holds the keys

As part of our Methodology, we ask: Is the product self-custodial?

The answer is "yes". The user has control of their own keys.
Read more

Source code

Public on github

Released

11th November 2021

Application build

The binary provided was reproducible from the code provided.
See the last Issue we created.

See test result
Tested 17th February 2025

Distribution

Google Play
4.6/5 stars via 26 ratings

Platform notes

On the Google Play Store, there are many apps that have Bitcoin in their name or description but don’t allow the user to use Bitcoin or they don’t look like Bitcoin wallets but turn out to be. We run our tests and document our findings.

Passed all 10 tests

We answered the following questions in this order:

Do many people use this product?

The answer is "yes".
If the answer was "no", we would mark it as "Few users" and the following would apply:

The answer is "no". We marked it as "Few users".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Few users" and the following would apply:

We focus on products that have the biggest impact if things go wrong and this one probably doesn’t have many users according to data publicly available.

Is this product the original?

The answer is "yes".
If the answer was "no", we would mark it as "Fake" and the following would apply:

The answer is "no". We marked it as "Fake".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Fake" and the following would apply:

The bigger wallets often get imitated by scammers that abuse the reputation of the product by imitating its name, logo or both.

Imitating a competitor is a huge red flag and we urge you to not put any money into this product!

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Is it a wallet?

The answer is "yes".
If the answer was "no", we would mark it as "Not a wallet" and the following would apply:

The answer is "no". We marked it as "Not a wallet".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Not a wallet" and the following would apply:

If it’s called “wallet” but is actually only a portfolio tracker, we don’t look any deeper, assuming it is not meant to control funds. What has no funds, can’t lose your coins. It might still leak your financial history!

If you can buy Bitcoins with this app but only into another wallet, it’s not a wallet itself.

Is it for bitcoins?

The answer is "yes".
If the answer was "no", we would mark it as "A wallet but not for Bitcoin" and the following would apply:

The answer is "no". We marked it as "A wallet but not for Bitcoin".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "A wallet but not for Bitcoin" and the following would apply:

At this point we only look into wallets that at least also support BTC.

Can it send and receive bitcoins?

The answer is "yes".
If the answer was "no", we would mark it as "Can't send or receive bitcoins" and the following would apply:

The answer is "no". We marked it as "Can't send or receive bitcoins".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Can't send or receive bitcoins" and the following would apply:

If it is for holding BTC but you can’t actually send or receive them with this product then it doesn’t function like a wallet for BTC but you might still be using it to hold your bitcoins with the intention to convert back to fiat when you “cash out”.

All products in this category are custodial and thus funds are at the mercy of the provider.

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Is the product self-custodial?

The answer is "yes".
If the answer was "no", we would mark it as "Custodial: The provider holds the keys" and the following would apply:

The answer is "no". We marked it as "Custodial: The provider holds the keys".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Custodial: The provider holds the keys" and the following would apply:

A custodial service is a service where the funds are held by a third party like the provider. The custodial service can at any point steal all the funds of all the users at their discretion. Our investigations stop there.

Some services might claim their setup is super secure, that they don’t actually have access to the funds, or that the access is shared between multiple parties. For our evaluation of it being a wallet, these details are irrelevant. They might be a trustworthy Bitcoin bank and they might be a better fit for certain users than being your own bank but our investigation still stops there as we are only interested in wallets.

Products that claim to be non-custodial but feature custodial accounts without very clearly marking those as custodial are also considered “custodial” as a whole to avoid misguiding users that follow our assessment.

This verdict means that the provider might or might not publish source code and maybe it is even possible to reproduce the build from the source code but as it is custodial, the provider already has control over the funds, so it is not a wallet where you would be in exclusive control of your funds.

We have to acknowledge that a huge majority of Bitcoiners are currently using custodial Bitcoin banks. If you do, please:

  • Do your own research if the provider is trust-worthy!
  • Check if you know at least enough about them so you can sue them when you have to!
  • Check if the provider is under a jurisdiction that will allow them to release your funds when you need them?
  • Check if the provider is taking security measures proportional to the amount of funds secured? If they have a million users and don’t use cold storage, that hot wallet is a million times more valuable for hackers to attack. A million times more effort will be taken by hackers to infiltrate their security systems.
The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Is the source code publicly available?

The answer is "yes".
If the answer was "no", we would mark it as "No source for current release found" and the following would apply:

The answer is "no". We marked it as "No source for current release found".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "No source for current release found" and the following would apply:

A wallet that claims to not give the provider the means to steal the users’ funds might actually be lying. In the spirit of “Don’t trust - verify!” you don’t want to take the provider at his word, but trust that people hunting for fame and bug bounties could actually find flaws and back-doors in the wallet so the provider doesn’t dare to put these in.

Back-doors and flaws are frequently found in closed source products but some remain hidden for years. And even in open source security software there might be catastrophic flaws undiscovered for years.

An evil wallet provider would certainly prefer not to publish the code, as hiding it makes audits orders of magnitude harder.

For your security, you thus want the code to be available for review.

If the wallet provider doesn’t share up to date code, our analysis stops there as the wallet could steal your funds at any time, and there is no protection except the provider’s word.

“Up to date” strictly means that any instance of the product being updated without the source code being updated counts as closed source. This puts the burden on the provider to always first release the source code before releasing the product’s update. This paragraph is a clarification to our rules following a little poll.

We are not concerned about the license as long as it allows us to perform our analysis. For a security audit, it is not necessary that the provider allows others to use their code for a competing wallet. You should still prefer actual open source licenses as a competing wallet won’t use the code without giving it careful scrutiny.

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Is the decompiled binary legible?

The answer is "yes".
If the answer was "no", we would mark it as "Obfuscated" and the following would apply:

The answer is "no". We marked it as "Obfuscated".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Obfuscated" and the following would apply:

When compiling source code to binary, usually a lot of meta information is retained. A variable storing a masterseed would usually still be called masterseed, so an auditor could inspect what happens to the masterseed. Does it get sent to some server? But obfuscation would rename it for example to _t12, making it harder to find what the product is doing with the masterseed.

In benign cases, code symbols are replaced by short strings to make the binary smaller but for the sake of transparency this should not be done for non-reproducible Bitcoin wallets. (Reproducible wallets could obfuscate the binary for size improvements as the reproducibility would assure the link between code and binary.)

Especially in the public source cases, obfuscation is a red flag. If the code is public, why obfuscate it?

As obfuscation is such a red flag when looking for transparency, we do also sometimes inspect the binaries of closed source apps.

As looking for code obfuscation is a more involved task, we do not inspect many apps but if we see other red flags, we might test this to then put the product into this red-flag category.

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Can the product be built from the source provided?

The answer is "yes".
If the answer was "no", we would mark it as "Failed to build from source provided!" and the following would apply:

The answer is "no". We marked it as "Failed to build from source provided!".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Failed to build from source provided!" and the following would apply:

Published code doesn’t help much if the app fails to compile.

We try to compile the published source code using the published build instructions into a binary. If that fails, we might try to work around issues but if we consistently fail to build the app, we give it this verdict and open an issue in the issue tracker of the provider to hopefully verify their app later.

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Does the published binary match the published source code?

The answer is "yes".
If the answer was "no", we would mark it as "Not reproducible from source provided" and the following would apply:

The answer is "no". We marked it as "Not reproducible from source provided".

We did not ask this question because we failed at a previous question.
If the answer was "no", we would mark it as "Not reproducible from source provided" and the following would apply:

Published code doesn’t help much if it is not what the published binary was built from. That is why we try to reproduce the binary. We

  1. obtain the binary from the provider
  2. compile the published source code using the published build instructions into a binary
  3. compare the two binaries
  4. 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 binary we can create and the binary we can find for download 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 binary 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.

The product cannot be independently verified. If the provider puts your funds at risk on purpose or by accident, you will probably not know about the issue before people start losing money. If the provider is more criminally inclined he might have collected all the backups of all the wallets, ready to be emptied at the press of a button. The product might have a formidable track record but out of distress or change in management turns out to be evil from some point on, with nobody outside ever knowing before it is too late.
Nunchuk Bitcoin Wallet  App Store

Application build test result

Update for v1.9.62

*** Summary of Differences ***
Contents of diff_armeabi_v7a.txt:
Binary files /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/armeabi_v7a/AndroidManifest.xml and /tmp/test_io.nunchuk.android_1.9.62/fromBuild-unzipped/armeabi_v7a/AndroidManifest.xml differ
Only in /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/armeabi_v7a: META-INF
Only in /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/armeabi_v7a: stamp-cert-sha256

Contents of diff_base.txt:
Binary files /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/base/AndroidManifest.xml and /tmp/test_io.nunchuk.android_1.9.62/fromBuild-unzipped/base/AndroidManifest.xml differ
Binary files /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/base/resources.arsc and /tmp/test_io.nunchuk.android_1.9.62/fromBuild-unzipped/base/resources.arsc differ
Only in /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/base: stamp-cert-sha256

Contents of diff_xhdpi.txt:
Binary files /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/xhdpi/AndroidManifest.xml and /tmp/test_io.nunchuk.android_1.9.62/fromBuild-unzipped/xhdpi/AndroidManifest.xml differ
Only in /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/xhdpi: META-INF
Binary files /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/xhdpi/resources.arsc and /tmp/test_io.nunchuk.android_1.9.62/fromBuild-unzipped/xhdpi/resources.arsc differ
Only in /tmp/test_io.nunchuk.android_1.9.62/fromPlay-unzipped/xhdpi: stamp-cert-sha256

Diffoscope results on the split apks

⛶ ✖

base.apk

  • Only the crashlytics ID differs in resources.arsc
  • The same ID diff is reflected in strings.xml
  • The file-size of the stamp-cert-sha256 file is 32 bytes
⛶ ✖

armeabi.apk

  • As expected <meta-data·android:name="com.android.vending.derived.apk.id"·android:value="6"/> only exists in the Play armeabi_v7a.apk
  • The stamp-cert-sha256 in the Play apk is only 32 bytes
⛶ ✖

base-xhdpi.apk

  • This AndroidManifest.xml line only exists in the Play xhdpi.apk:
<meta-data·android:name="com.android.vending.derived.apk.id"·android:value="6"/>	 
</application>
  • The stamp-cert-sha256 in the Google Play apk is only 32 bytes

Like in the previous review there was a problematic diffoscope result on xhdpi’s resources.arsc

$ diffoscope fromBuild-unzipped/xhdpi/resources.arsc fromPlay-unzipped/xhdpi/resources.arsc 
--- fromBuild-unzipped/xhdpi/resources.arsc
+++ fromPlay-unzipped/xhdpi/resources.arsc
│┄ Format-specific differences are supported for Android package resource table (ARSC) but no file-specific differences were detected; falling back to a binary diff. file(1) reports: Android package resource table (ARSC), 88 string(s), utf8
@@ -548,15 +548,15 @@
 00002230: 6e6f 7469 6669 6361 7469 6f6e 5f62 675f  notification_bg_
 00002240: 6e6f 726d 616c 5f70 7265 7373 6564 0021  normal_pressed.!
 00002250: 216e 6f74 6966 795f 7061 6e65 6c5f 6e6f  !notify_panel_no
 00002260: 7469 6669 6361 7469 6f6e 5f69 636f 6e5f  tification_icon_
 00002270: 6267 0027 2742 6173 652e 5769 6467 6574  bg.''Base.Widget
 00002280: 2e41 7070 436f 6d70 6174 2e44 7261 7765  .AppCompat.Drawe
 00002290: 7241 7272 6f77 546f 6767 6c65 0000 0000  rArrowToggle....
-000022a0: 0202 1000 480c 0000 0800 0000 0e03 0000  ....H...........
+000022a0: 0202 1000 480c 0000 0800 0300 0e03 0000  ....H...........
 000022b0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 000022c0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 000022d0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 000022e0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 000022f0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00002300: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00002310: 0000 0000 0000 0000 0000 0000 0000 0000  ................
@@ -1435,15 +1435,15 @@
 000059a0: 4b00 0000 0800 0003 5000 0000 0800 0000  K.......P.......
 000059b0: 4c00 0000 0800 0003 5100 0000 0800 0000  L.......Q.......
 000059c0: 4d00 0000 0800 0003 5200 0000 0800 0000  M.......R.......
 000059d0: 4e00 0000 0800 0003 5300 0000 0800 0000  N.......S.......
 000059e0: 4f00 0000 0800 0003 5400 0000 0800 0000  O.......T.......
 000059f0: 5000 0000 0800 0003 5500 0000 0800 0000  P.......U.......
 00005a00: 5100 0000 0800 0003 5700 0000 0202 1000  Q.......W.......
-00005a10: 3403 0000 1500 0000 c900 0000 0000 0000  4...............
+00005a10: 3403 0000 1500 0100 c900 0000 0000 0000  4...............
 00005a20: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a30: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a40: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a50: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a60: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a70: 0000 0000 0000 0000 0000 0000 0000 0000  ................
 00005a80: 0000 0000 0000 0000 0000 0000 0000 0000  ................

Explanation

The key difference here is 0000 0000 becoming 0000 0300.

-000022a0: 0202 1000 480c 0000 0800 0000 0e03 0000  ....H...........
+000022a0: 0202 1000 480c 0000 0800 0300 0e03 0000  ....H...........

The second difference is 1500 0000 becoming 1500 0100.

-00005a10: 3403 0000 1500 0000 c900 0000 0000 0000  4...............
+00005a10: 3403 0000 1500 0100 c900 0000 0000 0000  4...............

These are byte-level differences. So we attempted another approach to understand these diffs:

We ran:

aapt2 dump resources built-split_apks/split_config.xhdpi.apk > fromBuild-xhdpi-resources.txt
aapt2 dump resources /var/shared/apk/io.nunchuk.android/1.9.62/split_config.xhdpi.apk  > fromPlay-xhdpi-resources.txt

We then ran diff and diffoscope, with no differing results:

danny@lw10:/tmp/test_io.nunchuk.android_1.9.62$ diff -r fromBuild-xhdpi-resources.txt fromPlay-xhdpi-resources.txt 
danny@lw10:/tmp/test_io.nunchuk.android_1.9.62$ diffoscope fromBuild-xhdpi-resources.txt fromPlay-xhdpi-resources.txt 
danny@lw10:/tmp/test_io.nunchuk.android_1.9.62$

A few observations:

  • When using aapt2, we found no diffs whatsoever.
  • Byte level differences exist when comparing unzipped xhdpi split apks.

We take note of this in another app’s (Bitkey) explanation for the resources.arsc mismatch

resources.arsc mismatch

Until recently, once we normalized the APK names and contents, we could just run diff -r to check for identity. Unfortunately Google Play has changed how they build resources.arsc. From our testing, it seems like they are using a previously reserved byte. When built using bundletool, that byte is always 0, thus making direct comparison using diff impossible.

Since resources are important part of the application, we’re using aapt2 diff to check for differences between APKs from device and from bundletool.

Thus, aapt2 dump resources and diff on its output demonstrated that the actual resources are the same.

While the app is nonverifiable due to the differences observed, these differences are largely benign. The diffs in resources.arsc when not using aapt2 are likely due to variations in the build toolchain.

Version 1.9.62 of Nunchuk Bitcoin Wallet is nonverifiable.

App Description

Nunchuk is a Bitcoin wallet offering advanced multisignature (multisig) functionality, inheritance planning, and robust privacy features. It eliminates single points of failure with multisig setups and provides key recovery options through encrypted cloud backups. The wallet ensures non-custodial control, allowing users to retain full ownership of their Bitcoin. Privacy is emphasized with features like end-to-end encrypted communication and inheritance planning without identity verification.

Additional capabilities include advanced coin control for managing transaction inputs, tagging, and filtering coins, as well as a multi-user multisig wallet for shared Bitcoin management. Nunchuk supports secure collaboration for families or businesses, ensuring that assets can be managed collectively with ease.

Provider’s Own Process

The provider has their own script and instructions for testing the reproducibility of the app.

Tests performed by Leo Wandersleb, Emanuel, mohammad, Daniel Andrei R. Garcia

Previous application build tests

27th January 2025 1.9.59  
15th January 2025 1.9.58  
19th November 2024 1.9.53  
19th November 2024 1.9.53  
5th July 2023 1.9.32  
6th January 2023 1.9.23  
15th December 2021 1.9.21  
Show more

Disclaimer

Our Analysis is not a full code review! We plan to make code reviews available in the future but even then it will never be a stamp of approval but rather a list of incidents and questionable coding practice. Nasa sends probes to space that crash due to software bugs despite a huge budget and stringent scrutiny.

Do your own research

In addition to reading our analysis, it is important to do your own checks. Before transferring any bitcoin to your wallet, look up reviews for the wallet you want to use. They should be easy to find. If they aren't, that itself is a reason to be extra careful.