[PATCH v2 24/28] mbedtls: add RSA helper layer on MbedTLS

[Raymond Mao]

Add RSA helper layer on top on MbedTLS PK and RSA library.

Signed-off-by: Raymond Mao <raymond.mao at linaro.org>
---
Changes in v2
- Initial patch.

 lib/mbedtls/Makefile     |  1 +
 lib/mbedtls/rsa_helper.c | 99 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 100 insertions(+)
 create mode 100644 lib/mbedtls/rsa_helper.c

diff --git a/lib/mbedtls/Makefile b/lib/mbedtls/Makefile
index f0b8a1c4003..dab110891af 100644
--- a/lib/mbedtls/Makefile
+++ b/lib/mbedtls/Makefile
@@ -27,6 +27,7 @@ x509_mbedtls-$(CONFIG_$(SPL_)ASYMMETRIC_PUBLIC_KEY_SUBTYPE) += public_key.o
 x509_mbedtls-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER) += x509_cert_parser.o
 x509_mbedtls-$(CONFIG_$(SPL_)PKCS7_MESSAGE_PARSER) += pkcs7_parser.o
 x509_mbedtls-$(CONFIG_$(SPL_)MSCODE_PARSER) += mscode_parser.o
+x509_mbedtls-$(CONFIG_$(SPL_)RSA_PUBLIC_KEY_PARSER) += rsa_helper.o
 
 obj-$(CONFIG_MBEDTLS_LIB_CRYPTO) += mbedtls_lib_crypto.o
 mbedtls_lib_crypto-y := \
diff --git a/lib/mbedtls/rsa_helper.c b/lib/mbedtls/rsa_helper.c
new file mode 100644
index 00000000000..956e550c856
--- /dev/null
+++ b/lib/mbedtls/rsa_helper.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * RSA helper functions using MbedTLS
+ *
+ * Copyright (c) 2024 Linaro Limited
+ * Author: Raymond Mao <raymond.mao at linaro.org>
+ */
+
+#include <linux/err.h>
+#include <crypto/internal/rsa.h>
+#include <external/mbedtls/library/common.h>
+#include <external/mbedtls/include/mbedtls/pk.h>
+#include <external/mbedtls/include/mbedtls/rsa.h>
+#include <external/mbedtls/include/mbedtls/asn1.h>
+
+/**
+ * rsa_parse_pub_key() - decodes the BER encoded buffer and stores in the
+ *                       provided struct rsa_key, pointers to the raw key as is,
+ *                       so that the caller can copy it or MPI parse it, etc.
+ *
+ * @rsa_key:	struct rsa_key key representation
+ * @key:	key in BER format
+ * @key_len:	length of key
+ *
+ * Return:	0 on success or error code in case of error
+ */
+int rsa_parse_pub_key(struct rsa_key *rsa_key, const void *key,
+		      unsigned int key_len)
+{
+	int ret = 0;
+	mbedtls_pk_context pk;
+	mbedtls_rsa_context *rsa;
+
+	mbedtls_pk_init(&pk);
+
+	ret = mbedtls_pk_parse_public_key(&pk, (const unsigned char *)key,
+					  key_len);
+	if (ret) {
+		pr_err("Failed to parse public key, ret:-0x%04x\n",
+		       (unsigned int)-ret);
+		ret = -EINVAL;
+		goto clean_pubkey;
+	}
+
+	/* Ensure that it is a RSA key */
+	if (mbedtls_pk_get_type(&pk) != MBEDTLS_PK_RSA) {
+		pr_err("Non-RSA keys are not supported\n");
+		ret = -EKEYREJECTED;
+		goto clean_pubkey;
+	}
+
+	/* Get RSA key context */
+	rsa = mbedtls_pk_rsa(pk);
+	if (!rsa) {
+		pr_err("Failed to get RSA key context, ret:-0x%04x\n",
+		       (unsigned int)-ret);
+		ret = -EINVAL;
+		goto clean_pubkey;
+	}
+
+	/* Parse modulus (n) */
+	rsa_key->n_sz = mbedtls_mpi_size(&rsa->N);
+	rsa_key->n = kzalloc(rsa_key->n_sz, GFP_KERNEL);
+	if (!rsa_key->n) {
+		ret = -ENOMEM;
+		goto clean_pubkey;
+	}
+	ret = mbedtls_mpi_write_binary(&rsa->N, (unsigned char *)rsa_key->n,
+				       rsa_key->n_sz);
+	if (ret) {
+		pr_err("Failed to parse modulus (n), ret:-0x%04x\n",
+		       (unsigned int)-ret);
+		ret = -EINVAL;
+		goto clean_modulus;
+	}
+
+	/* Parse public exponent (e) */
+	rsa_key->e_sz = mbedtls_mpi_size(&rsa->E);
+	rsa_key->e = kzalloc(rsa_key->e_sz, GFP_KERNEL);
+	if (!rsa_key->e) {
+		ret = -ENOMEM;
+		goto clean_modulus;
+	}
+	ret = mbedtls_mpi_write_binary(&rsa->E, (unsigned char *)rsa_key->e,
+				       rsa_key->e_sz);
+	if (!ret)
+		return 0;
+
+	pr_err("Failed to parse public exponent (e), ret:-0x%04x\n",
+	       (unsigned int)-ret);
+	ret = -EINVAL;
+
+	kfree(rsa_key->e);
+clean_modulus:
+	kfree(rsa_key->n);
+clean_pubkey:
+	mbedtls_pk_free(&pk);
+	return ret;
+}
-- 
2.25.1