When using the quartus_cpf command via the command line you will need to specify the same 256-bit key twice as the quartus_cpf command is still in the process of being updated for Stratix® V, Arria® V, and Cyclone® V devices which only require that one key be used.
You can safely ignore the syntax error because the envrypted key programming file (.ekp) will be generated. If you want to avoid this syntax error, you can follow the workaround below.
At this time if you use only one key you will get a syntax error so to generate a .ekp file using the quartus_cpf command you will need to specify the same key twice.
Example 1 from Application Note 556: Using the Design Security Features in Altera FPGAs (PDF) can be modified as shown below to show the same key used twice:
quartus_cpf --key D:\SIV_DS\key1.key:key1 --key D:\SIV_DS\key1.key:key1 D:\SIV_DS\test.sof D:\SIV_DS\test.ekp
The Convert Programming Files utility within the Quartus® II software has been updated and only requires one key when generating a .ekp file.
The quartus_cpf command is scheduled to be updated in a future release of the Quartus II software.