bootutil: Refactor encrypted.c to have the same logic as image_validate

Currently encrypted.c contains all the implementation of the
functions inside enc_key.h for each configuration (RSA, EC256
and X25519).

The purpose of this pr is to adopt a logic similar to the
file organization of image_validate to make it easier to
add other confifurations.

Signed-off-by: Raphael Dupont <raphael7dup@gmail.com>

boot/bootutil/CMakeLists.txt

@@ -20,6 +20,11 @@ target_sources(bootutil
         src/bootutil_misc.c
         src/bootutil_public.c
         src/caps.c
+        src/encrypt_ec256.c
+        src/encrypt_kw.c
+        src/encrypt_misc.c
+        src/encrypt_rsa.c
+        src/encrypt_x25519.c
         src/encrypted.c
         src/fault_injection_hardening.c
         src/fault_injection_hardening_delay_rng_mbedtls.c

boot/bootutil/include/bootutil/crypto/ecdh_p256.h

@@ -71,7 +71,7 @@ static inline int bootutil_ecdh_p256_shared_secret(bootutil_ecdh_p256_context *c
 #define NUM_ECC_BYTES 32
 
 #if MBEDTLS_VERSION_NUMBER >= 0x03000000
-static int fake_rng(void *p_rng, unsigned char *output, size_t len);
+int fake_rng(void *p_rng, unsigned char *output, size_t len);
 #endif
 
 typedef struct bootutil_ecdh_p256_context {

boot/bootutil/src/encrypt_ec256.c

@@ -0,0 +1,144 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2018-2019 JUUL Labs
+ * Copyright (c) 2019-2021 Arm Limited
+ */
+
+#include "encrypted_priv.h"
+
+#if defined(MCUBOOT_ENC_IMAGES) && defined(MCUBOOT_ENCRYPT_EC256)
+
+static const uint8_t ec_pubkey_oid[] = MBEDTLS_OID_EC_ALG_UNRESTRICTED;
+static const uint8_t ec_secp256r1_oid[] = MBEDTLS_OID_EC_GRP_SECP256R1;
+
+/*
+ * Parses the output of `imgtool keygen`, which produces a PKCS#8 elliptic
+ * curve keypair. See RFC5208 and RFC5915.
+ */
+static int
+parse_ec256_enckey(uint8_t **p, uint8_t *end, uint8_t *private_key)
+{
+    int rc;
+    size_t len;
+    int version;
+    mbedtls_asn1_buf alg;
+    mbedtls_asn1_buf param;
+
+    if ((rc = mbedtls_asn1_get_tag(p, end, &len,
+                    MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE)) != 0) {
+        return -1;
+    }
+
+    if (*p + len != end) {
+        return -2;
+    }
+
+    version = 0;
+    if (mbedtls_asn1_get_int(p, end, &version) || version != 0) {
+        return -3;
+    }
+
+    if ((rc = mbedtls_asn1_get_alg(p, end, &alg, &param)) != 0) {
+        return -5;
+    }
+
+    if (alg.MBEDTLS_CONTEXT_MEMBER(len) != sizeof(ec_pubkey_oid) - 1 ||
+        memcmp(alg.MBEDTLS_CONTEXT_MEMBER(p), ec_pubkey_oid, sizeof(ec_pubkey_oid) - 1)) {
+        return -6;
+    }
+    if (param.MBEDTLS_CONTEXT_MEMBER(len) != sizeof(ec_secp256r1_oid) - 1 ||
+        memcmp(param.MBEDTLS_CONTEXT_MEMBER(p), ec_secp256r1_oid, sizeof(ec_secp256r1_oid) - 1)) {
+        return -7;
+    }
+
+    if ((rc = mbedtls_asn1_get_tag(p, end, &len, MBEDTLS_ASN1_OCTET_STRING)) != 0) {
+        return -8;
+    }
+
+    /* RFC5915 - ECPrivateKey */
+
+    if ((rc = mbedtls_asn1_get_tag(p, end, &len,
+                    MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE)) != 0) {
+        return -9;
+    }
+
+    version = 0;
+    if (mbedtls_asn1_get_int(p, end, &version) || version != 1) {
+        return -10;
+    }
+
+    /* privateKey */
+
+    if ((rc = mbedtls_asn1_get_tag(p, end, &len, MBEDTLS_ASN1_OCTET_STRING)) != 0) {
+        return -11;
+    }
+
+    if (len != NUM_ECC_BYTES) {
+        return -12;
+    }
+
+    memcpy(private_key, *p, len);
+
+    /* publicKey usually follows but is not parsed here */
+
+    return 0;
+}
+
+/*
+ * Decrypt an encryption key TLV.
+ *
+ * @param buf An encryption TLV read from flash (build time fixed length)
+ * @param enckey An AES-128 or AES-256 key sized buffer to store to plain key.
+ */
+int
+boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey)
+{
+    bootutil_ecdh_p256_context ecdh_p256;
+    uint8_t shared[SHARED_KEY_LEN];
+    uint8_t *cp;
+    uint8_t *cpend;
+    uint8_t derived_key[BOOTUTIL_CRYPTO_AES_CTR_KEY_SIZE + BOOTUTIL_CRYPTO_SHA256_DIGEST_SIZE];
+    uint8_t private_key[PRIV_KEY_LEN];
+    uint8_t counter[BOOTUTIL_CRYPTO_AES_CTR_BLOCK_SIZE];
+    int rc = -1;
+
+    /*
+    * Load the stored EC256 decryption private key
+    */
+
+    cp = (uint8_t *)bootutil_enc_key.key;
+    cpend = cp + *bootutil_enc_key.len;
+
+    rc = parse_ec256_enckey(&cp, cpend, private_key);
+    if (rc) {
+        return -1;
+    }
+
+    /*
+    * First "element" in the TLV is the curve point (public key)
+    */
+
+    bootutil_ecdh_p256_init(&ecdh_p256);
+
+    rc = bootutil_ecdh_p256_shared_secret(&ecdh_p256, &buf[EC_PUBK_INDEX], private_key, shared);
+    bootutil_ecdh_p256_drop(&ecdh_p256);
+    if (rc) {
+        return -1;
+    }
+
+    rc = expand_secret(derived_key, shared);
+    if (rc) {
+        return -1;
+    }
+
+    rc = HMAC_key(buf, derived_key);
+    if (rc) {
+        return -1;
+    }
+
+    return decrypt_ciphered_key(buf, counter, derived_key, enckey);
+
+}
+
+#endif /* MCUBOOT_ENC_IMAGES && MCUBOOT_ENCRYPT_EC256 */

boot/bootutil/src/encrypt_kw.c

@@ -0,0 +1,43 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Copyright (c) 2018-2019 JUUL Labs
+ * Copyright (c) 2019-2021 Arm Limited
+ */
+
+#include "encrypted_priv.h"
+
+#if defined(MCUBOOT_ENC_IMAGES) && defined(MCUBOOT_ENCRYPT_KW)
+
+#include "bootutil/crypto/aes_kw.h"
+
+int key_unwrap(const uint8_t *wrapped, uint8_t *enckey)
+{
+    bootutil_aes_kw_context aes_kw;
+    int rc;
+
+    bootutil_aes_kw_init(&aes_kw);
+    rc = bootutil_aes_kw_set_unwrap_key(&aes_kw, bootutil_enc_key.key, *bootutil_enc_key.len);
+    if (rc != 0) {
+        goto done;
+    }
+    rc = bootutil_aes_kw_unwrap(&aes_kw, wrapped, TLV_ENC_KW_SZ, enckey, BOOT_ENC_KEY_SIZE);
+    if (rc != 0) {
+        goto done;
+    }
+
+done:
+    bootutil_aes_kw_drop(&aes_kw);
+    return rc;
+}
+
+int
+boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey)
+{
+    assert(*bootutil_enc_key.len == BOOT_ENC_KEY_SIZE);
+    return key_unwrap(buf, enckey);
+}
+
+
+#endif /* MCUBOOT_ENC_IMAGES && MCUBOOT_ENCRYPT_KW */
+