generalised scalar extension

[finiteprods]

Currently, the PET protocol requires that model scalars be in the range [0,1] and that they are essentially public data (in particular, known by the coordinator). This merge request extends the protocol to deal with private scalars, local to participants in a similar way that local models are. In addition, the extension generalises scalars to be arbitrarily large i.e. [0, infinity).

The changes are summarised as follows.

• Removal of existing scalar distribution mechanism. Instead, scalars are now computed on the participant side.
• Adjustment of masking. Masking a model now additionally produces the masked scalar.
• Adjustment of update message. The message payload now additionally contains a masked scalar.
• Aggregation of masked scalars. The coordinator now additionally tracks the aggregation of masked scalars.
• Aggregation of scalar masks. Seeds are now used to additionally compute a scalar mask, aggregated by sum participants.
• Adjustment of sum2 message. The message payload now additionally contains an aggregated scalar mask.
• Adjustment of unmasking. Unmasking now additionally includes that of the scalar.
• Application of correction. An extra step is applied to correctly scale an unmasked model by the unmasked scalar.

[little-dude]

The code changes look good to me. The message parsing tests are going to be annoying to fix, let me know if you need help with that.

[codecov]

Codecov Report

Merging #496 into master will increase coverage by 1.24%.
The diff coverage is 56.42%.

@@            Coverage Diff             @@
##           master     #496      +/-   ##
==========================================
+ Coverage   54.37%   55.61%   +1.24%     
==========================================
  Files          67       67              
  Lines        3187     3231      +44     
==========================================
+ Hits         1733     1797      +64     
+ Misses       1454     1434      -20     

Impacted Files	Coverage Δ
rust/src/client/mobile_client/client.rs	0.00% <0.00%> (ø)
rust/src/client/mobile_client/participant/sum2.rs	17.85% <0.00%> (-3.89%)	⬇️
...ust/src/client/mobile_client/participant/update.rs	38.46% <0.00%> (ø)
rust/src/client/mod.rs	14.91% <0.00%> (+0.74%)	⬆️
rust/src/mask/object/serialization.rs	80.28% <ø> (ø)
rust/src/rest.rs	0.00% <ø> (ø)
rust/src/services/fetchers/mask_length.rs	70.00% <0.00%> (-17.50%)	⬇️
rust/src/services/fetchers/mod.rs	0.00% <0.00%> (ø)
rust/src/services/fetchers/model.rs	70.00% <0.00%> (-17.50%)	⬇️
rust/src/services/fetchers/round_parameters.rs	62.50% <0.00%> (-20.84%)	⬇️
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ef99441...cb66c7d. Read the comment docs.

[Robert-Steiner]

Looks good to me👍
I think it makes sense if we have a ticket to remove the expected_participants field in our code base so that we don't forget this

[little-dude]

Very nice. Let's squash or split this into a few meaningful commits before merging though.

[finiteprods]

Looks good to me+1
I think it makes sense if we have a ticket to remove the expected_participants field in our code base so that we don't forget this

indeed, added this additional task to the follow-up story.