Everybody is talking that data in contracts are public, but not everybody knows how to read it.
All contracts deployed to ethereum VM has dedicated storage where stores state. Here is an example how to read this storage with the web3.js library using eth.getStorageAt() method.
Contract example which we examine:
This contract is deployed in ropsten test net: at 0xf1f5896ace3a78c347eb7eab503450bc93bd0c3b
All parameters in the storage are indexed from the beginning. One index takes 256 bytes ant it fits 64 symbols. In this example, we have 10 parameters and we can iterate them:
let contractAddress = '0xf1f5896ace3a78c347eb7eab503450bc93bd0c3b'
for (index = 0; index < 10; index++){
console.log(`[${index}]` +
web3.eth.getStorageAt(contractAddress, index))
}result:
[0] 0x000000000000000000000000000000000000000000000000000000000000000f
[1] 0x00000000000000000000000059b92d9a0000000000000000000000000000429f
[2] 0x0000000000000000000000000000000074657374310000000000000000000000
[3] 0x7465737431323336000000000000000000000000000000000000000000000000
[4] 0x6c65747320737472696e6720736f6d657468696e67000000000000000000002a
[5] 0x0000000000000000000000000000000000000000000000000000000000000000
[6] 0x0000000000000000000000000000000000000000000000000000000000000000
[7] 0x0000000000000000000000000000000000000000000000000000000000000002
[8] 0x0000000000000000000000000000000000000000000000000000000000000002
[9] 0x0000000000000000000000000000000000000000000000000000000000000000
Let`s have a deeper look into each parameter
Index 0 — storeduint1
let contractAddress = '0xf1f5896ace3a78c347eb7eab503450bc93bd0c3b'
let index = 0console.log(web3.eth.getStorageAt(contractAddress, index))
console.log('DEC:' + web3.toDecimal(web3.eth.getStorageAt(contractAddress, index)))result:
0x000000000000000000000000000000000000000000000000000000000000000f
DEC:15
constuint
Constants are not stored in a storage. Available only in code.
Index 1 — investmentsLimit, investmentsDeadlineTimeStamp
let index = 1
console.log(web3.eth.getStorageAt(contractAddress, index))result:
0x00000000000000000000000059b92d9a0000000000000000000000000000429f
DEC: 1505308058 and 17055
In index 1 is merged 2 properties to optimize storage usage.
index 2 — string1
index = 2
console.log(web3.eth.getStorageAt(contractAddress, index))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, index)))result:
0x0000000000000000000000000000000074657374310000000000000000000000
ASCII: test1
index 3 — string2
index = 3
console.log(web3.eth.getStorageAt(contractAddress, index))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, index)))result:
0x7465737431323336000000000000000000000000000000000000000000000000
ASCII: test1236
index 4— string3
index = 4
console.log(web3.eth.getStorageAt(contractAddress, index))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, index)))result:
0x6c65747320737472696e6720736f6d657468696e67000000000000000000002a
ASCII: lets string something * (42)
End symbol 2a (dec 42) is length of the stored string. (more details http://solidity.readthedocs.io/en/latest/miscellaneous.html#layout-of-state-variables-in-storage)
index 5 — uints1
index = 5
console.log(web3.eth.getStorageAt(contractAddress, index))result:
0x0000000000000000000000000000000000000000000000000000000000000000PROBLEM!!!!
Mappings have a different indexation and should be read in other way. To read mapping value you should know the key value. Otherwise, read mapping value is impossible.
index = '0000000000000000000000000000000000000000000000000000000000000005'
key = '00000000000000000000000xbccc714d56bc0da0fd33d96d2a87b680dd6d0df6'let newKey = web3.sha3(key + index, {"encoding":"hex"})console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('DEC: ' + web3.toDecimal(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x0000000000000000000000000000000000000000000000000000000000000058
DEC: 88
index6 — structs1
index = "0000000000000000000000000000000000000000000000000000000000000006"
key = "00000000000000000000000xbccc714d56bc0da0fd33d96d2a87b680dd6d0df6"let newKey = web3.sha3(key + index, {"encoding":"hex"})
console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x6465766963654272616e64000000000000000000000000000000000000000016
ASCII: deviceBrand
To read second struct value you need to increase newKey value by 1
function increaseHexByOne(hex) {
let x = new BigNumber(hex)
let sum = x.add(1)
let result = '0x' + sum.toString(16)
return result
}index = "0000000000000000000000000000000000000000000000000000000000000006"
key = "00000000000000000000000xbccc714d56bc0da0fd33d96d2a87b680dd6d0df6"let newKey = increaseHexByOne(
web3.sha3(key + index, {"encoding":"hex"}))console.log(web3.eth.getStorageAt(contractAddress,newKey))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x6465766963655965617200000000000000000000000000000000000000000014
ASCII: deviceYear
If you want third struct value increase newKey once more.
index 7— uintarray
index = "7"
console.log(web3.eth.getStorageAt(contractAddress, index))result:
0x0000000000000000000000000000000000000000000000000000000000000002
This array has 2 items
index = "0000000000000000000000000000000000000000000000000000000000000007"
let newKey = web3.sha3(index, {"encoding":"hex"})console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('DEC: ' +
web3.toDecimal(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x0000000000000000000000000000000000000000000000000000000000001f40
DEC: 8000
newKey = increaseHexByOne(web3.sha3(index, {"encoding":"hex"}))console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('DEC: ' +
web3.toDecimal(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x0000000000000000000000000000000000000000000000000000000000002328
DEC: 9000
index 8— deviceDataArray
index = "0000000000000000000000000000000000000000000000000000000000000008"let newKey = web3.sha3(index, {"encoding":"hex"})console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x6465766963654272616e64000000000000000000000000000000000000000016
ASCII: deviceBrand
index = "0000000000000000000000000000000000000000000000000000000000000008"let newKey = increaseHexByOne(web3.sha3(index, {"encoding":"hex"}))console.log(web3.eth.getStorageAt(contractAddress, newKey))
console.log('ASCII: ' +
web3.toAscii(web3.eth.getStorageAt(contractAddress, newKey)))result:
0x6465766963655965617200000000000000000000000000000000000000000014
ASCII: deviceYear
Increase by 2 result:
0x776561724c6576656c0000000000000000000000000000000000000000000012
ASCII: wearLevel
Increase by 3 you enter to second item into array result:
0x6465766963654272616e64320000000000000000000000000000000000000018
ASCII: deviceBrand2
Sources:
https://github.com/ethereum/solidity/issues/1550
https://github.com/ethereum/wiki/wiki/JSON-RPC#eth_getstorageat
https://ethereum.stackexchange.com/questions/13910/how-to-read-a-private-variable-from-a-contract
https://github.com/ethereum/web3.js/issues/445
Check out our testing ground — Aigang!
More about Aigang — http://bit.ly/AigangWebsite
Aigang Whitepaper — http://bit.ly/2hwAtkt
Aigang on Telegram — http://bit.ly/AigangTelegram
Get Aigang Latest Updates — http://bit.ly/AigangUpdates
