[tbb-bugs] #26536 [Applications/Tor Browser]: Create APK signing keys

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#26536: Create APK signing keys
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 Reporter:  sysrqb                    |          Owner:  tbb-team
     Type:  task                      |         Status:  needs_information
 Priority:  Medium                    |      Milestone:
Component:  Applications/Tor Browser  |        Version:
 Severity:  Normal                    |     Resolution:
 Keywords:  tbb-mobile                |  Actual Points:
Parent ID:  #26531                    |         Points:
 Reviewer:                            |        Sponsor:
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Changes (by sysrqb):

 * status:  new => needs_information


Comment:

 It appears we can create and store the key offline.

 I was hoping we could create an "identity" key and a "signing" key for
 Android, but it seems like this won't work. Specifically,
 [[https://source.android.com/security/apksigning/v2|newer versions]] of
 Android support signing an app where the public key for verifying the
 signature is stored in two places. The first place is at the end of the
 signing block. This key has only one purpose - for verifying the signing
 block signatures are valid. The second place is the public key is stored
 within the signing block but here we may include a certificate chain. I
 was hoping we could create a long-term identity key and then a short-term
 signing keys, similar to PGP primary key and subkeys. However, from my
 code diving, Android does not verify the certificate chain embedded in the
 app. Android only verifies the
 [[https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/util/apk/ApkSignatureSchemeV2Verifier.java#376|first
 (leaf) certificate]] in the embedded certificate chain contains the same
 public key as the public key provided at the end of the signing block used
 for verifying the signature.

 We should generate the key offline - Hans published a nice script for this
 (although its a little old) https://github.com/guardianproject/smartcard-
 apk-signing/blob/master/openssl-gen/gen.sh

 We can use a Yubikey or Nitrokey for storing the key. I'll feel more
 comfortable if we have more than one copy of the key.

 Newer versions of Android support something called
 [[https://android.googlesource.com/platform/frameworks/base/+/master/services/core/java/com/android/server/pm/PackageManagerService.java#17745|(upgrade)
 keysets]] for verifying the apps authenticity. I'm not sure how we can use
 it yet. I think it allows for adding more signatures using more keys, but
 I'm not sure if there's a way we can use it for rotating keys.

 With all this being said, we can likely generate our first APK signing key
 using a similar method as the Tor Browser PGP signing key - using an
 offline laptop booted with TAILS, etc.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26536#comment:1>
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