Fix vulnerability in DParameterMP

[colll78]

partner-chains-smart-contracts/onchain/src/TrustlessSidechain/DParameter.hs

Line 33 in b18f693

mkMintingPolicy ::

Currently there is a vulnerability by which the operator burns and mints in the same transaction which allows him to by-pass the validator's checks that all tokens must go to the dParameter validator and smuggle tokens out of the contracts. This is due to the contract's dangerous use of currencySymbolValueOf.

I suggest you introduce the following checks to the DParameterMP:

1. Each output to the dParameter validator address:
   i. contains exactly 1 DParameterToken
   ii. contains an inline datum that correctly decodes into DParameterValidatorDatum

           case (txOutDatum datum) of 
             OutputDatum d -> 
               let fields = (BI.snd . BI.unsafeDataAsConstr) d 
                   permissionedCandidatesCount = BI.head fields
                   pccTail = BI.tail fields 
                   registeredCandidatesCount = BI.head pccTail
               in (BI.null pccTail) && (BI.unsafeDataAsI permissionedCandidatesCount > 0) && (BI.unsafeDataAsI registeredCandidatesCount > 0)

3. The transaction either mints or burns tokens, it does not do both. The current use of currencySymbolValueOf allows for an exploit where the operator burns and mints DParameter tokens in the same transaction thus tricking the script into believing that the number of tokens minted is less than it actually is. Instead of currencySymbolValueOf you should use variants that only sums the quantities that are positive and another variant that only sums the quantities which are negative and then you can check that the tx is only minting or only burning ie:

(positiveSymbolValueOf txInfoMint > 0 && negativeSymbolValueOf txInfoMint == 0) || (negativeSymbolValueOf txInfoMint < 0 && positiveSymbolValueOf txInfoMint == 0)

or just use variants onlyPositiveValueOf and onlyNegativeValueOf that error upon encountering non-positive / non-negative quantities and then use two redeemers (MintAct / BurnAct) to branch on minting and burning. The PermissionedCandidatesMint minting policy also suffers from this exploit.