a tough nut to crack

例句：

1. 1.It's a tough nut to crack, but I think I can solve it.
   这是一个棘手的问题，但我认为我能解决。
2. 2.He was determined to crack the code, despite the difficulty.
   他决心破解密码，尽管困难重重。
3. 3. The task of cracking the encryption was a tough nut for the team.
   破解加密的任务对团队来说是一个巨大的挑战。