diff --git a/boot/boot_serial/src/boot_serial_encryption.c b/boot/boot_serial/src/boot_serial_encryption.c
index cf9040b..7d3b47c 100644
--- a/boot/boot_serial/src/boot_serial_encryption.c
+++ b/boot/boot_serial/src/boot_serial_encryption.c
@@ -125,10 +125,11 @@
     size_t blk_off;
     uint16_t idx;
     uint32_t blk_sz;
-    uint8_t image_index;
-
+    int slot = flash_area_id_to_multi_image_slot(BOOT_CURR_IMG(state),
+                                                 flash_area_get_id(fap));
     uint8_t buf[sz] __attribute__((aligned));
     assert(sz <= sizeof buf);
+    assert(slot >= 0);
 
     bytes_copied = 0;
     while (bytes_copied < sz) {
@@ -143,7 +144,6 @@
             return BOOT_EFLASH;
         }
 
-        image_index = BOOT_CURR_IMG(state);
         if (IS_ENCRYPTED(hdr)) {
             blk_sz = chunk_sz;
             idx = 0;
@@ -171,7 +171,7 @@
                     blk_sz = tlv_off - (off + bytes_copied);
                 }
             }
-            boot_encrypt(BOOT_CURR_ENC(state), image_index, flash_area_get_id(fap),
+            boot_encrypt(BOOT_CURR_ENC(state), slot,
                     (off + bytes_copied + idx) - hdr->ih_hdr_size, blk_sz,
                     blk_off, &buf[idx]);
         }
diff --git a/boot/bootutil/include/bootutil/enc_key.h b/boot/bootutil/include/bootutil/enc_key.h
index 297381f..57ac244 100644
--- a/boot/bootutil/include/bootutil/enc_key.h
+++ b/boot/bootutil/include/bootutil/enc_key.h
@@ -71,9 +71,8 @@
         struct boot_status *bs);
 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,
-        int fa_id, uint32_t off, uint32_t sz,
-        uint32_t blk_off, uint8_t *buf);
+void boot_encrypt(struct enc_key_data *enc_state, int slot,
+        uint32_t off, uint32_t sz, uint32_t blk_off, uint8_t *buf);
 void boot_enc_zeroize(struct enc_key_data *enc_state);
 
 #ifdef __cplusplus
diff --git a/boot/bootutil/src/encrypted.c b/boot/bootutil/src/encrypted.c
index 08df0fe..760deef 100644
--- a/boot/bootutil/src/encrypted.c
+++ b/boot/bootutil/src/encrypted.c
@@ -698,13 +698,11 @@
 }
 
 void
-boot_encrypt(struct enc_key_data *enc_state, int image_index,
-        int fa_id, uint32_t off, uint32_t sz,
-        uint32_t blk_off, uint8_t *buf)
+boot_encrypt(struct enc_key_data *enc_state, int slot, uint32_t off,
+             uint32_t sz, uint32_t blk_off, uint8_t *buf)
 {
     struct enc_key_data *enc;
     uint8_t nonce[16];
-    int rc;
 
     /* boot_copy_region will call boot_encrypt with sz = 0 when skipping over
        the TLVs. */
@@ -719,13 +717,7 @@
     nonce[14] = (uint8_t)(off >> 8);
     nonce[15] = (uint8_t)off;
 
-    rc = flash_area_id_to_multi_image_slot(image_index, fa_id);
-    if (rc < 0) {
-        assert(0);
-        return;
-    }
-
-    enc = &enc_state[rc];
+    enc = &enc_state[slot];
     assert(enc->valid == 1);
     bootutil_aes_ctr_encrypt(&enc->aes_ctr, nonce, buf, sz, blk_off, buf);
 }
diff --git a/boot/bootutil/src/image_validate.c b/boot/bootutil/src/image_validate.c
index b4e8b79..239f369 100644
--- a/boot/bootutil/src/image_validate.c
+++ b/boot/bootutil/src/image_validate.c
@@ -148,10 +148,13 @@
 #ifdef MCUBOOT_ENC_IMAGES
         if (MUST_DECRYPT(fap, image_index, hdr)) {
             /* Only payload is encrypted (area between header and TLVs) */
+            int slot = flash_area_id_to_multi_image_slot(image_index,
+                            flash_area_get_id(fap));
+
             if (off >= hdr_size && off < tlv_off) {
                 blk_off = (off - hdr_size) & 0xf;
-                boot_encrypt(enc_state, image_index, flash_area_get_id(fap), off - hdr_size,
-                        blk_sz, blk_off, tmp_buf);
+                boot_encrypt(enc_state, slot, off - hdr_size,
+                             blk_sz, blk_off, tmp_buf);
             }
         }
 #endif
diff --git a/boot/bootutil/src/loader.c b/boot/bootutil/src/loader.c
index e30a83f..6a2323e 100644
--- a/boot/bootutil/src/loader.c
+++ b/boot/bootutil/src/loader.c
@@ -1216,13 +1216,14 @@
     uint32_t off;
     uint32_t tlv_off;
     size_t blk_off;
-    int enc_area_id;
     struct image_header *hdr;
     uint16_t idx;
     uint32_t blk_sz;
     uint8_t image_index;
     bool encrypted_src;
     bool encrypted_dst;
+    /* Assuming the secondary slot is source and needs decryption */
+    int source_slot = 1;
 #endif
 
     TARGET_STATIC uint8_t buf[BUF_SZ] __attribute__((aligned(4)));
@@ -1255,11 +1256,11 @@
             if (encrypted_dst) {
                 /* Need encryption, metadata from the primary slot */
                 hdr = boot_img_hdr(state, BOOT_PRIMARY_SLOT);
-                enc_area_id = FLASH_AREA_IMAGE_PRIMARY(image_index);
+                source_slot = 0;
             } else {
                 /* Need decryption, metadata from the secondary slot */
                 hdr = boot_img_hdr(state, BOOT_SECONDARY_SLOT);
-                enc_area_id = FLASH_AREA_IMAGE_SECONDARY(image_index);
+                source_slot = 1;
             }
 
             if (IS_ENCRYPTED(hdr)) {
@@ -1291,7 +1292,7 @@
                             blk_sz = tlv_off - abs_off;
                         }
                     }
-                    boot_encrypt(BOOT_CURR_ENC(state), image_index, enc_area_id,
+                    boot_encrypt(BOOT_CURR_ENC(state), source_slot,
                             (abs_off + idx) - hdr->ih_hdr_size, blk_sz,
                             blk_off, &buf[idx]);
                 }
@@ -2726,13 +2727,11 @@
     uint32_t chunk_sz;
     uint32_t max_sz = 1024;
     uint16_t idx;
-    uint8_t image_index;
     uint8_t * cur_dst;
     int area_id;
     int rc;
     uint8_t * ram_dst = (void *)(IMAGE_RAM_BASE + img_dst);
 
-    image_index = BOOT_CURR_IMG(state);
     area_id = flash_area_id_from_multi_image_slot(BOOT_CURR_IMG(state), slot);
     rc = flash_area_open(area_id, &fap_src);
     if (rc != 0){
@@ -2774,7 +2773,7 @@
              * Part of the chunk is encrypted payload */
             blk_sz = tlv_off - (bytes_copied);
         }
-        boot_encrypt(BOOT_CURR_ENC(state), image_index, area_id,
+        boot_encrypt(BOOT_CURR_ENC(state), slot,
             (bytes_copied + idx) - hdr->ih_hdr_size, blk_sz,
             blk_off, cur_dst);