Boot Time Security

Example BIF File

To create a boot image with encrypted partitions, the AES key file is specified in the BIF using the aeskeyfile attribute. Specify an encryption=aes attribute for each image file listed in the BIF file to be encrypted. The example BIF file (secure.bif) is shown below:

image: 
{
	[aeskeyfile] secretkey.nky
	[keysrc_encryption] efuse
	[bootloader, encryption=aes] fsbl.elf
	[encryption=aes] uboot.elf
}

bootgen -arch zynq -image secure.bif -w -o BOOT.bin

Key Generation

Bootgen can generate AES-CBC keys. Bootgen uses the AES key file specified in the BIF for encrypting the partitions. If the key file is empty or non-existent, Bootgen generates the keys in the file specified in the BIF file. If the key file is not specified in the BIF, and encryption is requested for any of the partitions, then Bootgen generates a key file with the name of the BIF file with extension .nky in the same directory as of BIF. The following is a sample key file.

Key Management

Good key management practice includes minimizing the use of secret or private keys. This can be accomplished this by using different key/IV pairs across different partitions in the boot image. The result is that the AES key stored on the device, in either the BBRAM or eFUSEs, is used for only 384 bits, which significantly limits its exposure to side channel attacks.

all: {
  image
  {
    {type=bootloader, encryption=aes, keysrc=bbram_red_key, aeskeyfile=plm.nky, dpacm_enable, file=plm.elf}
    {type=pmcdata, load=0xf2000000, aeskeyfile = pmc_data.nky, file=pmc_data.cdo}
    {core=psm, file=psm.elf}
    {type=cdo, encryption=aes, keysrc=bbram_red_key, aeskeyfile=ps_data.nky, file=ps_data.cdo}
    {type=cdo, file=subsystem.cdo}
    {core=a72-0, exception_level = el-3, file=a72-app.elf}
  }
}

Meta Header Authentication

For a Versal ACAP, Bootgen authenticates the meta header based on the parameters under the bif attribute "metaheader". A snippet of the usage is shown below.

metaheader
{
	authentication = rsa,
	pskfile = psk.pem,
	sskfile = ssk.pem
}

PPK Hash for eFUSE

Bootgen generates the PPK hash for storing in eFUSE for PPK to be trusted. This step is required only for authentication with eFUSE mode, and can be skipped for Boot Header Authentication. The value from efuseppksha.txt can be programmed to eFUSE for authentication with the eFUSE mode.

BIF File Example

The following is a sample BIF file, generate_hash_ppk.bif.

generate_hash_ppk:
{
    pskfile = primary0.pem
    sskfile = secondary0.pem
    image
    {
        name = pmc_ss, id = 0x1c000001
        { type=bootloader, authentication=rsa, file=plm.elf}
        { type=pmcdata, load=0xf2000000, file=pmc_cdo.bin}
    }
}

Command

The command to generate PPK hash for eFUSE programming is:

bootgen –image generate_hash_ppk.bif –arch versal –w –o test.bin –efuseppkbits efuseppksha.txt

Cumulative Secure Boot Operations for Versal ACAP

The following figure shows a Stage 0 to Stage 2 Boot stack that uses the HSM mode. It reduces the number of steps by distributing the SSK.

This figure uses the Zynq® UltraScale+™ MPSoC device to illustrate the stages.

Boot Process

Creating a boot image using HSM mode is similar to creating a boot image using a standard flow with following BIF file.

all:
{
	[auth_params] ppk_select=1;spk_id=0x8
	[keysrc_encryption]bbram_red_key
	[pskfile]primary.pem
	[sskfile]secondary.pem
	[
	 bootloader,
	 encryption=aes,
 	aeskeyfile=aes.nky,
	 authentication=rsa
	]fsbl.elf
	[destination_cpu=a53-0,authentication=rsa]hello_a53_0_64.elf
}

Stage 0: Create a boot image using HSM Mode

A trusted individual creates the SPK signature using the Primary Secret Key. The SPK Signature is on the Authentication Certificate Header, SPK, and SPK ID. To generate a hash for the above, use the following BIF file snippet.

stage 0:
{
	[auth_params] ppk_select=1;spk_id=0x3
	[spkfile]keys/secondary.pub
}

The following is the Bootgen command:

bootgen -arch zynqmp -image stage0.bif -generate_hashes

The output of this command is: secondary.pub.sha384.

Stage 1: Distribute the SPK Signature

The trusted individual distributes the SPK Signature to the development teams.

openssl rsautl -raw -sign -inkey keys/primary0.pem -in secondary.pub.sha384 > secondary.pub.sha384.sig 

The output of this command is: secondary.pub.sha384.sig

Stage 2: Encrypt using AES in FSBL

The development teams use Bootgen to create as many boot images as needed. The development teams use:

• The SPK Signature from the Trusted Individual.
• The Secondary Secret Key (SSK), SPK, and SPKID

Stage2:
{
	[keysrc_encryption]bbram_red_key 
	[auth_params] ppk_select=1;spk_id=0x3
	[ppkfile]keys/primary.pub
	[sskfile]keys/secondary0.pem
	[spksignature]secondary.pub.sha384.sig 
	[bootloader,destination_cpu=a53-0, encryption=aes, aeskeyfile=aes0.nky, authentication=rsa] fsbl.elf 
	[destination_cpu=a53-0, authentication=rsa] hello_a53_0_64.elf 
}

bootgen -arch zynqmp -image stage2.bif -o final.bin

The following figure provides a diagram of an HSM mode boot image for a Zynq®-7000 SoC device. The steps to create this boot image are immediately after the diagram.

The process to create a boot image using HSM mode for a Zynq®-7000 SoC device is similar to that of a boot image created using a standard flow with the following BIF file. These examples, where needed, use the OpenSSL program to generate hash files.

all:
{
	[aeskeyfile]my_efuse.nky 
	[pskfile]primary.pem 
	[sskfile]secondary.pem
	[bootloader,encryption=aes,authentication=rsa] zynq_fsbl_0.elf
	[authentication=rsa]system.bit
}

Stage 0: Generate a hash for SPK

This stage generates the hash of the SPK key.

stage0:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
}

The following is the Bootgen command.

bootgen -image stage0.bif –w -generate_hashes

Stage 1: Sign the SPK Hash

This stage creates the signatures by signing the SPK hash

xil_rsa_sign.exe -gensig -sk primary.pem -data secondary.pub.sha256 -out secondary.pub.sha256.sig

#Swap the bytes in SPK hash
objcopy -I binary -O binary --reverse-bytes=256 secondary.pub.sha256

#Generate SPK signature using OpenSSL
openssl rsautl -raw -sign -inkey primary.pem -in secondary.pub.sha256 > secondary.pub.sha256.sig

#Swap the bytes in SPK signature
objcopy -I binary -O binary --reverse-bytes=256 secondary.pub.sha256.sig

Stage 2: Encrypt using AES

This stage encrypts the partition. The stage2.bif is as follows.

stage2:
{
	[aeskeyfile] my_efuse.nky
	[bootloader, encryption=aes] zynq_fsbl_0.elf
}

bootgen -image stage2.bif -w -o fsbl_e.bin -encrypt efuse

Stage 3: Generate Partition Hashes

This stage generates the hashes of different partitions.

The BIF file is as follows:

stage3a:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature] secondary.pub.sha256.sig
	[bootimage, authentication=rsa] fsbl_e.bin
}

bootgen -image stage3a.bif -w -generate_hashes

The output is the hash file zynq_fsbl_0.elf.0.sha256.

The stage3b BIF file is as follows:

stage3b:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature] secondary.pub.sha256.sig
	[authentication=rsa] system.bit
}

bootgen -image stage3b.bif -w -generate_hashes

Stage 4: Sign the Hashes

This stage creates signatures from the partition hash files created.

xil_rsa_sign.exe -gensig -sk secondary.pem -data zynq_fsbl_0.elf.0.sha256 -out zynq_fsbl_0.elf.0.sha256.sig

#Swap the bytes in FSBL hash
objcopy -I binary -O binary --reverse-bytes=256 zynq_fsbl_0.elf.0.sha256

#Generate FSBL signature using OpenSSL
openssl rsautl -raw -sign -inkey secondary.pem -in zynq_fsbl_0.elf.0.sha256 > zynq_fsbl_0.elf.0.sha256.sig

#Swap the bytes in FSBL signature
objcopy -I binary -O binary --reverse-bytes=256 zynq_fsbl_0.elf.0.sha256.sig

The output is the signature file zynq_fsbl_0.elf.0.sha256.sig.

xil_rsa_sign.exe -gensig -sk secondary.pem -data system.bit.0.sha256 -out system.bit.0.sha256.sig

#Swap the bytes in bitstream hash
objcopy -I binary -O binary --reverse-bytes=256 system.bit.0.sha256

#Generate bitstream signature using OpenSSL
openssl rsautl -raw -sign -inkey secondary.pem -in system.bit.0.sha256 > system.bit.0.sha256.sig

#Swap the bytes in bitstream signature
objcopy -I binary -O binary --reverse-bytes=256 system.bit.0.sha256.sig

Stage 5: Insert Partition Signatures

Insert partition signatures created above are changed into authentication certificates.

The stage5a.bif is as follows.

stage5a:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature] secondary.pub.sha256.sig
	[bootimage, authentication=rsa, presign=zynq_fsbl_0.elf.0.sha256.sig] fsbl_e.bin
}

bootgen -image stage5a.bif -w -o fsbl_e_ac.bin -efuseppkbits efuseppkbits.txt -nonbooting

stage5b:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature] secondary.pub.sha256.sig
	[authentication=rsa, presign=system.bit.0.sha256.sig] system.bit
}

bootgen -image stage5b.bif -o system_e_ac.bin –nonbooting

Stage 6: Generate Header Table Hash

This stage generates the hash for the header tables.

stage6:
{
	[bootimage] fsbl_e_ac.bin
	[bootimage] system_e_ac.bin
}

bootgen -image stage6.bif -generate_hashes

Stage 7: Generate Header Table Signature

This stage generates the header table signature.

xil_rsa_sign.exe -gensig -sk secondary.pem -data ImageHeaderTable.sha256 -out ImageHeaderTable.sha256.sig

#Swap the bytes in header table hash
objcopy -I binary -O binary --reverse-bytes=256 ImageHeaderTable.sha256

#Generate header table signature using OpenSSL
openssl rsautl -raw -sign -inkey secondary.pem -in ImageHeaderTable.sha256 > ImageHeaderTable.sha256.sig

#Swap the bytes in header table signature
objcopy -I binary -O binary --reverse-bytes=256 ImageHeaderTable.sha256.sig

Stage 8: Combine Partitions, Insert Header Table Signature

The stage8.bif is as follows:

stage8:
{
	[headersignature] ImageHeaderTable.sha256.sig
	[bootimage] fsbl_e_ac.bin
	[bootimage] system_e_ac.bin
}

bootgen -image stage8.bif -w -o final.bin

The following figure provides a diagram of an HSM mode boot image.

To create a boot image using HSM mode for a Zynq® UltraScale+™ MPSoC device, it would be similar to a boot image created using a standard flow with the following BIF file. These examples, where needed, use the OpenSSL program to generate hash files.

all:
{
	[fsbl_config] bh_auth_enable
	[keysrc_encryption] bbram_red_key
	[pskfile] primary0.pem
	[sskfile] secondary0.pem
	
	[
	  bootloader,
	  destination_cpu=a53-0,
	  encryption=aes,
	  aeskeyfile=aes0.nky,
	  authentication=rsa
	] fsbl.elf

	[
	  destination_device=pl,
	  encryption=aes,
	  aeskeyfile=aes1.nky,
 	 authentication=rsa
	] system.bit

	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-3,
	  trustzone=secure
	] bl31.elf

	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-2
	] u-boot.elf
}

Stage 0: Generate a hash for SPK

The following is the snippet from the BIF file.

stage0:
{
	[ppkfile]primary.pub
	[spkfile]secondary.pub
}

The following is the Bootgen command:

bootgen -arch zynqmp -image stage0.bif -generate_hashes -w on -log error

Stage 1: Sign the SPK Hash (encrypt the partitions)

The following is a code snippet using OpenSSL to generate the SPK hash:

openssl rsautl -raw -sign -inkey primary0.pem -in secondary.pub.sha384 > secondary.pub.sha384.sig

The output of this command is secondary.pub.sha384.sig.

Stage 2a: Encrypt the FSBL

Encrypt the FSBL using the following snippet in the BIF file.

Stage 2a:
{
	[keysrc_encryption] bbram_red_key

	[
	  bootloader,destination_cpu=a53-0,
	  encryption=aes,
	  aeskeyfile=aes0.nky
	] fsbl.elf
}

The Bootgen command is:

bootgen -arch zynqmp -image stage2a.bif -o fsbl_e.bin -w on -log error

Stage 2b: Encrypt Bitstream

Generate the following BIF file entry:

stage2b:
{
	[
	  encryption=aes,
	  aeskeyfile=aes1.nky,
	  destination_device=pl,
	  pid=1
	] system.bit
}

The Bootgen command is:

bootgen -arch zynqmp -image stage2b.bif -o system_e.bin -w on -log error

Stage 3: Generate Boot Header Hash

Generate the boot header hash using the following BIF file:

stage3:
{
 	[fsbl_config] bh_auth_enable
 	[ppkfile] primary.pub
 	[spkfile] secondary.pub
 	[spksignature]secondary.pub.sha384.sig
 	[bootimage,authentication=rsa]fsbl_e.bin
}

The Bootgen command is:

bootgen -arch zynqmp -image stage3.bif -generate_hashes -w on -log error

Stage 4: Sign Boot Header Hash

Generate the boot header hash with the following OpenSSL command:

openssl rsautl -raw -sign -inkey secondary0.pem -in bootheader.sha384 > bootheader.sha384.sig

Stage 5: Get Partition Hashes

Get partition hashes using the following command in a BIF file:

stage5:
{		
	[ppkfile]primary.pub
	[spkfile]secondary.pub
	[spksignature]secondary.pub.sha384.sig
	[bhsignature]bootheader.sha384.sig 
	[bootimage,authentication=rsa]fsbl_e.bin
	[bootimage,authentication=rsa]system_e.bin
	
	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-3,
	  trustzone=secure
	] bl31.elf

	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-2
	] u-boot.elf
}

The Bootgen command is:

bootgen -arch zynqmp -image stage5.bif -generate_hashes -w on -log error

Multiple hashes will be generated for a bitstream partition. For more details, see Bitstream Authentication Using External Memory.

The Boot Header hash is also generated from in this stage5; which is different from the one generated in stage3, because the parameter bh_auth_enable is not used in stage5. This can be added in stage5 if needed, but does not have a significant impact because the Boot Header hash generated using stage3 is signed in stage4 and this signature will only be used in the HSM mode flow.

Stage 6: Sign Partition Hashes

Create the following files using OpenSSL:

openssl rsautl -raw -sign -inkey secondary0.pem -in fsbl.elf.0.sha384 > fsbl.elf.0.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in system.bit.0.sha384 > system.bit.0.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in system.bit.1.sha384 > system.bit.1.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in system.bit.2.sha384 > system.bit.2.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in system.bit.3.sha384 > system.bit.3.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in u-boot.elf.0.sha384 > u-boot.elf.0.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in bl31.elf.0.sha384 > bl31.elf.0.sha384.sig
openssl rsautl -raw -sign -inkey secondary0.pem -in bl31.elf.1.sha384 > bl31.elf.1.sha384.sig

Stage 7: Insert Partition Signatures into Authentication Certificate

Stage 7a: Insert the FSBL signature by adding this code to a BIF file:

Stage7a:
{
	[fsbl_config] bh_auth_enable
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature]secondary.pub.sha384.sig
	[bhsignature]bootheader.sha384.sig
	[bootimage,authentication=rsa,presign=fsbl.elf.0.sha384.sig]fsbl_e.bin
}

bootgen -arch zynqmp -image stage7a.bif -o fsbl_e_ac.bin -efuseppkbits 
efuseppkbits.txt -nonbooting -w on -log error

Stage 7b: Insert the bitstream signature by adding the following to the BIF file:

stage7b:
{
	[ppkfile]primary.pub
	[spkfile]secondary.pub
	[spksignature]secondary.pub.sha384.sig
	[bhsignature]bootheader.sha384.sig
	[
	  bootimage,
	  authentication=rsa,
	  presign=system.bit.0.sha384.sig
	] system_e.bin
}

The Bootgen command is:

bootgen -arch zynqmp -image stage7b.bif -o system_e_ac.bin -nonbooting -w on -log error

Stage 7c: Insert the U-Boot signature by adding the following to the BIF file:

stage7c:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature]secondary.pub.sha384.sig
	[bhsignature]bootheader.sha384.sig
	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-2,
	  presign=u-boot.elf.0.sha384.sig
	] u-boot.elf
}

The Bootgen command is:

bootgen -arch zynqmp -image stage7c.bif -o u-boot_ac.bin -nonbooting -w on -log error

Stage 7d: Insert the ATF signature by entering the following into a BIF file:

stage7d:
{
	[ppkfile] primary.pub
	[spkfile] secondary.pub
	[spksignature]secondary.pub.sha384.sig
	[bhsignature]bootheader.sha384.sig
	[
	  destination_cpu=a53-0,
	  authentication=rsa,
	  exception_level=el-3,
	  trustzone=secure,
	  presign=bl31.elf.0.sha384.sig
	] bl31.elf
}

The Bootgen command is:

bootgen -arch zynqmp -image stage7d.bif -o bl31_ac.bin -nonbooting -w on -log error

Stage 8: Combine Partitions, Get Header Table Hash

Enter the following in a BIF file:

stage8: 
{
	[bootimage]fsbl_e_ac.bin
	[bootimage]system_e_ac.bin
	[bootimage]bl31_ac.bin
	[bootimage]u-boot_ac.bin
}

The Bootgen command is:

bootgen -arch zynqmp -image stage8.bif -generate_hashes -o stage8.bin -w on -log error

Stage 9: Sign Header Table Hash

Generate the following files using OpenSSL:

openssl rsautl -raw -sign -inkey secondary0.pem -in ImageHeaderTable.sha384 > ImageHeaderTable.sha384.sig

Stage 10: Combine Partitions, Insert Header Table Signature

Enter the following in a BIF file:

stage10: 
{
	[headersignature]ImageHeaderTable.sha384.sig
	[bootimage]fsbl_e_ac.bin
	[bootimage]system_e_ac.bin
	[bootimage]bl31_ac.bin
	[bootimage]u-boot_ac.bin
}

The Bootgen command is:

bootgen -arch zynqmp -image stage10.bif -o final.bin -w on -log error