First draft of OMEMO

[daniele-athome]

A first very draft implementation of OMEMO, trying to fullfil #132.

The OMEMO model (in Smack) requires authentication to be handled tightly with encryption, so our current abstractions and separation between encryption and authentication struggles to work with smack-omemo.

I would rather not tighten Keyring with Coder, instead I would leave it to the higher functions (MessageCenter and UI stuff). Blind Trust Before Verification might be harder to do in a transparent way for the Coder interface, unless it's done in a separate manner (e.g. when receiving key bundles for users, through some listeners. Also PGP trust changes can be a trigger to trust or distrust OMEMO devices).

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OMEMO doesn't support stanza encryption yet (only the body is encrypted), therefore group messages will leak group metadata to servers and to unauthorized parties. Same thing with non-body parts of the message (chat state, attachments). HTTP-uploaded attachments will still be encrypted with OpenPGP.

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The current prototype uses Smack 4.3 which contains an alpha version of the OMEMO modules. The OMEMO API was rewritten in Smack 4.4 which is currently under development and raises Java minimum version to 1.8, which requires Android API 19. Sorry pre-KitKat users :-(

(Some notes on this decision to raise Java and Android versions here: https://discourse.igniterealtime.org/t/smack-android-api-requirements/85767)

[codecov]

Codecov Report

Merging #1256 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##             master   #1256      +/-   ##
===========================================
- Coverage      1.70%   1.68%   -0.02%     
  Complexity      113     113              
===========================================
  Files           359     357       -2     
  Lines         25143   25330     +187     
  Branches       3267    3297      +30     
===========================================
  Hits            428     428              
- Misses        24666   24853     +187     
  Partials         49      49              

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/org/kontalk/MessagesController.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../java/org/kontalk/authenticator/Authenticator.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...ain/java/org/kontalk/client/KontalkConnection.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...main/java/org/kontalk/client/SmackInitializer.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
app/src/main/java/org/kontalk/crypto/Coder.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...p/src/main/java/org/kontalk/crypto/OmemoCoder.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
app/src/main/java/org/kontalk/crypto/PGPCoder.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...pp/src/main/java/org/kontalk/crypto/PGPUserID.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
.../src/main/java/org/kontalk/crypto/PersonalKey.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...n/java/org/kontalk/crypto/PersonalKeyExporter.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
... and 21 more

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