Add Secure Aggregation functionalities

[panh99]

Issue

Description

The current Flower framework does not have intrinsic support for Secure Aggregation (SA).

Related issues/PRs

This PR is copied from #2132 , to change the source branch to secaggplus in adap/flower other than the forked repo.

This PR is based on the following PRs:
Add SecureAggregation field to Task message #1893
Adopt TaskIns/TaskRes-based communications on the client side #2007
Update serde.py for Secure Aggregation #2008
Update validator.py for SA #2151
Add PyCryptoDome and Cryptograhpy dependencies #2154

Proposal

Explanation

> Structure

Add the following python files and modules for SA functionalities.

├── client
|   └── secure_aggregation
|       ├── __init__.py
|       ├── handler.py
|       └── secaggplus_handler.py
└── common
    └── secure_aggregation
        ├── __init__.py
        ├── quantization.py
        ├── secaggplus_constants.py
        ├── secaggplus_utils.py
        ├── weights_arithmetic.py
        └── crypto
            ├── __init__.py
            ├── shamir.py
            └── symmetric_encryption.py

> Message Handler for the SecAgg+ protocol

Add abstract class SecureAggregationHandler in client/secure_aggregation/handler.py. To enable SA, a client class can inherit a subclass of SecureAggregationHandler.

Add subclass SecAggPlusHandler in client/secure_aggregation/secaggplus_handler.py for the SecAgg+ protocol.

> Utils

Modify message_handler.py to route a Task message to SA message handler if its sa field is set.

Add basic quantization utils in common/secure_aggregation/quantization.py

Add weights operations in common/secure_aggregation/weights_arithmetic.py

Add wrapper functions for Shamir's secret sharing APIs from PyCryptoDome in common/secure_aggregation/crypto/shamir.py

Add wrapper functions for ECDH key agreement and symmetric encryption APIs from Cryptography in common/secure_aggregation/crypto/symmetric_encryption.py

> Constants

Constants for the secagg/secagg+ protocols are all stored in common/secure_aggregation/secaggplus_constants.py.

Blocker

The PyCryptoDome package and the Cryptography package are not in the dependencies.

No blockers

Checklist

• Implement proposed change
• Write tests
• Update documentation
• Update changelog
• Make CI checks pass
• Ping maintainers on Slack (channel #contributions)

Any other comments?

[danieljanes]

Lgtm! 🎉