Enhance the BLS library to be fit-for-purpose

[paulhauner]

Presently the code-base is missing the following features for BLS signatures and public keys:

1. Support for aggregate pub keys. Presently there is support for aggregate signatures, but not aggregate public keys, meaning each pubkey needs to be individually checked against the aggregate sig.
2. Ability to be cloned (i.e., they don't have the Clone trait)
3. Ability to be encoded to some primitive then have that be decoded back to a proper struct. For example, writing to a byte-array/integer then reading back again.
4. Closely related to (and dependent on) (2), they need to support RLP encoding and decoding. This probably requires the EF researchers to pick a format for this.

With regards to (1), I'm not even certain if aggregating the keys then verifying them even gives a better result. I.e., is it actually quicker to add them all together then verify once, or is it quicker to just verify them all individually? I'm presently assuming the latter is quicker but I don't know for sure.

[paulhauner]

The ability to clone a PublicKey was added to the SigP BLS repo here:

sigp/bls@cc0c75c

We likely don't need to implement clone for SecretKey/KeyPair.

[paulhauner]

Some resources for you @AgeManning:

• ethereum/beacon_chain implemenation: https://github.com/ethereum/beacon_chain/blob/master/beacon_chain/utils/bls.py
• Our fork of str4d's BLS lib: https://github.com/sigp/bls
• Some other implementation (which I have no reason to trust): https://github.com/lovesh/signature-schemes

Note: tests take ages to run on the BLS lib. Use cargo's pattern matching to skip the long tests or drop the 0..500 for loops down to something more reasonable.

[paulhauner]

There's some interesting stuff going on here:

https://github.com/sigp/bls/blob/cc0c75cdf12ac46454a1be406e23e2f3e8dc99df/src/lib.rs#L97-L111

It seems to require a different message for each public key.

[paulhauner]

Issue is no longer current -- this can be addressed in the future.