lib: Add support for ECDSA image signing

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>
Reviewed-by: Simon Glass <sjg@chromium.org>

1 files changed, 306 insertions, 0 deletions

diff --git a/lib/ecdsa/ecdsa-libcrypto.c b/lib/ecdsa/ecdsa-libcrypto.c
new file mode 100644
index 0000000..3228809
--- /dev/null
+++ b/lib/ecdsa/ecdsa-libcrypto.c
@@ -0,0 +1,306 @@
+// 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) 2020,2021, 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>
+
+/* Image signing context for openssl-libcrypto */
+struct signer {
+	EVP_PKEY *evp_key;	/* Pointer to EVP_PKEY object */
+	EC_KEY *ecdsa_key;	/* Pointer to EC_KEY object */
+	void *hash;		/* Pointer to hash used for verification */
+	void *signature;	/* Pointer to output 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 -ENOMEM;
+
+	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);
+}
+
+/* Get a signature from a raw encoding */
+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;
+}
+
+/* ECDSA key size in bytes */
+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 -ENOENT;
+	}
+
+	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 -EIO;
+	}
+
+	if (EVP_PKEY_id(ctx->evp_key) != EVP_PKEY_EC) {
+		fprintf(stderr, "'%s' is not an ECDSA key\n", key_name);
+		return -EINVAL;
+	}
+
+	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 : -EINVAL;
+}
+
+/* Prepare a 'signer' context that's ready to sign and verify. */
+static int prepare_ctx(struct signer *ctx, const struct image_sign_info *info)
+{
+	const char *kname = info->keydir;
+	int key_len_bytes, ret;
+
+	ret = alloc_ctx(ctx, info);
+	if (ret)
+		return ret;
+
+	ret = read_key(ctx, kname);
+	if (ret)
+		return ret;
+
+	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 -EINVAL;
+	}
+
+	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 -ENOMEM;
+
+	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 -EINVAL;
+	}
+
+	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;
+}