Commit 3e569a6b authored by Simon Glass's avatar Simon Glass Committed by Tom Rini
Browse files

image: Add signing infrastructure

Add a structure to describe an algorithm which can sign and (later) verify
Signed-off-by: default avatarSimon Glass <>
parent b5f31937
......@@ -2805,6 +2805,11 @@ FIT uImage format:
most specific compatibility entry of U-Boot's fdt's root node.
The order of entries in the configuration's fdt is ignored.
This option enables signature verification of FIT uImages,
using a hash signed and verified using RSA. See
doc/uImage.FIT/signature.txt for more details.
- Standalone program support:
......@@ -243,6 +243,7 @@ COBJS-y += dlmalloc.o
COBJS-y += image.o
COBJS-$(CONFIG_OF_LIBFDT) += image-fdt.o
COBJS-$(CONFIG_FIT) += image-fit.o
COBJS-y += memsize.o
COBJS-y += stdio.o
* Copyright (c) 2013, Google Inc.
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License as
* published by the Free Software Foundation; either version 2 of
* the License, or (at your option) any later version.
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* GNU General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston,
* MA 02111-1307 USA
#include "mkimage.h"
#include <time.h>
#include <common.h>
#endif /* !USE_HOSTCC*/
#include <errno.h>
#include <image.h>
struct image_sig_algo image_sig_algos[] = {
struct image_sig_algo *image_get_sig_algo(const char *name)
int i;
for (i = 0; i < ARRAY_SIZE(image_sig_algos); i++) {
if (!strcmp(image_sig_algos[i].name, name))
return &image_sig_algos[i];
return NULL;
U-Boot FIT Signature Verification
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.
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
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
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
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
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
Example: See sign-images.its for an example image tree source file.
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
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
An easy way to test signing and verfication is to use the test script
provided in test/vboot/ 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/
Simple Verified Boot Test
Please see doc/uImage.FIT/verified-boot.txt for more information
Build keys
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
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
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
......@@ -46,6 +46,9 @@ struct lmb;
#define CONFIG_FIT_VERBOSE 1 /* enable fit_format_{error,warning}() */
/* Support FIT image signing on host */
......@@ -670,11 +673,12 @@ int image_setup_linux(bootm_headers_t *images);
#define FIT_IMAGES_PATH "/images"
#define FIT_CONFS_PATH "/configurations"
/* hash node */
/* hash/signature node */
#define FIT_HASH_NODENAME "hash"
#define FIT_ALGO_PROP "algo"
#define FIT_VALUE_PROP "value"
#define FIT_IGNORE_PROP "uboot-ignore"
#define FIT_SIG_NODENAME "signature"
/* image node */
#define FIT_DATA_PROP "data"
......@@ -804,15 +808,19 @@ int calculate_hash(const void *data, int data_len, const char *algo,
uint8_t *value, int *value_len);
* At present we only support verification on the device
* At present we only support signing on the host, and verification on the
* device
# ifdef USE_HOSTCC
# endif
......@@ -828,6 +836,84 @@ int calculate_hash(const void *data, int data_len, const char *algo,
/* Information passed to the signing routines */
struct image_sign_info {
const char *keydir; /* Directory conaining keys */
const char *keyname; /* Name of key to use */
void *fit; /* Pointer to FIT blob */
int node_offset; /* Offset of signature node */
struct image_sig_algo *algo; /* Algorithm information */
const void *fdt_blob; /* FDT containing public keys */
int required_keynode; /* Node offset of key to use: -1=any */
const char *require_keys; /* Value for 'required' property */
/* A part of an image, used for hashing */
struct image_region {
const void *data;
int size;
struct image_sig_algo {
const char *name; /* Name of algorithm */
* sign() - calculate and return signature for given input data
* @info: Specifies key and FIT information
* @data: Pointer to the input data
* @data_len: Data length
* @sigp: Set to an allocated buffer holding the signature
* @sig_len: Set to length of the calculated hash
* This computes input data signature according to selected algorithm.
* Resulting signature value is placed in an allocated buffer, the
* pointer is returned as *sigp. The length of the calculated
* signature is returned via the sig_len pointer argument. The caller
* should free *sigp.
* @return: 0, on success, -ve on error
int (*sign)(struct image_sign_info *info,
const struct image_region region[],
int region_count, uint8_t **sigp, uint *sig_len);
* add_verify_data() - Add verification information to FDT
* Add public key information to the FDT node, suitable for
* verification at run-time. The information added depends on the
* algorithm being used.
* @info: Specifies key and FIT information
* @keydest: Destination FDT blob for public key data
* @return: 0, on success, -ve on error
int (*add_verify_data)(struct image_sign_info *info, void *keydest);
* verify() - Verify a signature against some data
* @info: Specifies key and FIT information
* @data: Pointer to the input data
* @data_len: Data length
* @sig: Signature
* @sig_len: Number of bytes in signature
* @return 0 if verified, -ve on error
int (*verify)(struct image_sign_info *info,
const struct image_region region[], int region_count,
uint8_t *sig, uint sig_len);
* image_get_sig_algo() - Look up a signature algortihm
* @param name Name of algorithm
* @return pointer to algorithm information, or NULL if not found
struct image_sig_algo *image_get_sig_algo(const char *name);
static inline int fit_image_check_target_arch(const void *fdt, int node)
return fit_image_check_arch(fdt, node, IH_ARCH_DEFAULT);
......@@ -80,6 +80,7 @@ BIN_FILES-y += proftool(SFX)
EXT_OBJ_FILES-$(CONFIG_BUILD_ENVCRC) += common/env_embedded.o
EXT_OBJ_FILES-y += common/image.o
EXT_OBJ_FILES-$(CONFIG_FIT) += common/image-fit.o
EXT_OBJ_FILES-y += common/image-sig.o
EXT_OBJ_FILES-y += lib/crc32.o
EXT_OBJ_FILES-y += lib/md5.o
EXT_OBJ_FILES-y += lib/sha1.o
......@@ -154,6 +155,10 @@ HOSTSRCS += $(addprefix $(SRCTREE)/lib/libfdt/,$(LIBFDT_OBJ_FILES-y:.o=.c))
BINS := $(addprefix $(obj),$(sort $(BIN_FILES-y)))
LIBFDT_OBJS := $(addprefix $(obj),$(LIBFDT_OBJ_FILES-y))
# We cannot check CONFIG_FIT_SIGNATURE here since it is not set on the host
FIT_SIG_OBJ_FILES := image-sig.o
FIT_SIG_OBJS := $(addprefix $(obj),$(FIT_SIG_OBJ_FILES))
HOSTOBJS := $(addprefix $(obj),$(OBJ_FILES-y))
NOPEDOBJS := $(addprefix $(obj),$(NOPED_OBJ_FILES-y))
......@@ -213,6 +218,7 @@ $(obj)mkimage$(SFX): $(obj)aisimage.o \
$(obj)image-fit.o \
$(obj)image.o \
$(obj)image-host.o \
$(obj)imximage.o \
$(obj)kwbimage.o \
$(obj)pblimage.o \
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment