doc/uImage.FIT/signature.txt - platform/external/u-boot - Gitiles

 U-Boot FIT Signature Verification
 =================================

 Introduction
 ------------
 FIT supports hashing of images so that these hashes can be checked on
 loading. This protects against corruption of the image. However it does not
 prevent the substitution of one image for another.

 The signature feature allows the hash to be signed with a private key such
 that it can be verified using a public key later. Provided that the private
 key is kept secret and the public key is stored in a non-volatile place,
 any image can be verified in this way.

 See verified-boot.txt for more general information on verified boot.


 Concepts
 --------
 Some familiarity with public key cryptography is assumed in this section.

 The procedure for signing is as follows:

    - hash an image in the FIT
    - sign the hash with a private key to produce a signature
    - store the resulting signature in the FIT

 The procedure for verification is:

    - read the FIT
    - obtain the public key
    - extract the signature from the FIT
    - hash the image from the FIT
    - verify (with the public key) that the extracted signature matches the
        hash

 The signing is generally performed by mkimage, as part of making a firmware
 image for the device. The verification is normally done in U-Boot on the
 device.


 Algorithms
 ----------
 In principle any suitable algorithm can be used to sign and verify a hash.
 At present only one class of algorithms is supported: SHA1 hashing with RSA.
 This works by hashing the image to produce a 20-byte hash.

 While it is acceptable to bring in large cryptographic libraries such as
 openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
 For the run-time verification side, it is important to keep code and data
 size as small as possible.

 For this reason the RSA image verification uses pre-processed public keys
 which can be used with a very small amount of code - just some extraction
 of data from the FDT and exponentiation mod n. Code size impact is a little
 under 5KB on Tegra Seaboard, for example.

 It is relatively straightforward to add new algorithms if required. If
 another RSA variant is needed, then it can be added to the table in
 image-sig.c. If another algorithm is needed (such as DSA) then it can be
 placed alongside rsa.c, and its functions added to the table in image-sig.c
 also.


 Creating an RSA key and certificate
 -----------------------------------
 To create a new public key, size 2048 bits:

 $ openssl genrsa -F4 -out keys/dev.key 2048

 To create a certificate for this:

 $ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt

 If you like you can look at the public key also:

 $ openssl rsa -in keys/dev.key -pubout


 Device Tree Bindings
 --------------------
 The following properties are required in the FIT's signature node(s) to
 allow thes signer to operate. These should be added to the .its file.
 Signature nodes sit at the same level as hash nodes and are called
 signature@1, signature@2, etc.

 - algo: Algorithm name (e.g. "sha1,rs2048")

 - key-name-hint: Name of key to use for signing. The keys will normally be in
 a single directory (parameter -k to mkimage). For a given key <name>, its
 private key is stored in <name>.key and the certificate is stored in
 <name>.crt.

 When the image is signed, the following properties are added (mandatory):

 - value: The signature data (e.g. 256 bytes for 2048-bit RSA)

 When the image is signed, the following properties are optional:

 - timestamp: Time when image was signed (standard Unix time_t format)

 - signer-name: Name of the signer (e.g. "mkimage")

 - signer-version: Version string of the signer (e.g. "2013.01")

 - comment: Additional information about the signer or image

 For config bindings (see Signed Configurations below), the following
 additional properties are optional:

 - sign-images: A list of images to sign, each being a property of the conf
 node that contains then. The default is "kernel,fdt" which means that these
 two images will be looked up in the config and signed if present.

 For config bindings, these properties are added by the signer:

 - hashed-nodes: A list of nodes which were hashed by the signer. Each is
 	a string - the full path to node. A typical value might be:

 	hashed-nodes = "/", "/configurations/conf@1", "/images/kernel@1",
 		"/images/kernel@1/hash@1", "/images/fdt@1",
 		"/images/fdt@1/hash@1";

 - hashed-strings: The start and size of the string region of the FIT that
 	was hashed

 Example: See sign-images.its for an example image tree source file and
 sign-configs.its for config signing.


 Public Key Storage
 ------------------
 In order to verify an image that has been signed with a public key we need to
 have a trusted public key. This cannot be stored in the signed image, since
 it would be easy to alter. For this implementation we choose to store the
 public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).

 Public keys should be stored as sub-nodes in a /signature node. Required
 properties are:

 - algo: Algorithm name (e.g. "sha1,rs2048")

 Optional properties are:

 - key-name-hint: Name of key used for signing. This is only a hint since it
 is possible for the name to be changed. Verification can proceed by checking
 all available signing keys until one matches.

 - required: If present this indicates that the key must be verified for the
 image / configuration to be considered valid. Only required keys are
 normally verified by the FIT image booting algorithm. Valid values are
 "image" to force verification of all images, and "conf" to force verfication
 of the selected configuration (which then relies on hashes in the images to
 verify those).

 Each signing algorithm has its own additional properties.

 For RSA the following are mandatory:

 - rsa,num-bits: Number of key bits (e.g. 2048)
 - rsa,modulus: Modulus (N) as a big-endian multi-word integer
 - rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
 - rsa,n0-inverse: -1 / modulus[0] mod 2^32


 Signed Configurations
 ---------------------
 While signing images is useful, it does not provide complete protection
 against several types of attack. For example, it it possible to create a
 FIT with the same signed images, but with the configuration changed such
 that a different one is selected (mix and match attack). It is also possible
 to substitute a signed image from an older FIT version into a newer FIT
 (roll-back attack).

 As an example, consider this FIT:

 / {
 	images {
 		kernel@1 {
 			data = <data for kernel1>
 			signature@1 {
 				algo = "sha1,rsa2048";
 				value = <...kernel signature 1...>
 			};
 		};
 		kernel@2 {
 			data = <data for kernel2>
 			signature@1 {
 				algo = "sha1,rsa2048";
 				value = <...kernel signature 2...>
 			};
 		};
 		fdt@1 {
 			data = <data for fdt1>;
 			signature@1 {
 				algo = "sha1,rsa2048";
 				vaue = <...fdt signature 1...>
 			};
 		};
 		fdt@2 {
 			data = <data for fdt2>;
 			signature@1 {
 				algo = "sha1,rsa2048";
 				vaue = <...fdt signature 2...>
 			};
 		};
 	};
 	configurations {
 		default = "conf@1";
 		conf@1 {
 			kernel = "kernel@1";
 			fdt = "fdt@1";
 		};
 		conf@1 {
 			kernel = "kernel@2";
 			fdt = "fdt@2";
 		};
 	};
 };

 Since both kernels are signed it is easy for an attacker to add a new
 configuration 3 with kernel 1 and fdt 2:

 	configurations {
 		default = "conf@1";
 		conf@1 {
 			kernel = "kernel@1";
 			fdt = "fdt@1";
 		};
 		conf@1 {
 			kernel = "kernel@2";
 			fdt = "fdt@2";
 		};
 		conf@3 {
 			kernel = "kernel@1";
 			fdt = "fdt@2";
 		};
 	};

 With signed images, nothing protects against this. Whether it gains an
 advantage for the attacker is debatable, but it is not secure.

 To solved this problem, we support signed configurations. In this case it
 is the configurations that are signed, not the image. Each image has its
 own hash, and we include the hash in the configuration signature.

 So the above example is adjusted to look like this:

 / {
 	images {
 		kernel@1 {
 			data = <data for kernel1>
 			hash@1 {
 				algo = "sha1";
 				value = <...kernel hash 1...>
 			};
 		};
 		kernel@2 {
 			data = <data for kernel2>
 			hash@1 {
 				algo = "sha1";
 				value = <...kernel hash 2...>
 			};
 		};
 		fdt@1 {
 			data = <data for fdt1>;
 			hash@1 {
 				algo = "sha1";
 				value = <...fdt hash 1...>
 			};
 		};
 		fdt@2 {
 			data = <data for fdt2>;
 			hash@1 {
 				algo = "sha1";
 				value = <...fdt hash 2...>
 			};
 		};
 	};
 	configurations {
 		default = "conf@1";
 		conf@1 {
 			kernel = "kernel@1";
 			fdt = "fdt@1";
 			signature@1 {
 				algo = "sha1,rsa2048";
 				value = <...conf 1 signature...>;
 			};
 		};
 		conf@2 {
 			kernel = "kernel@2";
 			fdt = "fdt@2";
 			signature@1 {
 				algo = "sha1,rsa2048";
 				value = <...conf 1 signature...>;
 			};
 		};
 	};
 };


 You can see that we have added hashes for all images (since they are no
 longer signed), and a signature to each configuration. In the above example,
 mkimage will sign configurations/conf@1, the kernel and fdt that are
 pointed to by the configuration (/images/kernel@1, /images/kernel@1/hash@1,
 /images/fdt@1, /images/fdt@1/hash@1) and the root structure of the image
 (so that it isn't possible to add or remove root nodes). The signature is
 written into /configurations/conf@1/signature@1/value. It can easily be
 verified later even if the FIT has been signed with other keys in the
 meantime.


 Verification
 ------------
 FITs are verified when loaded. After the configuration is selected a list
 of required images is produced. If there are 'required' public keys, then
 each image must be verified against those keys. This means that every image
 that might be used by the target needs to be signed with 'required' keys.

 This happens automatically as part of a bootm command when FITs are used.


 Enabling FIT Verification
 -------------------------
 In addition to the options to enable FIT itself, the following CONFIGs must
 be enabled:

 CONFIG_FIT_SIGNATURE - enable signing and verfication in FITs
 CONFIG_RSA - enable RSA algorithm for signing

 WARNING: When relying on signed FIT images with required signature check
 the legacy image format is default disabled by not defining
 CONFIG_IMAGE_FORMAT_LEGACY

 Testing
 -------
 An easy way to test signing and verfication is to use the test script
 provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
 of U-Boot which runs under Linux) to show the operation of a 'bootm'
 command loading and verifying images.

 A sample run is show below:

 $ make O=sandbox sandbox_config
 $ make O=sandbox
 $ O=sandbox ./test/vboot/vboot_test.sh
 Simple Verified Boot Test
 =========================

 Please see doc/uImage.FIT/verified-boot.txt for more information

 /home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
 Build keys
 do sha1 test
 Build FIT with signed images
 Test Verified Boot Run: unsigned signatures:: OK
 Sign images
 Test Verified Boot Run: signed images: OK
 Build FIT with signed configuration
 Test Verified Boot Run: unsigned config: OK
 Sign images
 Test Verified Boot Run: signed config: OK
 check signed config on the host
 Signature check OK
 OK
 Test Verified Boot Run: signed config: OK
 Test Verified Boot Run: signed config with bad hash: OK
 do sha256 test
 Build FIT with signed images
 Test Verified Boot Run: unsigned signatures:: OK
 Sign images
 Test Verified Boot Run: signed images: OK
 Build FIT with signed configuration
 Test Verified Boot Run: unsigned config: OK
 Sign images
 Test Verified Boot Run: signed config: OK
 check signed config on the host
 Signature check OK
 OK
 Test Verified Boot Run: signed config: OK
 Test Verified Boot Run: signed config with bad hash: OK

 Test passed


 Future Work
 -----------
 - Roll-back protection using a TPM is done using the tpm command. This can
 be scripted, but we might consider a default way of doing this, built into
 bootm.


 Possible Future Work
 --------------------
 - Add support for other RSA/SHA variants, such as rsa4096,sha512.
 - Other algorithms besides RSA
 - More sandbox tests for failure modes
 - Passwords for keys/certificates
 - Perhaps implement OAEP
 - Enhance bootm to permit scripted signature verification (so that a script
 can verify an image but not actually boot it)


 Simon Glass
 sjg@chromium.org
 1-1-13
	U-Boot FIT Signature Verification
	=================================

	Introduction
	------------
	FIT supports hashing of images so that these hashes can be checked on
	loading. This protects against corruption of the image. However it does not
	prevent the substitution of one image for another.

	The signature feature allows the hash to be signed with a private key such
	that it can be verified using a public key later. Provided that the private
	key is kept secret and the public key is stored in a non-volatile place,
	any image can be verified in this way.

	See verified-boot.txt for more general information on verified boot.


	Concepts
	--------
	Some familiarity with public key cryptography is assumed in this section.

	The procedure for signing is as follows:

	- hash an image in the FIT
	- sign the hash with a private key to produce a signature
	- store the resulting signature in the FIT

	The procedure for verification is:

	- read the FIT
	- obtain the public key
	- extract the signature from the FIT
	- hash the image from the FIT
	- verify (with the public key) that the extracted signature matches the
	hash

	The signing is generally performed by mkimage, as part of making a firmware
	image for the device. The verification is normally done in U-Boot on the
	device.


	Algorithms
	----------
	In principle any suitable algorithm can be used to sign and verify a hash.
	At present only one class of algorithms is supported: SHA1 hashing with RSA.
	This works by hashing the image to produce a 20-byte hash.

	While it is acceptable to bring in large cryptographic libraries such as
	openssl on the host side (e.g. mkimage), it is not desirable for U-Boot.
	For the run-time verification side, it is important to keep code and data
	size as small as possible.

	For this reason the RSA image verification uses pre-processed public keys
	which can be used with a very small amount of code - just some extraction
	of data from the FDT and exponentiation mod n. Code size impact is a little
	under 5KB on Tegra Seaboard, for example.

	It is relatively straightforward to add new algorithms if required. If
	another RSA variant is needed, then it can be added to the table in
	image-sig.c. If another algorithm is needed (such as DSA) then it can be
	placed alongside rsa.c, and its functions added to the table in image-sig.c
	also.


	Creating an RSA key and certificate
	-----------------------------------
	To create a new public key, size 2048 bits:

	$ openssl genrsa -F4 -out keys/dev.key 2048

	To create a certificate for this:

	$ openssl req -batch -new -x509 -key keys/dev.key -out keys/dev.crt

	If you like you can look at the public key also:

	$ openssl rsa -in keys/dev.key -pubout


	Device Tree Bindings
	--------------------
	The following properties are required in the FIT's signature node(s) to
	allow thes signer to operate. These should be added to the .its file.
	Signature nodes sit at the same level as hash nodes and are called
	signature@1, signature@2, etc.

	- algo: Algorithm name (e.g. "sha1,rs2048")

	- key-name-hint: Name of key to use for signing. The keys will normally be in
	a single directory (parameter -k to mkimage). For a given key <name>, its
	private key is stored in <name>.key and the certificate is stored in
	<name>.crt.

	When the image is signed, the following properties are added (mandatory):

	- value: The signature data (e.g. 256 bytes for 2048-bit RSA)

	When the image is signed, the following properties are optional:

	- timestamp: Time when image was signed (standard Unix time_t format)

	- signer-name: Name of the signer (e.g. "mkimage")

	- signer-version: Version string of the signer (e.g. "2013.01")

	- comment: Additional information about the signer or image

	For config bindings (see Signed Configurations below), the following
	additional properties are optional:

	- sign-images: A list of images to sign, each being a property of the conf
	node that contains then. The default is "kernel,fdt" which means that these
	two images will be looked up in the config and signed if present.

	For config bindings, these properties are added by the signer:

	- hashed-nodes: A list of nodes which were hashed by the signer. Each is
	a string - the full path to node. A typical value might be:

	hashed-nodes = "/", "/configurations/conf@1", "/images/kernel@1",
	"/images/kernel@1/hash@1", "/images/fdt@1",
	"/images/fdt@1/hash@1";

	- hashed-strings: The start and size of the string region of the FIT that
	was hashed

	Example: See sign-images.its for an example image tree source file and
	sign-configs.its for config signing.


	Public Key Storage
	------------------
	In order to verify an image that has been signed with a public key we need to
	have a trusted public key. This cannot be stored in the signed image, since
	it would be easy to alter. For this implementation we choose to store the
	public key in U-Boot's control FDT (using CONFIG_OF_CONTROL).

	Public keys should be stored as sub-nodes in a /signature node. Required
	properties are:

	- algo: Algorithm name (e.g. "sha1,rs2048")

	Optional properties are:

	- key-name-hint: Name of key used for signing. This is only a hint since it
	is possible for the name to be changed. Verification can proceed by checking
	all available signing keys until one matches.

	- required: If present this indicates that the key must be verified for the
	image / configuration to be considered valid. Only required keys are
	normally verified by the FIT image booting algorithm. Valid values are
	"image" to force verification of all images, and "conf" to force verfication
	of the selected configuration (which then relies on hashes in the images to
	verify those).

	Each signing algorithm has its own additional properties.

	For RSA the following are mandatory:

	- rsa,num-bits: Number of key bits (e.g. 2048)
	- rsa,modulus: Modulus (N) as a big-endian multi-word integer
	- rsa,r-squared: (2^num-bits)^2 as a big-endian multi-word integer
	- rsa,n0-inverse: -1 / modulus[0] mod 2^32


	Signed Configurations
	---------------------
	While signing images is useful, it does not provide complete protection
	against several types of attack. For example, it it possible to create a
	FIT with the same signed images, but with the configuration changed such
	that a different one is selected (mix and match attack). It is also possible
	to substitute a signed image from an older FIT version into a newer FIT
	(roll-back attack).

	As an example, consider this FIT:

	/ {
	images {
	kernel@1 {
	data = <data for kernel1>
	signature@1 {
	algo = "sha1,rsa2048";
	value = <...kernel signature 1...>
	};
	};
	kernel@2 {
	data = <data for kernel2>
	signature@1 {
	algo = "sha1,rsa2048";
	value = <...kernel signature 2...>
	};
	};
	fdt@1 {
	data = <data for fdt1>;
	signature@1 {
	algo = "sha1,rsa2048";
	vaue = <...fdt signature 1...>
	};
	};
	fdt@2 {
	data = <data for fdt2>;
	signature@1 {
	algo = "sha1,rsa2048";
	vaue = <...fdt signature 2...>
	};
	};
	};
	configurations {
	default = "conf@1";
	conf@1 {
	kernel = "kernel@1";
	fdt = "fdt@1";
	};
	conf@1 {
	kernel = "kernel@2";
	fdt = "fdt@2";
	};
	};
	};

	Since both kernels are signed it is easy for an attacker to add a new
	configuration 3 with kernel 1 and fdt 2:

	configurations {
	default = "conf@1";
	conf@1 {
	kernel = "kernel@1";
	fdt = "fdt@1";
	};
	conf@1 {
	kernel = "kernel@2";
	fdt = "fdt@2";
	};
	conf@3 {
	kernel = "kernel@1";
	fdt = "fdt@2";
	};
	};

	With signed images, nothing protects against this. Whether it gains an
	advantage for the attacker is debatable, but it is not secure.

	To solved this problem, we support signed configurations. In this case it
	is the configurations that are signed, not the image. Each image has its
	own hash, and we include the hash in the configuration signature.

	So the above example is adjusted to look like this:

	/ {
	images {
	kernel@1 {
	data = <data for kernel1>
	hash@1 {
	algo = "sha1";
	value = <...kernel hash 1...>
	};
	};
	kernel@2 {
	data = <data for kernel2>
	hash@1 {
	algo = "sha1";
	value = <...kernel hash 2...>
	};
	};
	fdt@1 {
	data = <data for fdt1>;
	hash@1 {
	algo = "sha1";
	value = <...fdt hash 1...>
	};
	};
	fdt@2 {
	data = <data for fdt2>;
	hash@1 {
	algo = "sha1";
	value = <...fdt hash 2...>
	};
	};
	};
	configurations {
	default = "conf@1";
	conf@1 {
	kernel = "kernel@1";
	fdt = "fdt@1";
	signature@1 {
	algo = "sha1,rsa2048";
	value = <...conf 1 signature...>;
	};
	};
	conf@2 {
	kernel = "kernel@2";
	fdt = "fdt@2";
	signature@1 {
	algo = "sha1,rsa2048";
	value = <...conf 1 signature...>;
	};
	};
	};
	};


	You can see that we have added hashes for all images (since they are no
	longer signed), and a signature to each configuration. In the above example,
	mkimage will sign configurations/conf@1, the kernel and fdt that are
	pointed to by the configuration (/images/kernel@1, /images/kernel@1/hash@1,
	/images/fdt@1, /images/fdt@1/hash@1) and the root structure of the image
	(so that it isn't possible to add or remove root nodes). The signature is
	written into /configurations/conf@1/signature@1/value. It can easily be
	verified later even if the FIT has been signed with other keys in the
	meantime.


	Verification
	------------
	FITs are verified when loaded. After the configuration is selected a list
	of required images is produced. If there are 'required' public keys, then
	each image must be verified against those keys. This means that every image
	that might be used by the target needs to be signed with 'required' keys.

	This happens automatically as part of a bootm command when FITs are used.


	Enabling FIT Verification
	-------------------------
	In addition to the options to enable FIT itself, the following CONFIGs must
	be enabled:

	CONFIG_FIT_SIGNATURE - enable signing and verfication in FITs
	CONFIG_RSA - enable RSA algorithm for signing

	WARNING: When relying on signed FIT images with required signature check
	the legacy image format is default disabled by not defining
	CONFIG_IMAGE_FORMAT_LEGACY

	Testing
	-------
	An easy way to test signing and verfication is to use the test script
	provided in test/vboot/vboot_test.sh. This uses sandbox (a special version
	of U-Boot which runs under Linux) to show the operation of a 'bootm'
	command loading and verifying images.

	A sample run is show below:

	$ make O=sandbox sandbox_config
	$ make O=sandbox
	$ O=sandbox ./test/vboot/vboot_test.sh
	Simple Verified Boot Test
	=========================

	Please see doc/uImage.FIT/verified-boot.txt for more information

	/home/hs/ids/u-boot/sandbox/tools/mkimage -D -I dts -O dtb -p 2000
	Build keys
	do sha1 test
	Build FIT with signed images
	Test Verified Boot Run: unsigned signatures:: OK
	Sign images
	Test Verified Boot Run: signed images: OK
	Build FIT with signed configuration
	Test Verified Boot Run: unsigned config: OK
	Sign images
	Test Verified Boot Run: signed config: OK
	check signed config on the host
	Signature check OK
	OK
	Test Verified Boot Run: signed config: OK
	Test Verified Boot Run: signed config with bad hash: OK
	do sha256 test
	Build FIT with signed images
	Test Verified Boot Run: unsigned signatures:: OK
	Sign images
	Test Verified Boot Run: signed images: OK
	Build FIT with signed configuration
	Test Verified Boot Run: unsigned config: OK
	Sign images
	Test Verified Boot Run: signed config: OK
	check signed config on the host
	Signature check OK
	OK
	Test Verified Boot Run: signed config: OK
	Test Verified Boot Run: signed config with bad hash: OK

	Test passed


	Future Work
	-----------
	- Roll-back protection using a TPM is done using the tpm command. This can
	be scripted, but we might consider a default way of doing this, built into
	bootm.


	Possible Future Work
	--------------------
	- Add support for other RSA/SHA variants, such as rsa4096,sha512.
	- Other algorithms besides RSA
	- More sandbox tests for failure modes
	- Passwords for keys/certificates
	- Perhaps implement OAEP
	- Enhance bootm to permit scripted signature verification (so that a script
	can verify an image but not actually boot it)


	Simon Glass
	sjg@chromium.org
	1-1-13