Cheap way to attack blockchain

amaclin (OP)

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Cheap way to attack blockchain

August 31, 2015, 07:58:03 AM
Last edit: August 31, 2015, 08:12:51 AM by amaclin

#1

Seems to me that I know new way to attack & flood bitcoin network.

The last attacks were based on filling the blocks with transactions.
This is because of limit of block size. (Consensus rule that the blocksize is below 1mb)

But there are another limits for block which can not be changed without hard fork.

There is a limit of SIGOPS in transactions included to a block.

consensus.h

Code:

/** The maximum allowed size for a serialized block, in bytes (network rule) */
static const unsigned int MAX_BLOCK_SIZE = 1000000;
/** The maximum allowed number of signature check operations in a block (network rule) */
static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;

So, MAX_BLOCK_SIGOPS is 20000

How does the client calculate the number of SIGOPS? Let us look to the sources.

main.cpp

Code:

            if (fStrictPayToScriptHash)
            {
                // Add in sigops done by pay-to-script-hash inputs;
                // this is to prevent a "rogue miner" from creating
                // an incredibly-expensive-to-validate block.
                nSigOps += GetP2SHSigOpCount(tx, view);
                if (nSigOps > MAX_BLOCK_SIGOPS)
                    return state.DoS(100, error("ConnectBlock(): too many sigops"),
                                     REJECT_INVALID, "bad-blk-sigops");
            }

Miner node includes transactions to a block while the nSigOps not exceeds 20000.
The block with nSigOps > 20000 will be invalid (consensus rule) and will be rejected by all other nodes.

Now let us look the transaction
https://blockchain.info/tx/6766e75d6166a0a14bd814921d0f903285e15779e648d7ec52a4f7c0868ec07d
and calculate the number of SIGOPS in it

All input scripts are redeeming from p2sh-outputs with the inner scripts build on the same template:

Code:

OP_0 
OP_IF
  OP_15
  OP_CHECKMULTISIG
OP_ENDIF
OP_SMALLINTEGER

The number of SIGOPS in this small script is 15 (this is maximum value to pass IsStandard)
And the total number of SIGOPS in 6766e75d6166a0a14bd814921d0f903285e15779e648d7ec52a4f7c0868ec07d is 15 * 15 = 225

So, the maximum number of such transactions in one block is only 88 (because floor ( 20000 / 225 ) = 88)
And inserting 88 such transactions in one block leaves only 200 SIGOPS for regular transactions.
Which leaves a room only for ~100 transactions in block for other persons

The attack vector should be:
1) create and fund a big number of such p2sh-utxo
2) redeem them to OP_RETURN or to regular output

Each such transaction costs 0.00045 for dishonest attacker (can be even less)
88 transactions (attack one block) will cost only 0.0396 BTC
Daily attack 5.7024 BTC - not a big deal

Wanna hire me for this dirty job? Grin

fairglu

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