Hey @Aziz Munawar
I am not sure that it is listed anywhere in the Cisco documentation for the FTD2100 family. I would hazard a strong guess that the SEDs use AES, likely AES-256, for FDE.
If you need audit-grade proof for compliance purposes, I would get a hold of your Cisco AM/SE and request it - otherwise, you might be able to find it from the OEMs.
Good luck!
- BW
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