diff --git a/demos/timing.c b/demos/timing.c
index c576a4f75..d92cae95f 100644
--- a/demos/timing.c
+++ b/demos/timing.c
@@ -1354,7 +1354,7 @@ static void time_ccm(eac_ctx *ctx)
         t_start();
         t1 = t_read();
         z = 16;
-        if ((err = ccm_memory(ctx->cipher_idx, ctx->key, 16, NULL, ctx->IV, 16, NULL, 0, ctx->buf, ctx->size, ctx->buf, ctx->tag, &z, CCM_ENCRYPT)) != CRYPT_OK) {
+        if ((err = ccm_memory(ctx->cipher_idx, ctx->key, 16, NULL, ctx->IV, 13, NULL, 0, ctx->buf, ctx->size, ctx->buf, ctx->tag, &z, CCM_ENCRYPT)) != CRYPT_OK) {
            fprintf(stderr, "\nCCM error... %s\n", error_to_string(err));
            exit(EXIT_FAILURE);
         }
@@ -1369,7 +1369,7 @@ static void time_ccm(eac_ctx *ctx)
         t_start();
         t1 = t_read();
         z = 16;
-        if ((err = ccm_memory(ctx->cipher_idx, ctx->key, 16, &skey, ctx->IV, 16, NULL, 0, ctx->buf, ctx->size, ctx->buf, ctx->tag, &z, CCM_ENCRYPT)) != CRYPT_OK) {
+        if ((err = ccm_memory(ctx->cipher_idx, ctx->key, 16, &skey, ctx->IV, 13, NULL, 0, ctx->buf, ctx->size, ctx->buf, ctx->tag, &z, CCM_ENCRYPT)) != CRYPT_OK) {
            fprintf(stderr, "\nCCM error... %s\n", error_to_string(err));
            exit(EXIT_FAILURE);
         }
diff --git a/src/encauth/ccm/ccm_add_nonce.c b/src/encauth/ccm/ccm_add_nonce.c
index 176836bbe..d187e143f 100644
--- a/src/encauth/ccm/ccm_add_nonce.c
+++ b/src/encauth/ccm/ccm_add_nonce.c
@@ -20,8 +20,11 @@ int ccm_add_nonce(ccm_state *ccm,
    LTC_ARGCHK(ccm   != NULL);
    LTC_ARGCHK(nonce != NULL);
 
-   /* increase L to match the nonce len */
-   ccm->noncelen = (noncelen > 13) ? 13 : noncelen;
+   /* CCM nonce is 7..13 bytes */
+   if (noncelen < 7 || noncelen > 13) {
+      return CRYPT_INVALID_ARG;
+   }
+   ccm->noncelen = noncelen;
    if ((15 - ccm->noncelen) > ccm->L) {
       ccm->L = 15 - ccm->noncelen;
    }
@@ -29,9 +32,9 @@ int ccm_add_nonce(ccm_state *ccm,
       return CRYPT_INVALID_ARG;
    }
 
-   /* decrease noncelen to match L */
+   /* flags byte + nonce + length field must fit the 16-byte block */
    if ((ccm->noncelen + ccm->L) > 15) {
-      ccm->noncelen = 15 - ccm->L;
+      return CRYPT_INVALID_ARG;
    }
 
    /* form B_0 == flags | Nonce N | l(m) */
diff --git a/src/encauth/ccm/ccm_memory.c b/src/encauth/ccm/ccm_memory.c
index d23904463..9cee5f400 100644
--- a/src/encauth/ccm/ccm_memory.c
+++ b/src/encauth/ccm/ccm_memory.c
@@ -78,7 +78,8 @@ int ccm_memory(int cipher,
    if (*taglen < 4 || *taglen > 16 || (*taglen % 2) == 1 || headerlen > 0x7fffffffu) {
       return CRYPT_INVALID_ARG;
    }
-   if (noncelen < 7) {
+   /* CCM nonce is 7..13 bytes */
+   if (noncelen < 7 || noncelen > 13) {
       return CRYPT_INVALID_ARG;
    }
 
@@ -107,7 +108,6 @@ int ccm_memory(int cipher,
    }
 
    /* increase L to match the nonce len */
-   noncelen = (noncelen > 13) ? 13 : noncelen;
    if ((15 - noncelen) > L) {
       L = 15 - noncelen;
    }
@@ -115,6 +115,11 @@ int ccm_memory(int cipher,
       return CRYPT_INVALID_ARG;
    }
 
+   /* flags byte + nonce + length field must fit the 16-byte block */
+   if ((noncelen + L) > 15) {
+      return CRYPT_INVALID_ARG;
+   }
+
    /* allocate mem for the symmetric key */
    if (uskey == NULL) {
       skey = XMALLOC(sizeof(*skey));
diff --git a/src/encauth/ccm/ccm_test.c b/src/encauth/ccm/ccm_test.c
index 3aae2307a..69cf8be5b 100644
--- a/src/encauth/ccm/ccm_test.c
+++ b/src/encauth/ccm/ccm_test.c
@@ -267,6 +267,50 @@ int ccm_test(void)
       }
    }
 
+   /* invalid nonce lengths and nonce/L combinations must be rejected not silently adjusted */
+   {
+      unsigned char key[16]   = { 0 };
+      unsigned char nonce[16] = { 0 };
+      unsigned char pt[16]    = { 0 };
+      unsigned char ct[16];
+
+      /* CCM nonce is 7..13 bytes */
+      taglen = 8;
+      if (ccm_memory(idx, key, sizeof(key), NULL, nonce, 6, NULL, 0, pt, sizeof(pt), ct, tag, &taglen, CCM_ENCRYPT) != CRYPT_INVALID_ARG) {
+         return CRYPT_FAIL_TESTVECTOR;
+      }
+      taglen = 8;
+      if (ccm_memory(idx, key, sizeof(key), NULL, nonce, 14, NULL, 0, pt, sizeof(pt), ct, tag, &taglen, CCM_ENCRYPT) != CRYPT_INVALID_ARG) {
+         return CRYPT_FAIL_TESTVECTOR;
+      }
+      if ((err = ccm_init(&ccm, idx, key, sizeof(key), sizeof(pt), 8, 0)) != CRYPT_OK) {
+         return err;
+      }
+      if (ccm_add_nonce(&ccm, nonce, 6) != CRYPT_INVALID_ARG) {
+         return CRYPT_FAIL_TESTVECTOR;
+      }
+      if ((err = ccm_init(&ccm, idx, key, sizeof(key), sizeof(pt), 8, 0)) != CRYPT_OK) {
+         return err;
+      }
+      if (ccm_add_nonce(&ccm, nonce, 14) != CRYPT_INVALID_ARG) {
+         return CRYPT_FAIL_TESTVECTOR;
+      }
+      /* ptlen >= 2^16 forces L=3, so a 13-byte nonce no longer fits B_0 (13+3 > 15) */
+      if ((err = ccm_init(&ccm, idx, key, sizeof(key), 65536, 8, 0)) != CRYPT_OK) {
+         return err;
+      }
+      if (ccm_add_nonce(&ccm, nonce, 13) != CRYPT_INVALID_ARG) {
+         return CRYPT_FAIL_TESTVECTOR;
+      }
+      /* 12-byte nonce still fits (12+3 == 15) */
+      if ((err = ccm_init(&ccm, idx, key, sizeof(key), 65536, 8, 0)) != CRYPT_OK) {
+         return err;
+      }
+      if ((err = ccm_add_nonce(&ccm, nonce, 12)) != CRYPT_OK) {
+         return err;
+      }
+   }
+
    return CRYPT_OK;
 #endif
 }