diff --git a/src/cryptography/AES.c b/src/cryptography/AES.c
index ba9be05..c92c6b1 100755
--- a/src/cryptography/AES.c
+++ b/src/cryptography/AES.c
@@ -172,13 +172,14 @@ Result(u32) AESStreamEncryptor_encrypt(AESStreamEncryptor* ptr,
     Array(u8) src, Array(u8) dst)
 {
     Deferral(4);
-    u32 encrypted_size = AESStreamEncryptor_calcDstSize(src.size);
-    try_assert(dst.size >= encrypted_size);
-    
+
+    u32 encrypted_size = src.size;
     // if it is the beginning of the stream, write IV
     if(ptr->block_counter == 0){
         __Array_writeNext(&dst, ptr->iv, __AES_STREAM_IV_SIZE);
+        encrypted_size = AESStreamEncryptor_calcDstSize(encrypted_size);
     }
+    try_assert(dst.size >= encrypted_size);
     
     // encrypt full buffers
     while(src.size > __AES_BUFFER_SIZE){
diff --git a/src/cryptography/AES.h b/src/cryptography/AES.h
index 2dd8fc7..9597f68 100644
--- a/src/cryptography/AES.h
+++ b/src/cryptography/AES.h
@@ -101,7 +101,7 @@ void AESStreamEncryptor_changeKey(AESStreamEncryptor* ptr, Array(u8) key);
 
 /// @brief If ptr->block_counter == 0, writes random IV to `dst`. After that writes encrypted data to dst.
 /// @param src array of any size
-/// @param dst array of size >= AESStreamEncryptor_calcDstSize(src.size)
+/// @param dst array of size >= `AESStreamEncryptor_calcDstSize(src.size)` for first block and `src.size `for other blocks
 /// @return size of encrypted data
 Result(u32) AESStreamEncryptor_encrypt(AESStreamEncryptor* ptr, Array(u8) src, Array(u8) dst);