Just use RsaCtfTool:
```
cd /opt/RsaCtfTool
./RsaCtfTool.py --publickey /media/DATA/CTF/yubitsec/RSA_II/PublicKey.pem --private --uncipher /media/DATA/CTF/yubitsec/RSA_II/ciphertext
```
After the launch it only remains to collect the flag:
```
gh0st3rs@leacher-pc:RsaCtfTool$ sudo ./RsaCtfTool.py --publickey /media/DATA/CTF/yubitsec/RSA_II/PublicKey.pem --private --uncipher /media/DATA/CTF/yubitsec/RSA_II/ciphertext
-----BEGIN RSA PRIVATE KEY-----
MIIQSgIBAAKCA6Y/////////////////////////////////////////////////
////////////////////////////////////////////////////////////////
.........
AAAAAAAgAAAAAAAAgAAAAAAAAQAAAAAAAAQAAAAAAAAIAAAAAAAAIAAAAAAAAEAA
AAAAAAEAAAAAAAACAAAAAAAACAAAAAAAABAAAAAAAABAAAAAAAAAgAAAAAAAAQ==
-----END RSA PRIVATE KEY-----
[+] Clear text : ?�?�?�;�?�z�?�:���??l�SM?�n?zӃ�ܠ�.??�o�Wy?|?��)3Z>7sC��?m�d(dO����?`ɗ�L�W���4;k�63۲?���?�����Dt��0?����.jp�i`4�??
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�'�]O�~YUBITSEC{G00D_J0B_BRO_Y0U_MUST_KN0W_H0W_D03S_1T_W0RKS}
```

Flag: YUBITSEC{G00D_J0B_BRO_Y0U_MUST_KN0W_H0W_D03S_1T_W0RKS}