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AEM

AEM notes.

Adversary capabilities

  • physical access
  • remove disk
  • re-fash BIOS
  • exfiltrate TPM

Anti Evil Maid (AEM) alternatives:

  • BIOS password
    • attack requires removal of disk
    • there might be a BIOS master password
  • boot partition on USB
    • attack on BIOS required
    • or USB persistent modifications
  • disk hasher -> attack on BIOS required

Anti Evil Maid security issues:

  • download binary file SINIT without verification and hope the processor will correctly verify it
  • rely on closed source TPM

Anti Evil Maid (AEM)

  • authenticates machine to the user (not user to machine)
  • change in BIOS -> reseal required
  • another complicated password
  • either text OR picture, not both
  • picture support is going to be deprecated
  • one time password support (2FA) coming
  • measurement of Xen, kernel, and initrd versions
  • only compatible with legacy boot
  • not compatible with UEFI
  • not compatible with USBVM
  • not compatible with USB 3.0
  • therefore install on internal device
  • do not pass "-z" option, i.e. set AEM password
  • hide USB from dom0 (more secure but also more fragile) (BadUSB attack) vs not-hide (able to use USB AEM)
  • AEM on USB encourages to boot from USB -> makes it easier for an evil maid (no need to change settings in BIOS)
  • only reason for AEM on USB -> as a keyfile (second factor -> better yubikey