boot: Rename boot_enc_decrypt to boot_decrypt_key

All of boot_enc_ function follow the same pattern where
they take encryption context as the first parameter, and the
boot_enc_decrypt stands out here as it does not work around
the encryption context, but is rather single-part decryption
function only used for decrypting of the image encryption
key.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

boot/bootutil/include/bootutil/enc_key.h

@@ -48,14 +48,16 @@ struct enc_key_data {
 extern const struct bootutil_key bootutil_enc_key;
 struct boot_status;
 
+/* Decrypt random, symmetric encryption key */
+int boot_decrypt_key(const uint8_t *buf, uint8_t *enckey);
+
 int boot_enc_init(struct enc_key_data *enc_state, uint8_t slot);
 int boot_enc_drop(struct enc_key_data *enc_state, uint8_t slot);
 int boot_enc_set_key(struct enc_key_data *enc_state, uint8_t slot,
  const struct boot_status *bs);
 int boot_enc_load(struct enc_key_data *enc_state, int image_index,
  const struct image_header *hdr, const struct flash_area *fap,
  struct boot_status *bs);
-int boot_enc_decrypt(const uint8_t *buf, uint8_t *enckey);
 bool boot_enc_valid(struct enc_key_data *enc_state, int image_index,
  const struct flash_area *fap);
 void boot_encrypt(struct enc_key_data *enc_state, int image_index,

boot/bootutil/src/bootutil_misc.c

@@ -283,7 +283,7 @@ boot_read_enc_key(const struct flash_area *fap, uint8_t slot, struct boot_status
  }
  /* Only try to decrypt non-erased TLV metadata */
  if (i != BOOT_ENC_TLV_ALIGN_SIZE) {
- rc = boot_enc_decrypt(bs->enctlv[slot], bs->enckey[slot]);
+ rc = boot_decrypt_key(bs->enctlv[slot], bs->enckey[slot]);
  }
  }
 #else

boot/bootutil/src/encrypted.c

@@ -415,7 +415,7 @@ static int fake_rng(void *p_rng, unsigned char *output, size_t len)
  * @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)
+boot_decrypt_key(const uint8_t *buf, uint8_t *enckey)
 {
 #if defined(MCUBOOT_ENCRYPT_RSA)
  bootutil_rsa_context rsa;
@@ -660,7 +660,7 @@ boot_enc_load(struct enc_key_data *enc_state, int image_index,
  return -1;
  }
 
- return boot_enc_decrypt(buf, bs->enckey[slot]);
+ return boot_decrypt_key(buf, bs->enckey[slot]);
 }
 
 bool