Tags: fpga lattice reversing 

Rating: 5.0

Original Writeup at: https://github.com/berndoJ/revenge-of-lllattice-writeup

## TL;DR

As no team solved this challenge during the CTF, we decided to create a detailed
writeup. For those who just want to know the overall steps - here's a TL;DR:

- Identify the FPGA bitstream part from `xxd` and google to find out it's a
lattice FPGA.
- Use `ecpunpack` ([Project Trellis](https://github.com/YosysHQ/prjtrellis)) and
VoidMercy's [Lattice ECP5 Bitstream Decompiler](https://github.com/VoidMercy/Lattice-ECP5-Bitstream-Decompiler)
to "decompile" the bitstream into Verilog.
- Use `yosys` to simplify and clean up the Verilog file, reduces line count from
over 650K to ~60K.
- Get a RTL schematic using a toolchain like Vivado to find out what the inputs
and outputs of the FPGA are used for (UART TX/RX, reset, clock).
- Code a testbench in Verilog to be able to communicate with the FPGA over
UART and send/receive data -> it encrypts 8-byte blocks and sends them back
over UART. Through changing the input we give to the FPGA we can also find out
that the encryption method used is a ECB block cipher.
- Use a timeline analysis tool (Vivado or iverilog with GTKWave) to find out that
the encryption happens over 32 clock cyles -> the block cipher most likely has
32 rounds. 64-bit, 32 rounds -> this could be TEA!
- Use the RTL-schematic and statistical analysis (or other methods) to find out
which FFs correspond to which bit in which encryption round (only first two
rounds needed). By statical analysis on the schematic we can also further
confirm our guess that the algo used is TEA because we find two 32-bit adders
between each encryption stage.
- Extract the two 32-bit data words for two consecutive TEA rounds and use
bruteforce to find out the encryption keys -> `b'bocchi za rock!\x00'`
- Decrypt the `flag.enc` file with this key and obtain the flag:
`pbctf{amazing job! now go enjoy an after school tea time break!}`

Original writeup (https://github.com/berndoJ/revenge-of-lllattice-writeup).