Implement Credential transfer API (CXF-based credentials import / export)

[Dunexus]

Checks

• I have read the Wiki, searched the open issues, and still think this is a new feature.

Explain the problem clearly and succinctly:

Migration from a password manager to another is tedious and may imply exporting / importing passwords in unencrypted files.

Describe the solution you'd like:

A new Android API, Credential transfer API, has been created to facilitate import / export of credentials from one password manager to another using the FIDO Credential Exchange Format (CXF) standard.

It may help people using other password manager software to migrate to KeePassDX.

Describe alternatives you've considered:

No response

Additional context:

No response

[J-Jamet]

Linked to #2598 #2612 #2579 (reply in thread) #505 #2131

https://fidoalliance.org/specs/cx/cxf-v1.0-ps-errata-20260309.html
keepassxreboot/keepassxc#11363

This will be a must-have feature. Historically, users have wanted a migration system using unsecure CSV, but the CXF protocol is clearly the best solution.

[J-Jamet]

It's great that there's a library that handles CXF, in practice, it requires:

• Android API 24 / Android Nougat for credentials-providerevents: This means a migration will be necessary, it's mandatory. KeePassDX currently supports min Android API 19 / KitKat. I was able to minimize the update gap by upgrading to Android API 21 / Lollipop using the overrideLibrary and filters Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP. This also allows to switch UI to Compose, which will be available starting version 4.6.0 of KeePassDX.

    <uses-sdk tools:overrideLibrary="
            androidx.credentials.providerevents,
            androidx.credentials"
        />

• Refactor the data mapping system to handle types and extensions: https://developer.android.com/identity/sign-in/credential-transfer#supported-credential-types It would have been nice to handle the templates better at the same time, we'll have to see if that can be easily incorporated into the migration.
  We will need to create data models based on the CXF type format and then use the new mapping. https://fidoalliance.org/specs/cx/cxf-v1.0-ps-errata-20260309.html#dict-person-name
  There will be extensive use of custom fields, so we'll need to add a layer of abstraction to the database to manage entities separately. This has already been implemented with OTP and Passkey, but it will need to be improved for all models.
• Implement the import and export functions, the Google documentation seems fine.
• Think about designing a suitable UI for importing and exporting data, with a filtering system by entry and by group. We have the advantage of a tree-based structure, which will make our lives easier. I’ve already thought this for a secure export system (KSS) between KeePass clients. I’ll give a presentation when it’s ready.

It will still require a lot of work, but it's good that the library is already available. We'll see what challenges come up as we go along, but I'm confident.