Runtime: Eras in transactions

[gavofyork]

This is to avoid fringe replay attacks on deleted accounts, since when an account is reaped the nonce is discarded, all pre-existing transactions could be replayed if the account becomes funded (it remains to be seen how one could use this to craft an attack in practice, but nonetheless remains an open question).

To avoid it, we ensure transactions are signed with a recent headerhash to identify them with a single era. The recentness of the hash can be adjusted to give a tradeoff between safety (i.e. how long can pass before the transaction becomes inherently un(re)playable) and liveness (i.e. how long it can float around off-chain and still remain valid).

What to do

Add an 8-bit version identifier on to the beginning of the transaction. That should be set to 0.

Append a 16-bit Age onto the inner Transaction format:

<CheckPoint (12 bits)><LogDuration (4 bits)>

This comes alongside a second requirement that the signed Transaction format (i.e. the data that is signed) have an additional hash field, which must be the hash of the correct header of the block number B given the current block number N:

B = quantize(N + C, D) - C

where

quantize(num, quot) := floor(num / quot) * quot

and

D = 2 ** (Duration + 2)

and

C = CheckPoint * (LogDuration + 2**12-1) / 2**12

This requirement fulfils three goals:

• The transaction is valid only for a range of blocks;
• that range of blocks itself has a wide variance from the very small (4 blocks) to very large (131,072 blocks);
• it adds a negligible additional payload into the transaction;
• it is only valid on a particular fork of the chain.

[gavofyork]

@rphmeier comments?

[gitcoinbot]

Issue Status: 1. Open 2. Started 3. Submitted 4. Done

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