Challenge

Given is a network service which simulates a tiny blockchain VM. The service prints a mined block summary and asks you to submit:

• Nonce (hex)
• A RISC-V assembly contract, terminated by an empty line

You can interact via netcat or a socket client. An ELF binary was provided for local analysis.

Step 1 --- Initial analysis

• I ran the provided ELF locally to inspect prompts and behavior. The binary prints a block summary and asks for the nonce and the RISC-V assembly contract. The service reads the assembly until it receives a completely empty line, then assembles and validates the block.
• Static inspection (strings, nm, objdump) revealed symbols for the ecall handler, block validation, hash calculation, and the RISC-V assembler/executor.

Observed prompt sequence:

Submit your new block's data.
Nonce (as hex):
Enter your RISC-V assembly contract (end with a blank line):

Step 2 --- Key reversing notes

• is_block_valid contains a short-circuit: if the block's nonce equals 0xdeadbeef the function returns valid immediately, bypassing PoW and other checks.
• ecall_handler checks a register for the syscall id. If the syscall id equals 1337, it prints a message and reads flag.txt then prints it. The rodata contains the string ECALL 1337: Nice! Here's your flag!.
• The assembler accepts a subset of RISC-V (including addi and ecall). The server requires the contract input to be terminated by a blank line; otherwise it waits for more input.

Attack idea: submit the magic nonce to bypass validation, then submit a tiny RISC-V contract that sets the ECALL number to 1337 and calls ecall.

Step 3 --- Exploit / manual solve

Manual steps using nc (interactive):

1. Connect:

nc riskychain.challs.infobahnc.tf 1337

1. When prompted, submit the nonce (hex):

deadbeef

1. Then submit the contract lines, and remember to end with one extra blank line so the server knows the contract is finished:

addi x10, x0, 1337
ecall

Why this works:

• addi x10, x0, 1337 sets register x10 to 1337. The ECALL handler reads the syscall number from x10.
• ecall triggers the handler. With the block validated via 0xdeadbeef, the handler prints the flag when it sees syscall 1337.

# illustrate what to send
s.sendall(b"deadbeef\n")
s.sendall(b"addi x10, x0, 1337\n")
s.sendall(b"ecall\n\n")

Flag

infobahn{Th3_futur3_15_m3t4l1c_4nd_RISC-V}

Notes

• The intentional backdoor is the 0xdeadbeef nonce in is_block_valid. That is the designed path to the flag for this challenge.
• The RISC-V interpreter executes the contract after block validation, and ECALL 1337 discloses the flag.
• Remember the final blank line to terminate the contract input.