2023 | WolvCTF | Crypto

TealyMan

#smart contract #algorand #teal #beginner

题目

Use the TestNet. Submit by sending enough testnet algos to cover the current transaction fee to the CTF_Address.

CTF_Address	OH4YZ4QXWOWLHIUKPQAOMPBZBELCANRPRX3NI7SQFL2OFASHLBW5DTLDZQ
AppID	163726037

解题思路

• 看地址首先排除 Ethereum uwu 根据 TestNet 和 algos 锁定 Algorand 生态 owo
• OH4YZ4QXWOWLHIUKPQAOMPBZBELCANRPRX3NI7SQFL2OFASHLBW5DTLDZQ 为普通账户，163726037 对应需要交互的智能合约账户
• Algorand 智能合约包含两部分，ApprovalProgram 和 ClearStateProgram。ApprovalProgram 负责处理主应用逻辑，ClearStateProgram 负责将对应智能合约从账户记录中移除，执行成功返回 \(1\)

the flag will be sent to u once ur done with the app and provide the address enough to cover current transaction fees

• 目标是与应用进行完整交互。与合约交互的方法包括 Opt-in、Call(NoOp)、Read state、Update、Close out、Delete 以及 Clear state
  • 在开始与使用本地状态的应用交互前，账户需要 Opt in

• 分析 ApprovalProgram，根据 OnCompletion 的类型执行相关操作，重点关注 OnCompletion == 0，即 NoOp。目标应用支持 setup 和 buy

  • 首先通过 setup 向应用注册 asset，并向应用转移一部分 asset 以便后续购买操作
  • 再进行 buy，需要发送一个交易组，其中第一个交易为 Payment（向应用支付购买 asset 的 algo），第二个是 Application Call
  Approval Program

  #pragma version 5
      intcblock 1 0 4 9223372036854775808 // prepare block of uint64 constants for use by intc
      bytecblock 0x7374617274 0x656e64 0x61646d696e5f6b6579 0x6e66745f6964 // prepare block of byte-array constants for use by bytec
  
      // Deploy or Call?
      txn ApplicationID
      intc_1 // 0
      ==
      bnz label1
  
      txn OnCompletion
      intc_1 // 0, NoOp, only execute the ApprovalProgram associated with this application ID, with no additional effects.
      ==
      bnz label2
  
      txn OnCompletion
      intc_0 // 1, OptIn, before executing the ApprovalProgram, allocate local state for this application into the sender's account data
      ==
      bnz label3
  
      txn OnCompletion
      pushint 2 // CloseOut, after executing the ApprovalProgram, clear any local state for this application out of the sender's account data.
      ==
      bnz label4
  
      txn OnCompletion
      pushint 5 // DeleteApplication
      ==
      bnz label5
  
      txn OnCompletion
      intc_2 // 4, UpdateApplication
      ==
      bnz label6
  
      err
  label6:
      intc_1 // 0
      return
  label5:
      txn Sender
      global CreatorAddress
      ==
      assert
  
      bytec_2 // "admin_key"
      app_global_get
      callsub label7
  
      intc_0 // 1
      return
  label4:
      intc_0 // 1
      return
  label3:
      intc_0 // 1
      return
  label2:
      txna ApplicationArgs 0 // 0-th value of the ApplicationArgs array of the current transaction
      pushbytes 0x7365747570 // "setup"
      ==
      bnz label8
  
      txna ApplicationArgs 0
      pushbytes 0x627579 // "buy"
      ==
      bnz label9
  
      err
  label9:
      global CurrentApplicationAddress // Address that the current application controls
      txna Assets 0 // Foreign Assets listed in the ApplicationCall transaction
      asset_holding_get AssetBalance
      store 1 // store is_opted_in to the 1-th scratch space
      store 0 // store asset balance to the 0-th scratch space
      load 1  // load is_opted_in from the 1-th scratch space
      intc_1 // 0
      load 0
      callsub label10
      &&  // if is_opted_in and asset balance > 0
  
      txn Sender
      bytec_0 // "start"
      app_local_get
      global LatestTimestamp
      callsub label10
      &&
  
      global LatestTimestamp
      txn Sender
      bytec_1 // "end"
      app_local_get
      callsub label10
      &&
  
      txn GroupIndex // index=1
      intc_0 // 1
      - // GroupIndex - 1
      gtxns TypeEnum // field F of the (GroupIndex - 1)-th transaction in the current group
  
      intc_0 // 1
      ==
      &&
  
      txn GroupIndex // index=1
      intc_0 // 1
      -
      gtxns Sender
  
      txn Sender
      ==
      &&
  
      txn GroupIndex // index=1
      intc_0 // 1
      -
      gtxns Receiver
  
      global CurrentApplicationAddress
      ==
      &&
  
      global MinTxnFee
  
      txn GroupIndex // index=1
      intc_0 // 1
      -
      gtxns Amount
  
      callsub label10
      &&
      assert
  
      txna ApplicationArgs 1
      btoi
      global LatestTimestamp
      +
  
      txn Sender
      bytec_0 // "start"
      app_local_get
      callsub label10
      bnz label11
  
      intc_1 // 0
      return
  label11:
      txn Sender
      bytec_3 // "nft_id"
      app_local_get
  
      txn Sender
      callsub label12
  
      intc_0 // 1
      return
  label8:
      itxn_begin // begin preparation of a new inner transaction in a new transaction group
  
      intc_2 // 4
      itxn_field TypeEnum // set field of the current inner transaction to AssetTransfer
  
      txna Assets 0
      itxn_field XferAsset
  
      global CurrentApplicationAddress
      itxn_field AssetReceiver
      itxn_submit // execute the current inner transaction group
  
      txn Sender
      bytec_3 // "nft_id"
      txna Assets 0
      app_local_put // store the nft_id in the sender's local state
  
      txn Sender
      pushbytes 0x73656c6c6572 // "seller"
      txn Sender
      app_local_put // store the seller address in the sender's local state
  
      txn Sender
      bytec_0 // "start"
      global LatestTimestamp
      app_local_put
  
      txn Sender
      bytec_1 // "end"
      txna ApplicationArgs 1
      btoi
      app_local_put
  
      txn Sender
      bytec_0 // "start"
      app_local_get
      txn Sender
      bytec_1 // "end"
      app_local_get
      <
      assert  // require start < end
  
      intc_0 // 1, approval
      return
  label1:
      bytec_2 // "admin_key"
      txn Sender
      app_global_put
  
      intc_0 // 1
      return
  label12:
      store 3
      store 2
      global CurrentApplicationAddress
      load 2
      asset_holding_get AssetBalance
      store 5
      store 4
      load 5
      bz label13
  
      itxn_begin
      intc_2 // 4
      itxn_field TypeEnum
      load 2
      itxn_field XferAsset
      load 3
      itxn_field AssetCloseTo
      itxn_submit
  label13:
      retsub
  label7:
      store 6
      global CurrentApplicationAddress
      balance
      intc_1 // 0
      !=
      bz label14
  
      itxn_begin
      intc_0 // 1
      itxn_field TypeEnum
      load 6
      itxn_field CloseRemainderTo
      itxn_submit
  label14:
      retsub
  label10:
      store 8
      store 7
      intc_3 // 9223372036854775808
      load 7
      &
      bnz label15
  
      intc_3 // 9223372036854775808, 0x8000000000000000
      load 8
      &
      bnz label16
  
      load 7
      load 8
      <
      bnz label17
  
      intc_1 // 0
      retsub
  label17:
      intc_0 // 1
      retsub
  label16:
      intc_1 // 0
      retsub
  label15:
      intc_3 // 9223372036854775808
      load 8
      &
      bnz label18
  
      intc_0 // 1
      retsub
  label18:
      load 7
      load 8
      >
      bnz label19
  
      intc_1 // 0
      retsub
  label19:
      intc_0 // 1
      retsub
  

• CTF_Address 会向完成交易的账户发起 Transfer 交易，其中包含使用 AlgoSMS 加密的 Flag

Exploit

from algosdk.v2client import algod
from algosdk import transaction, account, constants
from Crypto.Util.number import long_to_bytes
from datetime import datetime
import json

def transact(txn):
    if not isinstance(txn, list):
        signed_txn = txn.sign(private_key)
        txid = algod_client.send_transaction(signed_txn)
    else:
        signed_txns = [t.sign(private_key) for t in txn]
        txid = algod_client.send_transactions(signed_txns)
    confirmed_txn = transaction.wait_for_confirmation(algod_client, txid, 4)
    print(f"Transaction information: {json.dumps(confirmed_txn, indent=4)}")
    return confirmed_txn

private_key = '<private>'
sender = account.address_from_private_key(private_key)

app_id = 163726037
app_address = "5K7JP6324NOEPDB5THIZY3FWHUKF4FBPJFYKIUX2SBNG264NUY6E3AI3BM"

algod_address = "https://testnet-algorand.api.purestake.io/ps2"
algod_token = "<api-key>"
headers = {
    "X-API-Key": algod_token,
}
algod_client = algod.AlgodClient(algod_token, algod_address, headers)

params = algod_client.suggested_params()

# OptIn
optin_txn = transaction.ApplicationOptInTxn(sender, params, app_id)
transact(optin_txn)

# Create asset
create_txn = transaction.AssetCreateTxn(
    sender,
    params,
    100,
    decimals=2,
    default_frozen=False,
    unit_name="CC",
    asset_name="Chicken Coin",
    manager=sender,
)
res = transact(create_txn)
asset_id = res['asset-index']

setup_txn = transaction.ApplicationNoOpTxn(
    sender,
    params,
    app_id,
    [b'setup', long_to_bytes(int(datetime.now().timestamp() * 1000) + 0x80000000)],
    foreign_assets=[asset_id],
    foreign_apps=[asset_id],
    )
transact(setup_txn)

transfer_txn = transaction.AssetTransferTxn(
    sender,
    params,
    app_address,
    1,
    asset_id,
)
transact(transfer_txn)

payment_txn = transaction.PaymentTxn(
    sender,
    params,
    app_address,
    constants.MIN_TXN_FEE * 10,
)
noop_txn = transaction.ApplicationNoOpTxn(
    sender,
    params,
    app_id,
    [b'buy', long_to_bytes(9223372036854775808)],
    foreign_assets=[asset_id],
    foreign_apps=[asset_id],
)
gid = transaction.calculate_group_id([payment_txn, noop_txn])
payment_txn.group = gid
noop_txn.group = gid
transact([payment_txn, noop_txn])

close_txn = transaction.ApplicationCloseOutTxn(
    sender,
    params,
    app_id,
)
transact(close_txn)

const algosdk = require('algosdk');
const { unsealMessageFromNote } = require('algosms');

async function main() {
    const baseServer = 'https://testnet-algorand.api.purestake.io/idx2'
    const port = '';
    const token = {
        'X-API-Key': '<api-key>'
    }

    let indexerClient = new algosdk.Indexer(token, baseServer, port);

    const accRcpt = algosdk.mnemonicToSecretKey('<mnemonic>');

    /* get the TXN with encrypted note from indexer */
    let SmsTXID = "DPHCQMJZ7BKJZHZO674VWGUO3J7JJAIKFKGSAEDHBSQBVYGV4I7A";
    const txn = await indexerClient.lookupTransactionByID(SmsTXID).do();

    /* convert base64 to bytes */
    const note = Buffer.from(txn.transaction.note, 'base64');
    const senderAddr = txn.transaction.sender;

    /* decrypt the note with recipient secret key */
    const msg = unsealMessageFromNote(note, senderAddr, accRcpt);
    console.log(msg);
}

main();

Flag

wctf{1_h0p3_y0u_d0nt_4cc1dent4lly_4get_t0_r3m0ve_th3_4pp}

参考资料

• Interact with smart contracts - Algorand Developer Portal
• Opcodes - Algorand Developer Portal
• PureStake Developer Portal
• algosms - npm

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最后更新: 2023年3月21日 20:33:27

Contributors: YanhuiJessica
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