[RFC PATCH 2/4] ecdsa: initial support of ecdsa using mbedtls

[Philippe Reynes]

Adds an initial support of ecdsa verify using mbedtls.

Signed-off-by: Philippe Reynes <philippe.reynes at softathome.com>
---
 include/crypto/internal/sw_ecdsa.h | 14 +++++
 lib/mbedtls/Makefile               |  3 +
 lib/mbedtls/sw_ecdsa.c             | 94 ++++++++++++++++++++++++++++++
 3 files changed, 111 insertions(+)
 create mode 100644 include/crypto/internal/sw_ecdsa.h
 create mode 100644 lib/mbedtls/sw_ecdsa.c

diff --git a/include/crypto/internal/sw_ecdsa.h b/include/crypto/internal/sw_ecdsa.h
new file mode 100644
index 00000000000..b1ca31da0f8
--- /dev/null
+++ b/include/crypto/internal/sw_ecdsa.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2026, Philippe Reynes <philippe.reynes at softathome.com>
+ */
+#ifndef _SW_ECDSA
+#define _SW_ECDSA
+
+struct ecdsa_public_key;
+
+int sw_ecdsa_verify(const struct ecdsa_public_key *pubkey,
+		    const void *hash, size_t hash_len,
+		    const void *signature, size_t sig_len);
+
+#endif
diff --git a/lib/mbedtls/Makefile b/lib/mbedtls/Makefile
index 54a893609cf..a5331313a60 100644
--- a/lib/mbedtls/Makefile
+++ b/lib/mbedtls/Makefile
@@ -11,6 +11,9 @@ obj-$(CONFIG_$(PHASE_)SHA1_MBEDTLS) += sha1.o
 obj-$(CONFIG_$(PHASE_)SHA256_MBEDTLS) += sha256.o
 obj-$(CONFIG_$(PHASE_)SHA512_MBEDTLS) += sha512.o
 
+# shim layer for crypto
+obj-$(CONFIG_$(PHASE_)ECDSA_MBEDTLS) += sw_ecdsa.o
+
 # x509 libraries
 obj-$(CONFIG_$(PHASE_)ASYMMETRIC_PUBLIC_KEY_MBEDTLS) += \
 	public_key.o
diff --git a/lib/mbedtls/sw_ecdsa.c b/lib/mbedtls/sw_ecdsa.c
new file mode 100644
index 00000000000..0ed95f4407f
--- /dev/null
+++ b/lib/mbedtls/sw_ecdsa.c
@@ -0,0 +1,94 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2026 Philippe Reynes <philippe.reynes at softathome.com>
+ */
+
+#include <crypto/ecdsa-uclass.h>
+#include "mbedtls_options.h" /* required to access private fields */
+#include <mbedtls/ecdsa.h>
+#include <mbedtls/ecp.h>
+
+static mbedtls_ecp_group_id sw_ecdsa_search_group_id(const char *curve_name)
+{
+	mbedtls_ecp_group_id grp_id = MBEDTLS_ECP_DP_NONE;
+	const mbedtls_ecp_curve_info *info;
+
+	if (!curve_name)
+		goto out;
+
+	if (!strcmp(curve_name, "prime256v1"))
+		grp_id = MBEDTLS_ECP_DP_SECP256R1;
+
+	info = mbedtls_ecp_curve_list();
+	while (info && info->name) {
+		if (!strcmp(curve_name, info->name))
+			grp_id = info->grp_id;
+		info++;
+	}
+
+ out:
+	return grp_id;
+}
+
+int sw_ecdsa_verify(const struct ecdsa_public_key *pubkey,
+		    const void *hash, size_t hash_len,
+		    const void *signature, size_t sig_len)
+{
+	mbedtls_ecp_group_id grp_id;
+	mbedtls_ecp_group grp;
+	const unsigned char *buf = hash;
+	size_t blen = hash_len;
+	mbedtls_ecp_point Q;
+	mbedtls_mpi r, s;
+	int key_len;
+	int err = -1;
+
+	if (!(pubkey->size_bits % 8))
+		key_len = pubkey->size_bits / 8;
+	else
+		key_len = pubkey->size_bits / 8 + 1;
+
+	/* search the group */
+	grp_id = sw_ecdsa_search_group_id(pubkey->curve_name);
+	if (grp_id == MBEDTLS_ECP_DP_NONE) {
+		printf("%s: curve name %s not found\n",
+		       __func__, pubkey->curve_name);
+		goto out;
+	}
+
+	/* init and load the group */
+	mbedtls_ecp_group_init(&grp);
+	err = mbedtls_ecp_group_load(&grp, grp_id);
+	if (err < 0)
+		goto out;
+
+	/* prepare the pubkey */
+	mbedtls_ecp_point_init(&Q);
+	mbedtls_mpi_init(&Q.X);
+	mbedtls_mpi_init(&Q.Y);
+	mbedtls_mpi_init(&Q.Z);
+	mbedtls_mpi_read_binary(&Q.X, pubkey->x, key_len);
+	mbedtls_mpi_read_binary(&Q.Y, pubkey->y, key_len);
+	mbedtls_mpi_lset(&Q.Z, 1);
+
+	/* check if the pubkey is valid */
+	err = mbedtls_ecp_check_pubkey(&grp, &Q);
+	if (err < 0) {
+		printf("%s: public key is invalid (err = %d)\n", __func__, err);
+		goto out;
+	}
+
+	/* compute r */
+	mbedtls_mpi_init(&r);
+	mbedtls_mpi_read_binary(&r, signature, key_len);
+
+	/* compute s */
+	mbedtls_mpi_init(&s);
+	mbedtls_mpi_read_binary(&s, signature + key_len, key_len);
+
+	/* check the signature */
+	err = mbedtls_ecdsa_verify(&grp, buf, blen, &Q, &r, &s);
+
+ out:
+	return err;
+}
-- 
2.43.0