diff --git a/src/dm/dm_crypto.c b/src/dm/dm_crypto.c
index 12deafe6..50226472 100644
--- a/src/dm/dm_crypto.c
+++ b/src/dm/dm_crypto.c
@@ -14,6 +14,19 @@
 
 #define DM_ID "dm_crypto"
 
+/* Медиа использует тот же ключ target, но отдельное пространство nonce. */
+void dm_build_media_nonce(uint64_t author, const uint8_t media_id[16], uint64_t part, uint8_t nonce[DM_NONCE_SIZE]) {
+    SHA256_CTX ctx;
+    uint8_t hash[32];
+    SHA256_Init(&ctx);
+    SHA256_Update(&ctx, "utun_dm_media", 13);
+    SHA256_Update(&ctx, &author, sizeof(author));
+    SHA256_Update(&ctx, media_id, 16);
+    SHA256_Update(&ctx, &part, sizeof(part));
+    SHA256_Final(hash, &ctx);
+    memcpy(nonce, hash, DM_NONCE_SIZE);
+}
+
 uint64_t dm_derive_conv_id(uint64_t a, uint64_t b) {
     uint64_t lo = a < b ? a : b;
     uint64_t hi = a < b ? b : a;
@@ -80,10 +93,17 @@ void dm_build_nonce(uint64_t conv_id, uint64_t author, uint64_t seq, uint8_t non
 static int dm_ccm_crypt(const uint8_t key[DM_CONTENT_KEY_SIZE], const uint8_t nonce[DM_NONCE_SIZE],
                         const uint8_t* in, size_t in_len, uint8_t* out, size_t* out_len,
                         int encrypt, const uint8_t* tag_in) {
-    if (!key || !nonce || !in || !out || !out_len) return -1;
+    if (!key || !nonce || (!in && in_len) || !out || !out_len ||
+        (encrypt ? in_len > DM_CCM_PLAIN_MAX : in_len < DM_TAG_SIZE || in_len - DM_TAG_SIZE > DM_CCM_PLAIN_MAX)) {
+        DEBUG_ERROR(DEBUG_CATEGORY_DM, "%s: invalid CCM arguments encrypt=%d bytes=%zu", DM_ID, encrypt, in_len);
+        return -1;
+    }
+    *out_len = 0;
+    static const uint8_t empty = 0;
+    if (!in) in = &empty;
 
     EVP_CIPHER_CTX* ectx = EVP_CIPHER_CTX_new();
-    if (!ectx) return -1;
+    if (!ectx) { DEBUG_ERROR(DEBUG_CATEGORY_DM, "%s: CCM context allocation failed", DM_ID); return -1; }
     const EVP_CIPHER* cipher = EVP_aes_256_ccm();
 
     int rc = -1;
@@ -93,6 +113,7 @@ static int dm_ccm_crypt(const uint8_t key[DM_CONTENT_KEY_SIZE], const uint8_t no
             || EVP_CIPHER_CTX_ctrl(ectx, EVP_CTRL_AEAD_SET_TAG, DM_TAG_SIZE, NULL) != 1
             || EVP_EncryptInit_ex(ectx, NULL, NULL, key, nonce) != 1) goto done;
         int outl;
+        if (EVP_EncryptUpdate(ectx, NULL, &outl, NULL, (int)in_len) != 1) goto done;
         if (EVP_EncryptUpdate(ectx, out, &outl, in, (int)in_len) != 1 || outl != (int)in_len) goto done;
         int tmpl;
         if (EVP_EncryptFinal_ex(ectx, out + outl, &tmpl) != 1) goto done;
@@ -106,14 +127,14 @@ static int dm_ccm_crypt(const uint8_t key[DM_CONTENT_KEY_SIZE], const uint8_t no
             || EVP_CIPHER_CTX_ctrl(ectx, EVP_CTRL_AEAD_SET_TAG, DM_TAG_SIZE, (void*)(tag_in ? tag_in : in + body)) != 1
             || EVP_DecryptInit_ex(ectx, NULL, NULL, key, nonce) != 1) goto done;
         int outl;
+        if (EVP_DecryptUpdate(ectx, NULL, &outl, NULL, (int)body) != 1) goto done;
         if (EVP_DecryptUpdate(ectx, out, &outl, in, (int)body) != 1 || outl != (int)body) goto done;
-        int tmpl;
-        if (EVP_DecryptFinal_ex(ectx, out + outl, &tmpl) != 1) goto done;
         *out_len = body;
     }
     rc = 0;
 done:
     EVP_CIPHER_CTX_free(ectx);
+    if (rc) DEBUG_ERROR(DEBUG_CATEGORY_DM, "%s: CCM %s failed bytes=%zu", DM_ID, encrypt ? "encrypt" : "decrypt", in_len);
     return rc;
 }
 
diff --git a/src/dm/dm_crypto.h b/src/dm/dm_crypto.h
index 6ca1ae3c..f59f6f42 100644
--- a/src/dm/dm_crypto.h
+++ b/src/dm/dm_crypto.h
@@ -20,6 +20,8 @@ extern "C" {
 #define DM_CONTENT_KEY_SIZE  32
 #define DM_NONCE_SIZE        13   /* AES-CCM требует ровно 13 байт */
 #define DM_TAG_SIZE          16
+#define DM_CCM_PLAIN_MAX     65535u
+#define DM_MEDIA_PLAIN_SIZE  32768u
 
 /* conv_id = SHA256("utun_dm_v1" || min(a,b) || max(a,b))[..8] & 0x7FFF...,
  * 63-битное число, одинаковое на обеих сторонах (одна ветка на пару). */
@@ -35,6 +37,9 @@ int dm_derive_content_key(const uint8_t my_priv[32], const uint8_t peer_pub[32],
  * не пересекается между направлениями (author различается). */
 void dm_build_nonce(uint64_t conv_id, uint64_t author, uint64_t seq, uint8_t nonce[DM_NONCE_SIZE]);
 
+/* Nonce порции медиа; media_id неизменен на всех повторах одной отправки. */
+void dm_build_media_nonce(uint64_t author, const uint8_t media_id[16], uint64_t part, uint8_t nonce[DM_NONCE_SIZE]);
+
 /* AES-256-CCM. ciphertext = ciphertext(plain_len) || tag(DM_TAG_SIZE).
  * out должен вмещать plain_len + DM_TAG_SIZE. 0=ок, <0=ошибка. */
 int dm_encrypt(const uint8_t key[DM_CONTENT_KEY_SIZE], const uint8_t nonce[DM_NONCE_SIZE],