2.9.3.4 Double Step Verification

In Double-Step mode, an additional check is done after image decryption. Instead of random data in the header padding field, an AES-CMAC tag with the customer key set is used to verify the plain text bootstrap image.

The following figure shows a tagged bootstrap image format.

Figure 2-29. Secure Mode Image Format (Tagged Version)

In this mode Auth. Data value is as follows:

ModeRFUAdditional Data Size
0x12 or 0x130x000x0000

Mode is encrypted, tagged, double-step. There is no additional data for this mode.

When DSS mode is used, image format is shown in the figure below.

Figure 2-30. Secure Mode Image Format (DSS Version)

Auth. Data field is as follows:

ModeRFUAdditional Data Size
0x14,0x150x000xyyyy

Mode is encrypted, signed with DSS, double-step. A security blob is added following the bootstrap image. Security data size specifies the size in bytes of this security chain used to certify the image.