EVM-Puzzles: Learn Ethereum By Solving Interactive Puzzles

is a collection of challenges that will help you to better understand the Ethereum Virtual Machine. Each puzzle starts out by giving you a series of opcodes and prompts you to input the correct transaction value or calldata that will allow the sequence to run without reverting. This walkthrough aims to be a low impact guide for each puzzle, making it easy for anyone with any experience level to fully understand the why and the how behind each solution. This walkthrough will assume that you are familiar with stack machines. If not, take a look at before starting. It’s helpful to know that each element on the stack in the EVM is 32 byes (ie. one word). In this repo, there are 10 puzzles. For someone with no experience with the EVM, this should take about 1-2 hours. For someone with basic EVM experience, this should take about 1 hour. If you are very comfortable with the EVM but you still want to go through the walkthrough, this should take somewhere around 30 minutes. With that note, we are ready to get started! EVM-Puzzles how stack machines work First, head over to the , clone the project and set up your local environment. Make sure you have hardhat installed. If you don’t, you can simply enter when you are in the root project folder. EVM-Puzzles repo npm install --save-dev hardhat Next, if you are newer to the EVM, take a brief look at the (don’t feel the need to understand everything, just get the general idea). EVM opcodes With all of that out of the way, let’s check out the first puzzle. To start the first puzzle, cd into the root directory of the project and enter into the terminal. npx hardhat play Puzzle #1 Let’s take a look at the first puzzle. You are given a series of opcodes that represent a contract. The puzzle prompts you to enter a value to send, or in other words if you sent a transaction to this contract, what would the transaction value need to be for this contract to run without hitting the ? Go ahead and give it a shot and then feel free to come back here if you get stuck or want an in depth look at the solution after solving the puzzle. REVERT instruction ############ # Puzzle 1 # ############ 00 34 CALLVALUE 01 56 JUMP 02 FD REVERT 03 FD REVERT 04 FD REVERT 05 FD REVERT 06 FD REVERT 07 FD REVERT 08 5B JUMPDEST 09 00 STOP ? Enter the value to send: (0) Ok, now for the explainer. First, we need to know what the does. This opcode gets the value of the current call (ie. the transaction value) in wei and pushes that value to the top of the stack. So if we entered a value of 10, before the instruction is CALLVALUE instruction CALLVALUE evaluated, the stack would look like this. [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] After the opcode is evaluated, the stack would look like this. CALLVALUE [10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Next, we need to know what the instruction does. This opcode consumes the top value on the stack and jumps to the th instruction in the sequence where is the value at the top of the stack. A quick example will make this more clear. Lets say we have the following sequence. JUMP n n 00 34 CALLVALUE 01 56 JUMP 02 FD REVERT 03 FD REVERT 04 80 JUMPDEST 05 80 DUP1 06 00 STOP ? Enter the value to send: (0) If we enter 4 in as the value to send, the opcode will push onto the stack. After , now our stack looks like this. CALLVALUE 4 CALLVALUE [4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Then the opcode consumes the top value on the stack and jumps to the instruction at that position. Since the value on the top of the stack is , the program counter jumps to the fourth instruction and continues. A opcode must alter the program counter to end up at a instruction. For the above example, we can think of the program looking like this after the instruction is evaluated. JUMP 4 JUMP JUMPDEST JUMP 05 80 DUP1 06 00 STOP Now that all of that is clear, let’s get back to the puzzle. We need to enter a value so that the program runs without hitting a instruction. REVERT 00 34 CALLVALUE 01 56 JUMP 02 FD REVERT 03 FD REVERT 04 FD REVERT 05 FD REVERT 06 FD REVERT 07 FD REVERT 08 5B JUMPDEST 09 00 STOP To do this, we can enter a call value of 8, which causes the instruction to push onto the stack where the instruction then consumes that value and jumps to the 8th instruction, skipping all of the instructions. Nice work, one puzzle down! CALLVALUE 8 JUMP REVERT Puzzle #2 Now that you have your feet wet, let’s take a look at the second puzzle. Give it a shot on your own and just like before, feel free to come back to check out the solution as well as the explanation. Here is the puzzle. ############ # Puzzle 2 # ############ 00 34 CALLVALUE 01 38 CODESIZE 02 03 SUB 03 56 JUMP 04 FD REVERT 05 FD REVERT 06 5B JUMPDEST 07 00 STOP 08 FD REVERT 09 FD REVERT ? Enter the value to send: (0) Just like before, we need to enter a transaction value to send that will cause the program to run without reverting. If we take a look at the sequence of instructions, we can see that we need the opcode to alter the program counter to the 6th instruction. Just like before, the first instruction is , so we know that the value we enter will end up on the top of the stack after the first instruction. JUMP CALLVALUE Let’s take a look at the . This opcode gets the size of the code running in the current environment. In this example, we can manually check the size of the code by looking at how many opcodes there are in the sequence. Each opcode is 1 byte, and in this puzzle we have 10 opcodes meaning that the size of the code is 10 bytes. As an important side note, the EVM uses hex numbers to represent byte code. If you are unfamiliar, check out work. With this in mind, we can know that gets pushed to the stack, representing 10 bytes. CODESIZE instruction how hex numbers 0a The next opcode we come across is the , which takes the first stack element minus the second stack element, pushing the result on the top of the stack. Both inputs at the top of the stack before the instruction are consumed. For example if we had a stack that looked like this. SUB instruction SUB [3 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Executing the instruction would produce the following stack result. SUB [1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] With this information lets get back to the puzzle. We now know that the program first evaluates the instruction, pushing the value we entered onto the stack. Then the program evaluates the instruction, which pushes (representing 10 bytes) onto the stack. We also know that we need to alter the program counter to the 6th instruction. This is what the stack looks like after the instruction. CALLVALUE CODESIZE 0a JUMP CODESIZE [a your_input 0 0 0 0 0 0 0 0 0 0 0 0 0 0] If you haven’t finished the puzzle already, go ahead and try to use the above information to enter the correct value. Otherwise, feel free to read on for the last step of the solution. Since we know the instruction is next, we need to enter a value such that equals , which makes our answer 4. SUB 0a - your_input 6 Puzzle #3 Get ready to switch gears a little. Instead of entering a transaction value to solve the puzzle, we are going to have to enter calldata. Calldata is a read-only byte-addressable space where the transaction data during a message or call is held. In plain english, this is byte code payload that is attached to a message ( ). Let’s take a look at the puzzle. click here to learn more about the anatomy of a transaction in Ethereum ############ # Puzzle 3 # ############ 00 36 CALLDATASIZE 01 56 JUMP 02 FD REVERT 03 FD REVERT 04 5B JUMPDEST 05 00 STOP ? Enter the calldata: For this puzzle, its helpful to know that 1 byte is 8 bits and that numbers 0-255 can represent one byte in the EVM. This puzzle also introduces us to a new opcode called . This instruction gets the size of the calldata in bytes and pushes it onto the stack. CALLDATASIZE With that knowledge, this makes the puzzle pretty straightforward. We will need to pass in calldata such that the instruction pushes 4 on the stack. From there, the instruction will jump to the fourth instruction in the sequence, reaching the . To keep it simple, can be used to represent 1 byte since in hexadecimal evaluates to 255 in decimal format. All we need to do is copy four times, making the byte code we should enter: . Another puzzle down! CALLDATASIZE JUMP JUMPDEST 0xff ff ff 0xffffffff Puzzle #4 Enter bitwise. In this puzzle we see our first instruction. As usual, feel free to give it a shot and figure it out on your own. When you’re ready, head back here for the solution and explanation. XOR ############ # Puzzle 4 # ############ 00 34 CALLVALUE 01 38 CODESIZE 02 18 XOR 03 56 JUMP 04 FD REVERT 05 FD REVERT 06 FD REVERT 07 FD REVERT 08 FD REVERT 09 FD REVERT 0A 5B JUMPDEST 0B 00 STOP ? Enter the value to send: (0) We know that will push the value we enter onto the top of the stack. Also we can know how big the is by taking a look at how many instructions there are. In this program, we have 12 instructions, which makes 12 bytes or in hexadecimal, which gets pushed to the stack. So now our stack looks like this. CALLVALUE CODESIZE 0c [c your_input 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Let’s take a look at the . This instruction evaluates two numbers in their binary representation and returns a in each bit position where the bits of operands are s. Let’s take a look at quick example. Say that we have two numbers on the top of the stack. XOR instruction 1 either, but not both 1 [5 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0] When executing the instruction, we can imagine the two numbers in binary representation like this. XOR 00000000000000000000000000000101 00000000000000000000000000000011 Then, bit by bit, the two numbers are evaluated against each other. If one bit is a and the other bit is a , the resulting bit will be a , if both bits are s or both bits are s, the resulting number is a . So the result of is this. 0 1 1 0 1 0 5 XOR 3 00000000000000000000000000000110 Back to the puzzle. We know that we have at the top of the stack and in the second stack position. After the , the opcode needs to send us to the 10th instruction. Now with all this information known, we just need to enter a callvalue so that results in hexadecimal . Go ahead and give it a shot on your own. 0c your_input XOR JUMP 0c XOR callvalue 10 Ok, now for the final steps. We know that we need the result of to be , which in binary is represented as . We also have on the stack, which in binary is represented as . So now we need to find a number such that results in , making the number we need to enter . This evaluates to the hex number . 6 is our answer! XOR 10 1010 0c 1100 c XOR your_input 1010 0110 06 Puzzle #5 Welcome to the next puzzle, where we are met with a few new opcodes. Feel free to give it a shot. In the meantime, let’s take a look at the sequence of instructions for this puzzle. ############ # Puzzle 5 # ############ 00 34 CALLVALUE 01 80 DUP1 02 02 MUL 03 610100 PUSH2 0100 06 14 EQ 07 600C PUSH1 0C 09 57 JUMPI 0A FD REVERT 0B FD REVERT 0C 5B JUMPDEST 0D 00 STOP 0E FD REVERT 0F FD REVERT ? Enter the value to send: (0) meet reader, reader meet . The is pretty straight forward. It duplicates the value at the 1st position on the stack and pushes the duplicate to the top of the stack. Similarly, would duplicate the value at the second position on the stack and push the duplicate value to the top. There are DUP instructions for all positions in the stack ( ). DUP1 DUP1 DUP1 instruction DUP2 DUP1-DUP16 Taking a look at the first two instructions of the puzzle, first is executed, pushing the value we enter to the top of the stack. Then is executed, duplicating the value we entered and pushing it to the top of the stack. So now after the first two instructions, our stack looks like this. CALLVALUE DUP1 [your_input your_input 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Then we are met with another new instruction. The takes the first two values on the stack, multiplies them together and pushes the result onto the top of the stack. So in this instance, is multiplied by and the resulting stack looks like this. MUL instruction your_input your_input [mul_result 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Next we encounter the . This instruction pushes a 2 byte value onto the top of the stack. When you see any instruction, it will always be accompanied by the value that it will push. For example, in our puzzle we have meaning that it will push the 2 byte hex number onto the top of the stack. There are push instructions from to . PUSH2 instruction PUSH PUSH2 0100 0100 PUSH1 PUSH32 Coming back to our puzzle, since the next instruction is , our resulting stack will now look like this. PUSH2 0100 [0100 mul_result 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Now we encounter the . This instruction takes the first two values on the stack, runs an equality comparison and pushes the result on the top of the stack. If the first two values are equal, is pushed to the top, otherwise is pushed to the stack instead. Both values at positions 1 and 2 on the stack are consumed from the instruction. EQ instruction 1 0 EQ For simplicity sake, let’s say that the is so when the instruction is evaluated, is pushed to the stack, making our stack now look like this. mul_result 0100 EQ 1 [1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] The next instruction that is evaluated is which pushes to the top of the stack. Following this instruction, we see yet another new instruction. The will conditionally alter the program counter. This instruction looks at the second stack element to know if it should jump or not, depending on if the second stack element is a or a . Then the first stack element is used to know what position to jump to. The instruction consumes both values at the top of the stack during this process. So taking a look at our puzzle, after the instruction, our stack looks like this. PUSH1 0C 0c JUMPI instruction 1 0 JUMPI PUSH1 0c [0c 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0] First, the instruction checks the second stack element. In this case it is indicating that the program should jump. Then checks the first stack element to know where it should jump to. The top stack value is meaning that it will jump to the 12th instruction, which is our . JUMPI 1 JUMPI 0c JUMPDEST And that will complete our puzzle! So with all this information we now know that we need to enter a callvalue so that when it gets duplicated once (making the first two elements on the stack the callvalue), and after the top stack values are multiplied, our result is the hex number . Feel free to give it a shot from here and see if you can figure it out. 0100 Ok now for the final steps. We can convert into a decimal number and get 256. Then we can take the square root of 256 since the is essentially multiplying the number by itself. The resulting number is 16, which is the answer to this puzzle! 0100 DUP1 MUL Puzzle #6 5 puzzles down, 5 to go! As usual, give the puzzle a try, then feel free to come back here for the solution and explanation. ############ # Puzzle 6 # ############ 00 6000 PUSH1 00 02 35 CALLDATALOAD 03 56 JUMP 04 FD REVERT 05 FD REVERT 06 FD REVERT 07 FD REVERT 08 FD REVERT 09 FD REVERT 0A 5B JUMPDEST 0B 00 STOP ? Enter the calldata: Say hello to the . This instruction gets the input data from the calldata attached to a transaction. There are a few important things to note about this opcode. expects an integer at the top of the stack to know what byte to start loading the calldata from. For example, if you send a transaction with a 32 byte sequence as calldata and you push to the top of the stack, when you execute , all the calldata from byte 8 to byte 32 will be pushed onto the top of the stack. As an additional note, if the calldata is 64 bytes and you need to access the second 32 byes of the sequence, you can push onto the stack and then use to get the second 32 byes of the sequence. CALLDATALOAD instruction CALLDATALOAD 08 CALLDATALOAD 20 CALLDATALOAD Now back to the puzzle. We can see that there is followed by meaning that the calldata will be loaded in starting from byte 0 and bytes 0-32 of the calldata will be pushed onto the top of the stack. We can see that the instruction needs to alter the program counter to (ie. the 10th instruction). Feel free to stop here and try to solve the rest of the puzzle. PUSH1 00 CALLDATALOAD JUMP 0a Ok let’s go over the final steps. We know that calldata is in hexadecimal, so it might seem intuitive to enter as the calldata to complete the puzzle, but you might have noticed that this doesn’t work. This is because when calldata is sent, since the byte sequence was not 32 bytes, it is padded to the right, so what we thought was , actually turns into . So what we need to do is pad our with 31 bytes to the left making it . There you go, that is our answer! 0x0a 0a a00000000000000000000000000000000000000000000000000000000000000 0x0a 0x000000000000000000000000000000000000000000000000000000000000000a Puzzle #7 You know the drill. Give the puzzle a shot and then come back to see the full solution / explanation. ############ # Puzzle 7 # ############ 00 36 CALLDATASIZE 01 6000 PUSH1 00 03 80 DUP1 04 37 CALLDATACOPY 05 36 CALLDATASIZE 06 6000 PUSH1 00 08 6000 PUSH1 00 0A F0 CREATE 0B 3B EXTCODESIZE 0C 6001 PUSH1 01 0E 14 EQ 0F 6013 PUSH1 13 11 57 JUMPI 12 FD REVERT 13 5B JUMPDEST 14 00 STOP ? Enter the calldata: First things first, we can see and know that we will need to enter calldata with a specific size to solve this puzzle. Let's take note of this and come back later. After the size of the calldata is pushed to the stack, there is , and making our stack at this point look like this. CALLDATASIZE PUSH1 00 DUP1 [0 0 calldata_size 0 0 0 0 0 0 0 0 0 0 0 0 0] Next we encounter the . This instruction copies the input data from the transaction and saves it into memory. This opcode expects three elements at the top of the stack which are , in this order. is the byte offset in the memory where the result will be copied. We haven’t talked much about memory at this point and if you want to learn more you can . The abbreviated version is that there is a temporary data structure that allocates space to hold values during the execution of a function and the tells the program which slot in memory to store the data that is copied from calldata. The dictates where to start copying the calldata from (just like does in the last example) and the tells the program how much of the byte sequence to store in memory. During this process, all three of the top elements on the stack are consumed. CALLDATACOPY instruction [destOffset offset size] destOffset read about it here destOffset offset CALLDATALOAD size With all of this known, let’s revisit our current stack. [0 0 calldata_size 0 0 0 0 0 0 0 0 0 0 0 0 0] When the instruction executes, it will store calldata in memory slot , starting at byte , and storing the size of the entire calldata. Our resulting stack after this instruction looks like this. CALLDATALOAD 0 0 [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Immediately after, are all executed, making the stack look just like we had it before . CALLDATASIZE PUSH1 00 PUSH1 00 CALLDATALOAD [0 0 calldata_size 0 0 0 0 0 0 0 0 0 0 0 0 0] Next we are introduced to another new opcode, the . This instruction creates a new account (ie. Contract or EOA). Let's get a little under the hood with the opcode, as this will come in handy later during the walkthrough (and is generally good to know). CREATE instruction CREATE When deploying a new contract with the opcode, the stack must have at the top of the stack, in this order. The is the amount of wei to send the new contract that is being created, the is the location in memory where the bytecode starts that will run on deployment and is the size of the bytecode that will run on deployment. When you deploy a contract with the opcode, the bytecode from the is not the new contract's bytecode, but rather the bytecode from the is executed during deployment and the becomes the newly created contract's bytecode. CREATE [value offset size] value offset size CREATE offset offset return value Let's run through a quick example that will make this easy to understand. If you use the opcode with deployment bytecode of , since the return value of this bytecode sequence is , the newly created contract's bytecode will be ie. . So when you call this contract, it will simply execute ! Make sure to take note of this concept as it will come in handy later. CREATE 0x6160016000526002601Ef3 6001 6001 PUSH1 01 PUSH1 01 When the instruction executes, all three values are consumed and the address that the account was deployed to is pushed to the top of the stack. After this opcode executes, our stack looks like this. CREATE [address_deployed 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Next, we come across the which expects an address on the top of the stack and returns the size of the code at that address. The address at the top of the stack is consumed in this process. After we see making our stack look like this. EXTCODESIZE instruction EXTCODESIZE PUSH1 01 [01 address_code_size 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Directly after, the instruction is executed, checking if the top two values are equal, pushing the result on the stack. From there and are used to get us to the ! So coming all the way back to the beginning of the puzzle, we will need to enter calldata such that the code size is equal to 01 byte! This is a little tricky so to understand this, we can look at the playground example from the . Here is what the example looks like. EQ PUSH1 13 JUMPI JUMPDEST EXTCODESIZE instruction // Creates a constructor that creates a contract with 32 FF as code PUSH32 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF PUSH1 0 MSTORE //Opcodes to return 32 ff PUSH32 0xFF60005260206000F30000000000000000000000000000000000000000000000 PUSH1 32 MSTORE // Create the contract with the constructor code above PUSH1 41 PUSH1 0 PUSH1 0 CREATE // Puts the new contract address on the stack // The address is on the stack, we can query the size EXTCODESIZE Lets take a closer look at the opcodes in the constructor. // Push a 32 byte value onto the stack PUSH32 FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF PUSH1 00 // Store the 32 byte value at memory slot 0 MSTORE // Return a 32 byte value starting at memory slot 0 PUSH1 20 PUSH1 00 RETURN STOP STOP ... When this code is run, it returns a value of which is 32 bytes. If we change the return size to 16 bytes instead of 32 bytes, the will be which is 16 bytes in hexadecimal. This indicates that uses the size of the return value to dictate the code size. ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff EXTCODESIZE 10 EXTCODESIZE Let’s finish the puzzle. Now we know that the evaluates the size of the return value from the deployed bytecode. With this information, we can pass in calldata such that when it is deployed, it returns a 1 byte value! You can use any sequence of opcodes that returns 1 byte, but for this walkthrough, we will use . And with that, another puzzle solved! EXTCODESIZE 0x60016000526001601ff3 Puzzle #8 Welcome to the eighth puzzle. Let’s take a look at what is in store. ############ # Puzzle 8 # ############ 00 36 CALLDATASIZE 01 6000 PUSH1 00 03 80 DUP1 04 37 CALLDATACOPY 05 36 CALLDATASIZE 06 6000 PUSH1 00 08 6000 PUSH1 00 0A F0 CREATE 0B 6000 PUSH1 00 0D 80 DUP1 0E 80 DUP1 0F 80 DUP1 10 80 DUP1 11 94 SWAP5 12 5A GAS 13 F1 CALL 14 6000 PUSH1 00 16 14 EQ 17 601B PUSH1 1B 19 57 JUMPI 1A FD REVERT 1B 5B JUMPDEST 1C 00 STOP ? Enter the calldata: This one might look more daunting but it is actually pretty simple. First we see a very similar which, just like the previous puzzle, creates a new contract from the calldata that you pass in and returns the deployment address. So right from the start, we know that we will have to enter calldata with bytecode for a contract to solve the puzzle. Lets take a quick mental note of what the stack looks like at this point. Since the instruction consumes the top three stack values and pushes the address that the account was deployed to, our stack now looks like this. CALLDATASIZE PUSH1 00 DUP1 CALLDATACOPY CALLDATASIZE PUSH1 00 PUSH1 00 CREATE CREATE [address_deployed 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] The next 5 instructions all relate to the . This instruction creates a new sub context and execute the code of the given account, then resumes the current one. In plain english, the instruction is used to interact with another contract. This opcode expects the stack to have a few values at the top of the stack , in this order. Lets walk through each of the arguments one by one. is the amount of gas that will be sent with the message call. is the address that the message will be sent to. is the amount of wei that will be sent with the message. is the location in memory within the current context (ie. the msg.sender) that will be used as calldata for the message call. is the size of the calldata to send with the message call. is the location in memory within the current context where the return value from the call will be stored. Finally, is the size of the return value that will be stored in memory. CALL instruction CALL [gas address value argsOffset argsSize retOffset retSize] gas address value argsOffset argsSize retOffset retSize Now let's take a look at the puzzle again. The next four opcodes are , which makes the stack look like this. PUSH1 00 DUP1 DUP1 DUP1 DUP1 [0 0 0 0 0 address_deployed 0 0 0 0 0 0 0 0 0 0] Then we see the . This instruction swap the 1st and 6th stack items. There are instructions for all positions in the stack ( - ). In this case, exchanges with making our stack now in the correct order to match . Here is what our stack looks like now. SWAP5 instruction SWAP SWAP1 SWAP16 SWAP5 0 address_deployed [gas address value argsOffset argsSize retOffset retSize] [address_deployed 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0] Then we execute the instruction, which returns if the sub context reverted and if it was a success. After the instruction we can see a meaning that we need to push a onto the stack. Go ahead and give the rest of the puzzle a shot, then feel free to come back to see the rest of the solution. CALL 0 1 CALL PUSH1 00 EQ CALL 0 Ok, so now we know that the instruction needs to return which means we need to enter calldata that causes to fail. To get to fail, there are three ways. One way it can fail is if there is not enough gas. The second way it can fail is if there are not enough values on the stack. The third way it can fail is if the current execution context is from a and the value in wei (stack index 2) is not 0 (since Byzantium fork). It is also important to note that will always succeed as true when you an account with no code (or codesize of 0). CALL 0 CALL CALL STATICCALL CALL CALL To finish this puzzle, let's refer back to how the opcode works. We know that the return value of the bytecode that is run on deployment becomes the bytecode for the newly created contract. With that information known, we can pass in calldata with a bytecode sequence such that the return value of the sequence causes a when run. CREATE REVERT You can pass in any that will result in a but for the walkthrough we will use as the deployment bytecode. Since the return value of this bytecode sequence is , the newly created contract's code will be ie. the instruction. So when you call this contract, it will execute the instruction, and since there are no values on the stack in the subcontext of the contract, the will fail (ie. )! There you go, is our answer! REVERT 0x60016000526001601ff3 01 01 ADD ADD CALL REVERT 0x60016000526001601ff3 Puzzle #9 We are in the home stretch, let's take a look at puzzle #9. This puzzle adds one more layer of complexity, requiring you to enter both a callvalue and calldata to solve the puzzle. ############ # Puzzle 9 # ############ 00 36 CALLDATASIZE 01 6003 PUSH1 03 03 10 LT 04 6009 PUSH1 09 06 57 JUMPI 07 FD REVERT 08 FD REVERT 09 5B JUMPDEST 0A 34 CALLVALUE 0B 36 CALLDATASIZE 0C 02 MUL 0D 6008 PUSH1 08 0F 14 EQ 10 6014 PUSH1 14 12 57 JUMPI 13 FD REVERT 14 5B JUMPDEST 15 00 STOP ? Enter the value to send: (0) We are already familiar with the first two opcodes so we can know that after the instructions, our stack looks like this. CALLDATASIZE PUSH1 03 [03 calldata_size 0 0 0 0 0 0 0 0 0 0 0 0 0 0] The runs a comparison on the first two stack values to see if the first stack element is less than the second stack element. If evaluates to true, is pushed on the stack, otherwise is pushed instead. The two values used in the comparison are consumed in this process. For the sake of the example, let's say that the is 4 bytes, so will push as a result. LT instruction LT 1 0 CALLDATASIZE LT 1 Since evaluated to true, the code then jumps to the at instruction . Following the jump, and are pushed onto the stack and multiplies them together, consuming the top two stack values in the process. pushes to the stack and then checks if the result of equals , consuming the values in the process. needs to push a to the stack to enable to get us to the end of the puzzle. LT JUMPDEST 09 CALLVALUE CALLDATASIZE MUL PUSH1 08 08 EQ MUL 08 EQ 1 JUMPI With all this information, we now know that we need to enter calldata such that the is greater than 3 bytes, and the product of is . CALLDATASIZE CALLDATASIZE * CALLVALUE 08 With some quick math, we can use any combination of values that evaluate to 8 when multiplied together that satisfy the conditions above. For the walkthrough, we will enter as the calldata and as the callvalue. One more puzzle to go! 0x00000001 2 Puzzle #10 Here it is, the final puzzle. Let's jump in. ############# # Puzzle 10 # ############# 00 38 CODESIZE 01 34 CALLVALUE 02 90 SWAP1 03 11 GT 04 6008 PUSH1 08 06 57 JUMPI 07 FD REVERT 08 5B JUMPDEST 09 36 CALLDATASIZE 0A 610003 PUSH2 0003 0D 90 SWAP1 0E 06 MOD 0F 15 ISZERO 10 34 CALLVALUE 11 600A PUSH1 0A 13 01 ADD 14 57 JUMPI 15 FD REVERT 16 FD REVERT 17 FD REVERT 18 FD REVERT 19 5B JUMPDEST 1A 00 STOP ? Enter the value to send: (0) In this puzzle, you will need to enter a callvalue as well as calldata. Let's take a look at the first few instructions. First we see which pushes the size of the code, followed by the callvalue you passed in and then swaps their positions. At this point our stack looks like this. CODESIZE CALLVALUE SWAP1 [1b callvalue 0 0 0 0 0 0 0 0 0 0 0 0 0] Next we se the which operates exactly like , but evaluates greater than instead of less than. For this puzzle, we need to push on the stack, so we know that our callvalue must be less than (ie. 27 in decimal notation). This will allow the program to jump down to the first at instruction . GT instruction LT GT 1 1b JUMPDEST 08 Now we see which pushes the calldata size as well as onto the stack, and swaps their positions. Now our stack looks like this. CALLDATASIZE PUSH2 0003 SWAP1 0003 [calldata_size 3 0 0 0 0 0 0 0 0 0 0 0 0 0] Next we see the . This instruction runs a modulo of the first stack element and the second stack element, pushing the remainder onto the stack. Following the instruction we see the , which pushes onto the stack if the top value on the stack is . If any other number is on the top of the stack, is pushed to the stack instead. In our case, we need to push to the stack (we will come back to this). We then see . The simply adds the first two values on the stack and pushes the result to the stack. Following this sequence, there is a , meaning that needs to push the position of the onto the stack. Feel free to give the rest of the puzzle a shot from here. MOD instruction MOD ISZERO instruction 1 0 0 ISZERO 1 CALLVALUE PUSH1 0A ADD ADD instruction JUMPI CALLVALUE PUSH1 0A ADD JUMPDEST With all this information, we now know a few things. First, we need to enter calldata such that the size of the calldata is divisible by 3 bytes, enabling the sequence to push onto the stack. This allows to push a to the stack, where the program can then jump to the second . Second, we need to enter a callvalue such that the value is less than 26 and equals . With these factors known, we can enter as calldata and (in decimal) as the callvalue. Just like that, we have completed the final puzzle! CALLDATASIZE PUSH2 0003 SWAP1 MOD 0 ISZERO 1 JUMPDEST callvalue + 0a 0x19 0x000001 15

EVM-Puzzles: Learn Ethereum By Solving Interactive Puzzles

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