Question

For partial file upload and encryption i’d like to read chunks (example: 10MB) from a large file (example: 100MB), encrypt the chunk with AES256 and upload the chunk to the server.
On the server the chunks are joined together to one big file (which then should represent the complete encrypted file).

I’d like to avoid a full encryption of the file (example: 100MB!) before taking out chunks and sending them to the server (CPU overhead, because i would then first need to copy the whole file and do AES256 over the whole file).

I tried the following (example for a 16MB File = 2 Chunks = 1*10MB + 1*6MB)

1st chunk is ~10MB, ensured then the chunksize is a multiple of 16 (AES_BLOCK_SIZE)

EVP_CIPHER_CTX e_ctx;
EVP_CIPHER_CTX_init(&e_ctx);
EVP_EncryptInit_ex(&e_ctx, EVP_aes_256_cbc(), NULL, key, iv);
EVP_EncryptUpdate(&e_ctx, enc_data, &outlen, unenc_data, chunksize_to_load_from_file_unencrypted);

During the 1st chunk, i do not call EVP_EncryptFinal_ex() because the encryption is not finished yet (maybe this is the problem?).

2nd chunk is ~6MB, there i encrypt the rest of the data offset from &outlen above

EVP_CIPHER_CTX e_ctx;
EVP_CIPHER_CTX_init(&e_ctx);
EVP_EncryptInit_ex(&e_ctx, EVP_aes_256_cbc(), NULL, key, iv);
EVP_EncryptUpdate(&e_ctx, enc_data, &outlen, unenc_data, chunksize_to_load_from_file_unencrypted);
EVP_EncryptFinal_ex(&e_ctx, enc_data+outlen, &final_length);

On the server the two file-parts get’s joined.
I then can decrypt the file “successfully”, same size of the unencrypted file. But at offset ~10MB, some bytes are different (16 Bytes! = AES_BLOCK_SIZE)

Any idea? Could that be the padding? Is that not only at the end?
Does EVP_EncryptFinal_ex act different when not pass through the whole file through EVP_EncryptUpdate?

How can i do transmission of encrypted partial-file-chunk and join them on the server to get a proper encrypted file?

Thanks!

Answer 1

You are using the wrong initialization vector for the second chunk. With CBC, the iv of the subsequent encryption should be the last 16 bytes of the previous ciphertext. That way, just concatenating the encrypted parts gives the encryption of the whole.