Reverse the program with disassembler, setup a breakpoint at location of interest and run against server to retrieve the flag (or just run strace).

```py
from pwn import *

# Allows you to switch between local/GDB/remote from terminal
def start(argv=[], *a, **kw):
if args.GDB: # Set GDBscript below
return gdb.debug([exe] + argv, gdbscript=gdbscript, *a, **kw)
elif args.REMOTE: # ('server', 'port')
return remote(sys.argv[1], sys.argv[2], *a, **kw)
else: # Run locally
return process([exe] + argv, *a, **kw)

# Specify GDB script here (breakpoints etc)
gdbscript = '''
init-pwndbg
breakrva 0x16a0
breakrva 0x1707
breakrva 0x1748
continue
'''.format(**locals())

# Binary filename
exe = './client'
# This will automatically get context arch, bits, os etc
elf = context.binary = ELF(exe, checksec=False)
# Change logging level to help with debugging (warning/info/debug)
context.log_level = 'debug'

# ===========================================================
# EXPLOIT GOES HERE
# ===========================================================

client = listen(1337) # Setup listener on port 1337
io = start(['127.0.0.1', '1337']) # Launch binary with localhost:1337
# Wait for the client to connect to the server
io = client.wait_for_connection()

# FOR TESTING - turns out solution was breakpoint/strace
io.send(b'HTB{0123456789abcdefghijklmnopq}')
print(io.recv())

io.send(b'HTB{0123456789abcdefghijklmnopq}')
print(io.recv())

io.send(b'1')

io.interactive()
```