Encrypting a USB Flash Drive

[mac_in_tosh]

I recently updated to Mac OS Sierra. I notice that in Disk Utility if you create an encrypted disk image it still gives the choice between 128-bit and 256-bit encryption but if you use that program to encrypt a mounted USB flash drive, it doesn't. Same if you just right click on the mounted flash drive - it just asks for a password and hint.

So which level of encryption is being used for flash drives?

Thanks.