bootutil: PureEdDSA using ED25519

The commit adds support for PureEdDSA, which validates signature
of image rather than hash. This is most secure, available, ED25519
usage in MCUboot, but due to requirement of PureEdDSA to be able
to calculate signature at whole message at once, here image,
it only works on setups where entire image can be mapped to
device address space, so that PSA functions calculating the
signature can see the whole image at once.

This option is enabled with Kconfig option:
 CONFIG_BOOT_SIGNATURE_TYPE_PURE
when the ED25519 signature type is already selected.

Note that the option will enable SHA512 for calculating public
key hash.

Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

boot/bootutil/src/bootutil_priv.h

@@ -268,6 +268,9 @@ struct boot_loader_state {
 fih_ret bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig,
                             size_t slen, uint8_t key_id);
 
+fih_ret bootutil_verify_img(const uint8_t *img, uint32_t size,
+                            uint8_t *sig, size_t slen, uint8_t key_id);
+
 fih_ret boot_fih_memequal(const void *s1, const void *s2, size_t n);
 
 int boot_find_status(int image_index, const struct flash_area **fap);

boot/bootutil/src/image_ed25519.c

@@ -105,4 +105,41 @@ bootutil_verify_sig(uint8_t *hash, uint32_t hlen, uint8_t *sig, size_t slen,
     FIH_RET(fih_rc);
 }
 
+fih_ret
+bootutil_verify_img(const uint8_t *img, uint32_t size,
+                    uint8_t *sig, size_t slen, uint8_t key_id)
+{
+    int rc;
+    FIH_DECLARE(fih_rc, FIH_FAILURE);
+    uint8_t *pubkey;
+    uint8_t *end;
+
+    if (slen != EDDSA_SIGNATURE_LENGTH) {
+        FIH_SET(fih_rc, FIH_FAILURE);
+        goto out;
+    }
+
+    pubkey = (uint8_t *)bootutil_keys[key_id].key;
+    end = pubkey + *bootutil_keys[key_id].len;
+
+    rc = bootutil_import_key(&pubkey, end);
+    if (rc) {
+        FIH_SET(fih_rc, FIH_FAILURE);
+        goto out;
+    }
+
+    rc = ED25519_verify(img, size, sig, pubkey);
+
+    if (rc == 0) {
+        /* if verify returns 0, there was an error. */
+        FIH_SET(fih_rc, FIH_FAILURE);
+        goto out;
+    }
+
+    FIH_SET(fih_rc, FIH_SUCCESS);
+out:
+
+    FIH_RET(fih_rc);
+}
+
 #endif /* MCUBOOT_SIGN_ED25519 */

boot/bootutil/src/image_validate.c

@@ -56,6 +56,7 @@
 
 #include "bootutil_priv.h"
 
+#ifndef MCUBOOT_SIGN_PURE
 /*
  * Compute SHA hash over the image.
  * (SHA384 if ECDSA-P384 is being used,
@@ -175,6 +176,7 @@ bootutil_img_hash(struct enc_key_data *enc_state, int image_index,
 
     return 0;
 }
+#endif
 
 /*
  * Currently, we only support being able to verify one type of
@@ -361,6 +363,35 @@ bootutil_get_img_security_cnt(struct image_header *hdr,
     return 0;
 }
 
+#if defined(MCUBOOT_SIGN_PURE)
+/* Returns:
+ *  0 -- found
+ *  1 -- not found
+ * -1 -- failed for some reason
+ *
+ * Value of TLV does not matter, presence decides.
+ */
+static int bootutil_check_for_pure(const struct image_header *hdr,
+                                   const struct flash_area *fap)
+{
+    struct image_tlv_iter it;
+    uint32_t off;
+    uint16_t len;
+    int32_t rc;
+
+    rc = bootutil_tlv_iter_begin(&it, hdr, fap, IMAGE_TLV_SIG_PURE, false);
+    if (rc) {
+        return rc;
+    }
+
+    /* Search for the TLV */
+    rc = bootutil_tlv_iter_next(&it, &off, &len, NULL);
+
+    return rc;
+}
+#endif
+
+
 #ifndef ALLOW_ROGUE_TLVS
 /*
  * The following list of TLVs are the only entries allowed in the unprotected
@@ -377,6 +408,9 @@ static const uint16_t allowed_unprot_tlvs[] = {
      IMAGE_TLV_ECDSA_SIG,
      IMAGE_TLV_RSA3072_PSS,
      IMAGE_TLV_ED25519,
+#if defined(MCUBOOT_SIGN_PURE)
+     IMAGE_TLV_SIG_PURE,
+#endif
      IMAGE_TLV_ENC_RSA2048,
      IMAGE_TLV_ENC_KW,
      IMAGE_TLV_ENC_EC256,
@@ -399,7 +433,6 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
     uint32_t off;
     uint16_t len;
     uint16_t type;
-    int image_hash_valid = 0;
 #ifdef EXPECTED_SIG_TLV
     FIH_DECLARE(valid_signature, FIH_FAILURE);
 #ifndef MCUBOOT_BUILTIN_KEY
@@ -416,7 +449,10 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
 #endif /* EXPECTED_SIG_TLV */
     struct image_tlv_iter it;
     uint8_t buf[SIG_BUF_SIZE];
+#if defined(EXPECTED_HASH_TLV) && !defined(MCUBOOT_SIGN_PURE)
+    int image_hash_valid = 0;
     uint8_t hash[IMAGE_HASH_SIZE];
+#endif
     int rc = 0;
     FIH_DECLARE(fih_rc, FIH_FAILURE);
 #ifdef MCUBOOT_HW_ROLLBACK_PROT
@@ -425,6 +461,7 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
     FIH_DECLARE(security_counter_valid, FIH_FAILURE);
 #endif
 
+#if defined(EXPECTED_HASH_TLV) && !defined(MCUBOOT_SIGN_PURE)
     rc = bootutil_img_hash(enc_state, image_index, hdr, fap, tmp_buf,
             tmp_buf_sz, hash, seed, seed_len);
     if (rc) {
@@ -434,6 +471,15 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
     if (out_hash) {
         memcpy(out_hash, hash, IMAGE_HASH_SIZE);
     }
+#endif
+
+#if defined(MCUBOOT_SIGN_PURE)
+    /* If Pure type signature is expected then it has to be there */
+    rc = bootutil_check_for_pure(hdr, fap);
+    if (rc != 0) {
+	goto out;
+    }
+#endif
 
     rc = bootutil_tlv_iter_begin(&it, hdr, fap, IMAGE_TLV_ANY, false);
     if (rc) {
@@ -477,8 +523,10 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
              }
         }
 #endif
-
-        if (type == EXPECTED_HASH_TLV) {
+        switch(type) {
+#if defined(EXPECTED_HASH_TLV) && !defined(MCUBOOT_SIGN_PURE)
+        case EXPECTED_HASH_TLV:
+        {
             /* Verify the image hash. This must always be present. */
             if (len != sizeof(hash)) {
                 rc = -1;
@@ -496,8 +544,12 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
             }
 
             image_hash_valid = 1;
+            break;
+        }
+#endif /* defined(EXPECTED_HASH_TLV) && !defined(MCUBOOT_SIGN_PURE) */
 #ifdef EXPECTED_KEY_TLV
-        } else if (type == EXPECTED_KEY_TLV) {
+        case EXPECTED_KEY_TLV:
+        {
             /*
              * Determine which key we should be checking.
              */
@@ -522,9 +574,12 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
              * The key may not be found, which is acceptable.  There
              * can be multiple signatures, each preceded by a key.
              */
+            break;
+        }
 #endif /* EXPECTED_KEY_TLV */
 #ifdef EXPECTED_SIG_TLV
-        } else if (type == EXPECTED_SIG_TLV) {
+        case EXPECTED_SIG_TLV:
+        {
             /* Ignore this signature if it is out of bounds. */
             if (key_id < 0 || key_id >= bootutil_key_cnt) {
                 key_id = -1;
@@ -538,12 +593,25 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
             if (rc) {
                 goto out;
             }
+#ifndef MCUBOOT_SIGN_PURE
             FIH_CALL(bootutil_verify_sig, valid_signature, hash, sizeof(hash),
                                                            buf, len, key_id);
+#else
+            /* Directly check signature on the image, by using the mapping of
+             * a device to memory. The pointer is beginning of image in flash,
+             * so offset of area, the range is header + image + protected tlvs.
+             */
+            FIH_CALL(bootutil_verify_img, valid_signature, (void *)flash_area_get_off(fap),
+                     hdr->ih_hdr_size + hdr->ih_img_size + hdr->ih_protect_tlv_size,
+                     buf, len, key_id);
+#endif
             key_id = -1;
+            break;
+        }
 #endif /* EXPECTED_SIG_TLV */
 #ifdef MCUBOOT_HW_ROLLBACK_PROT
-        } else if (type == IMAGE_TLV_SEC_CNT) {
+        case IMAGE_TLV_SEC_CNT:
+        {
             /*
              * Verify the image's security counter.
              * This must always be present.
@@ -578,14 +646,21 @@ bootutil_img_validate(struct enc_key_data *enc_state, int image_index,
 
             /* The image's security counter has been successfully verified. */
             security_counter_valid = fih_rc;
+            break;
+        }
 #endif /* MCUBOOT_HW_ROLLBACK_PROT */
         }
     }
 
+#if defined(EXPECTED_HASH_TLV) && !defined(MCUBOOT_SIGN_PURE)
     rc = !image_hash_valid;
     if (rc) {
         goto out;
     }
+#elif defined(MCUBOOT_SIGN_PURE)
+    /* This returns true on EQ, rc is err on non-0 */
+    rc = !FIH_EQ(valid_signature, FIH_SUCCESS);
+#endif
 #ifdef EXPECTED_SIG_TLV
     FIH_SET(fih_rc, valid_signature);
 #endif

boot/zephyr/Kconfig

@@ -134,6 +134,14 @@ config BOOT_IMG_HASH_ALG_SHA512
 
 endchoice # BOOT_IMG_HASH_ALG
 
+config BOOT_SIGNATURE_TYPE_PURE_ALLOW
+	bool
+	help
+	  Hidden option set by configurations that allow Pure variant,
+	  for example ed25519. The pure variant means that image
+	  signature is calculated over entire image instead of hash
+	  of an image.
+
 choice BOOT_SIGNATURE_TYPE
 	prompt "Signature type"
 	default BOOT_SIGNATURE_TYPE_RSA
@@ -183,10 +191,31 @@ endif
 
 config BOOT_SIGNATURE_TYPE_ED25519
 	bool "Edwards curve digital signatures using ed25519"
-	select BOOT_ENCRYPTION_SUPPORT
-	select BOOT_IMG_HASH_ALG_SHA256_ALLOW
+	select BOOT_ENCRYPTION_SUPPORT if !BOOT_SIGNATURE_TYPE_PURE
+	select BOOT_IMG_HASH_ALG_SHA256_ALLOW if !BOOT_SIGNATURE_TYPE_PURE
+	# The SHA is used only for key hashing, not for images.
+	select BOOT_SIGNATURE_TYPE_PURE_ALLOW
+	help
+	  This is ed25519 signature calculated over SHA512 of SHA256 of application
+	  image; that is not completely correct approach as the SHA512 should be
+	  rather directly calculated over an image.
+	  Select BOOT_SIGNATURE_TYPE_PURE to have a PureEdDSA calculating image
+	  signature directly on image, rather than hash of the image.
 
 if BOOT_SIGNATURE_TYPE_ED25519
+
+config BOOT_SIGNATURE_TYPE_PURE
+	bool "Use Pure signature of image"
+	depends on BOOT_SIGNATURE_TYPE_PURE_ALLOW
+	help
+	  The Pure signature is calculated directly over image rather than
+	  hash of an image.
+	  This is more secure signature, specifically if hardware can do the
+	  verification without need to share key.
+	  Note that this requires that all slots for which signature is to be
+	  verified need to be accessible through memory address space that
+	  cryptography can access.
+
 choice BOOT_ED25519_IMPLEMENTATION
 	prompt "Ecdsa implementation"
 	default BOOT_ED25519_TINYCRYPT

boot/zephyr/include/mcuboot_config/mcuboot_config.h

@@ -148,6 +148,10 @@
 #define MCUBOOT_HASH_STORAGE_DIRECTLY
 #endif
 
+#ifdef CONFIG_BOOT_SIGNATURE_TYPE_PURE
+#define MCUBOOT_SIGN_PURE
+#endif
+
 #ifdef CONFIG_BOOT_BOOTSTRAP
 #define MCUBOOT_BOOTSTRAP 1
 #endif