Doc where extra gas is coming from for privacy marker transaction

[MadelineMurray]

The public transaction gas usage is calculated in the following way:

Gas usage = TX_BASE_COST + (TX_DATA_ZERO * NO_OF_ZEROS) + (TX_DATA_NON_ZERO * NO_OF_NON_ZEROS)

where,

TX_BASE_COST = 21000
TX_DATA_ZERO = 4
NO_OF_ZEROS = number of "00" in the hex payload of the transaction
TX_DATA_NON_ZERO = 68 pre-istanbul / 16 post-istanbul (as in istanbul hard fork)
NO_OF_NON_ZEROS = number of non-"00" in the hex payload of the transaction i.e. number of pairs in the hex that are not "00"

The PMT has 32 bytes of non-zero hex. It therefore consumes:

Gas usage of PMT = 21000 + (4 * 0) + (68 * 32) = 21000 + 2176 = 23176

Post-Instanbul the PMT will consume:

Gas usage of PMT post-istanbul = 21000 + (4 * 0) + (16 * 32) = 21000 + 512 = 21512

Invoking the Privacy Precompiled Contract consumes 0 gas.

More info: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2028.md

Comments from the original JI https://jira.hyperledger.org/browse/BESU-75:

Madeline Murray - Need to confirm where the extra 2992 is coming from 
 
Madeline Murray - Document where extra gas is coming from.

Also update in Besu and web3js, and web3j
  
Ivaylo Kirilov September 16, 20the changes required in Besu are documentation related only
 
Ivaylo Kirilov September 17, 2019, 12:14 AM
 
fixed in web3j - hyperledger/web3j#1029

[bgravenorst]

This issue must also address, "Doc that gas price on private transaction passed through privacy marker transaction"

See: #289

[MadelineMurray]

Putting this back on the pile until we finish privacy interop.