[PATCH RFC v2 3/5] lib: Add support for ECDSA image signing

Alexandru Gagniuc
2020-12-30T21:00:25Z

mkimage supports rsa2048, and rsa4096 signatures. With newer silicon
now supporting hardware-accelerated ECDSA, it makes sense to expand
signing support to elliptic curves.

Implement host-side ECDSA signing and verification with libcrypto.
Device-side implementation of signature verification is beyond the
scope of this patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
---
common/image-sig.c | 14 +-
include/u-boot/ecdsa.h | 27 ++++
lib/ecdsa/ecdsa-libcrypto.c | 300 ++++++++++++++++++++++++++++++++++++
tools/Makefile | 3 +
4 files changed, 342 insertions(+), 2 deletions(-)
create mode 100644 include/u-boot/ecdsa.h
create mode 100644 lib/ecdsa/ecdsa-libcrypto.c

diff --git a/common/image-sig.c b/common/image-sig.c
index 21dafe6b91..2548b3eb0f 100644
--- a/common/image-sig.c
+++ b/common/image-sig.c
@@ -15,6 +15,7 @@
DECLARE_GLOBAL_DATA_PTR;
#endif /* !USE_HOSTCC*/
#include <image.h>
+#include <u-boot/ecdsa.h>
#include <u-boot/rsa.h>
#include <u-boot/hash-checksum.h>

@@ -82,8 +83,17 @@ struct crypto_algo crypto_algos[] = {
.sign = rsa_sign,
.add_verify_data = rsa_add_verify_data,
.verify = rsa_verify,
- }
-
+ },
+#if defined(USE_HOSTCC)
+ /* Currently, only host support exists for ECDSA */
+ {
+ .name = "ecdsa256",
+ .key_len = ECDSA256_BYTES,
+ .sign = ecdsa_sign,
+ .add_verify_data = ecdsa_add_verify_data,
+ .verify = ecdsa_verify,
+ },
+#endif
};

struct padding_algo padding_algos[] = {
diff --git a/include/u-boot/ecdsa.h b/include/u-boot/ecdsa.h
new file mode 100644
index 0000000000..a13a7267e1
--- /dev/null
+++ b/include/u-boot/ecdsa.h
@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2020, Alexandru Gagniuc <mr.nuke.me@gmail.com>.
+ */
+
+#ifndef _ECDSA_H
+#define _ECDSA_H
+
+#include <errno.h>
+#include <image.h>
+
+/**
+ * crypto_algo API impementation for ECDSA;
+ * @see "struct crypto_algo"
+ * @{
+ */
+int ecdsa_sign(struct image_sign_info *info, const struct image_region region[],
+ int region_count, uint8_t **sigp, uint *sig_len);
+int ecdsa_verify(struct image_sign_info *info,
+ const struct image_region region[], int region_count,
+ uint8_t *sig, uint sig_len);
+int ecdsa_add_verify_data(struct image_sign_info *info, void *keydest);
+/** @} */
+
+#define ECDSA256_BYTES (256 / 8)
+
+#endif
diff --git a/lib/ecdsa/ecdsa-libcrypto.c b/lib/ecdsa/ecdsa-libcrypto.c
new file mode 100644
index 0000000000..ff491411d0
--- /dev/null
+++ b/lib/ecdsa/ecdsa-libcrypto.c
@@ -0,0 +1,300 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * ECDSA image signing implementation using libcrypto backend
+ *
+ * The signature is a binary representation of the (R, S) points, padded to the
+ * key size. The signature will be (2 * key_size_bits) / 8 bytes.
+ *
+ * Deviations from behavior of RSA equivalent:
+ * - Verification uses private key. This is not technically required, but a
+ * limitation on how clumsy the openssl API is to use.
+ * - Handling of keys and key paths:
+ * - The '-K' key directory option must contain path to the key file,
+ * instead of the key directory.
+ * - No assumptions are made about the file extension of the key
+ * - The 'key-name-hint' property is only used for naming devicetree nodes,
+ * but is not used for looking up keys on the filesystem.
+ *
+ * Copyright (c) 2020, Alexandru Gagniuc <mr.nuke.me@gmail.com>
+ */
+
+#include <u-boot/ecdsa.h>
+#include <u-boot/fdt-libcrypto.h>
+#include <openssl/ssl.h>
+#include <openssl/ec.h>
+#include <openssl/bn.h>
+
+struct signer {
+ EVP_PKEY *evp_key;
+ EC_KEY *ecdsa_key;
+ void *hash;
+ void *signature; /* Do not free() !*/
+};
+
+static int alloc_ctx(struct signer *ctx, const struct image_sign_info *info)
+{
+ memset(ctx, 0, sizeof(*ctx));
+
+ if (!OPENSSL_init_ssl(0, NULL)) {
+ fprintf(stderr, "Failure to init SSL library\n");
+ return -1;
+ }
+
+ ctx->hash = malloc(info->checksum->checksum_len);
+ ctx->signature = malloc(info->crypto->key_len * 2);
+
+ if (!ctx->hash || !ctx->signature)
+ return -1;
+
+ return 0;
+}
+
+static void free_ctx(struct signer *ctx)
+{
+ if (ctx->ecdsa_key)
+ EC_KEY_free(ctx->ecdsa_key);
+
+ if (ctx->evp_key)
+ EVP_PKEY_free(ctx->evp_key);
+
+ if (ctx->hash)
+ free(ctx->hash);
+}
+
+/*
+ * Convert an ECDSA signature to raw format
+ *
+ * openssl DER-encodes 'binary' signatures. We want the signature in a raw
+ * (R, S) point pair. So we have to dance a bit.
+ */
+static void ecdsa_sig_encode_raw(void *buf, const ECDSA_SIG *sig, size_t order)
+{
+ int point_bytes = order;
+ const BIGNUM *r, *s;
+ uintptr_t s_buf;
+
+ ECDSA_SIG_get0(sig, &r, &s);
+ s_buf = (uintptr_t)buf + point_bytes;
+ BN_bn2binpad(r, buf, point_bytes);
+ BN_bn2binpad(s, (void *)s_buf, point_bytes);
+}
+
+static ECDSA_SIG *ecdsa_sig_from_raw(void *buf, size_t order)
+{
+ int point_bytes = order;
+ uintptr_t s_buf;
+ ECDSA_SIG *sig;
+ BIGNUM *r, *s;
+
+ sig = ECDSA_SIG_new();
+ if (!sig)
+ return NULL;
+
+ s_buf = (uintptr_t)buf + point_bytes;
+ r = BN_bin2bn(buf, point_bytes, NULL);
+ s = BN_bin2bn((void *)s_buf, point_bytes, NULL);
+ ECDSA_SIG_set0(sig, r, s);
+
+ return sig;
+}
+
+static size_t ecdsa_key_size_bytes(const EC_KEY *key)
+{
+ const EC_GROUP *group;
+
+ group = EC_KEY_get0_group(key);
+ return EC_GROUP_order_bits(group) / 8;
+}
+
+static int read_key(struct signer *ctx, const char *key_name)
+{
+ FILE *f = fopen(key_name, "r");
+
+ if (!f) {
+ fprintf(stderr, "Can not get key file '%s'\n", key_name);
+ return -1;
+ }
+
+ ctx->evp_key = PEM_read_PrivateKey(f, NULL, NULL, NULL);
+ fclose(f);
+ if (!ctx->evp_key) {
+ fprintf(stderr, "Can not read key from '%s'\n", key_name);
+ return -1;
+ }
+
+ if (EVP_PKEY_id(ctx->evp_key) != EVP_PKEY_EC) {
+ fprintf(stderr, "'%s' is not an ECDSA key\n", key_name);
+ return -1;
+ }
+
+ ctx->ecdsa_key = EVP_PKEY_get1_EC_KEY(ctx->evp_key);
+ if (!ctx->ecdsa_key)
+ fprintf(stderr, "Can not extract ECDSA key\n");
+
+ return (ctx->ecdsa_key) ? 0 : -1;
+}
+
+static int prepare_ctx(struct signer *ctx, const struct image_sign_info *info)
+{
+ const char *kname = info->keydir;
+ int key_len_bytes;
+
+ if (alloc_ctx(ctx, info) < 0)
+ return -1;
+
+ if (read_key(ctx, kname) < 0)
+ return -1;
+
+ key_len_bytes = ecdsa_key_size_bytes(ctx->ecdsa_key);
+ if (key_len_bytes != info->crypto->key_len) {
+ fprintf(stderr, "Expected a %u-bit key, got %u-bit key\n",
+ info->crypto->key_len * 8, key_len_bytes * 8);
+ return -1;
+ }
+
+ return 0;
+}
+
+static int do_sign(struct signer *ctx, struct image_sign_info *info,
+ const struct image_region region[], int region_count)
+{
+ const struct checksum_algo *algo = info->checksum;
+ ECDSA_SIG *sig;
+
+ algo->calculate(algo->name, region, region_count, ctx->hash);
+ sig = ECDSA_do_sign(ctx->hash, algo->checksum_len, ctx->ecdsa_key);
+
+ ecdsa_sig_encode_raw(ctx->signature, sig, info->crypto->key_len);
+
+ return 0;
+}
+
+static int ecdsa_check_signature(struct signer *ctx, struct image_sign_info *info)
+{
+ ECDSA_SIG *sig;
+ int okay;
+
+ sig = ecdsa_sig_from_raw(ctx->signature, info->crypto->key_len);
+ if (!sig)
+ return -1;
+
+ okay = ECDSA_do_verify(ctx->hash, info->checksum->checksum_len,
+ sig, ctx->ecdsa_key);
+ if (!okay)
+ fprintf(stderr, "WARNING: Signature is fake news!\n");
+
+ ECDSA_SIG_free(sig);
+ return !okay;
+}
+
+static int do_verify(struct signer *ctx, struct image_sign_info *info,
+ const struct image_region region[], int region_count,
+ uint8_t *raw_sig, uint sig_len)
+{
+ const struct checksum_algo *algo = info->checksum;
+
+ if (sig_len != info->crypto->key_len * 2) {
+ fprintf(stderr, "Signature has wrong length\n");
+ return -1;
+ }
+
+ memcpy(ctx->signature, raw_sig, sig_len);
+ algo->calculate(algo->name, region, region_count, ctx->hash);
+
+ return ecdsa_check_signature(ctx, info);
+}
+
+int ecdsa_sign(struct image_sign_info *info, const struct image_region region[],
+ int region_count, uint8_t **sigp, uint *sig_len)
+{
+ struct signer ctx;
+ int ret;
+
+ ret = prepare_ctx(&ctx, info);
+ if (ret >= 0) {
+ do_sign(&ctx, info, region, region_count);
+ *sigp = ctx.signature;
+ *sig_len = info->crypto->key_len * 2;
+
+ ret = ecdsa_check_signature(&ctx, info);
+ }
+
+ free_ctx(&ctx);
+ return ret;
+}
+
+int ecdsa_verify(struct image_sign_info *info,
+ const struct image_region region[], int region_count,
+ uint8_t *sig, uint sig_len)
+{
+ struct signer ctx;
+ int ret;
+
+ ret = prepare_ctx(&ctx, info);
+ if (ret >= 0)
+ ret = do_verify(&ctx, info, region, region_count, sig, sig_len);
+
+ free_ctx(&ctx);
+ return ret;
+}
+
+static int do_add(struct signer *ctx, void *fdt, const char *key_node_name)
+{
+ int signature_node, key_node, ret, key_bits;
+ const char *curve_name;
+ const EC_GROUP *group;
+ const EC_POINT *point;
+ BIGNUM *x, *y;
+
+ signature_node = fdt_subnode_offset(fdt, 0, FIT_SIG_NODENAME);
+ if (signature_node < 0) {
+ fprintf(stderr, "Could not find 'signature node: %s\n",
+ fdt_strerror(signature_node));
+ return signature_node;
+ }
+
+ key_node = fdt_add_subnode(fdt, signature_node, key_node_name);
+ if (key_node < 0) {
+ fprintf(stderr, "Could not create '%s' node: %s\n",
+ key_node_name, fdt_strerror(key_node));
+ return key_node;
+ }
+
+ group = EC_KEY_get0_group(ctx->ecdsa_key);
+ key_bits = EC_GROUP_order_bits(group);
+ curve_name = OBJ_nid2sn(EC_GROUP_get_curve_name(group));
+ /* Let 'x' and 'y' memory leak by not BN_free()'ing them. */
+ x = BN_new();
+ y = BN_new();
+ point = EC_KEY_get0_public_key(ctx->ecdsa_key);
+ EC_POINT_get_affine_coordinates(group, point, x, y, NULL);
+
+ ret = fdt_setprop_string(fdt, key_node, "ecdsa,curve", curve_name);
+ if (ret < 0)
+ return ret;
+
+ ret = fdt_add_bignum(fdt, key_node, "ecdsa,x-point", x, key_bits);
+ if (ret < 0)
+ return ret;
+
+ ret = fdt_add_bignum(fdt, key_node, "ecdsa,y-point", y, key_bits);
+ if (ret < 0)
+ return ret;
+
+ return 0;
+}
+
+int ecdsa_add_verify_data(struct image_sign_info *info, void *fdt)
+{
+ const char *fdt_key_name;
+ struct signer ctx;
+ int ret;
+
+ fdt_key_name = info->keyname ? info->keyname : "default-key";
+ ret = prepare_ctx(&ctx, info);
+ if (ret >= 0)
+ do_add(&ctx, fdt, fdt_key_name);
+
+ free_ctx(&ctx);
+ return ret;
+}
diff --git a/tools/Makefile b/tools/Makefile
index af7698fd01..a4f7b7e80c 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -70,6 +70,8 @@ RSA_OBJS-$(CONFIG_FIT_SIGNATURE) := $(addprefix lib/rsa/, \
rsa-sign.o rsa-verify.o \
rsa-mod-exp.o)

+ECDSA_OBJS-$(CONFIG_FIT_SIGNATURE) := $(addprefix lib/ecdsa/, ecdsa-libcrypto.o)
+
AES_OBJS-$(CONFIG_FIT_CIPHER) := $(addprefix lib/aes/, \
aes-encrypt.o aes-decrypt.o)

@@ -119,6 +121,7 @@ dumpimage-mkimage-objs := aisimage.o \
gpimage.o \
gpimage-common.o \
mtk_image.o \
+ $(ECDSA_OBJS-y) \
$(RSA_OBJS-y) \
$(AES_OBJS-y)

--
2.26.2