Written by Dmitry Chestnykh in 2014. Public domain. https://github.com/dchest/nacl-stream-js

Based on https://www.imperialviolet.org/2014/06/27/streamingencryption.html

• Inputs: 32-byte key, 16-byte nonce, a stream (or a file).

• Stream is split into chunks of the specified length.

• 24-byte fullNonce is acquired by concatenating 16-byte nonce and 8-byte little-endian chunk number.

• Each chunk except for the last one is encrypted like this, starting with chunk number 0:

  fullNonce0 := nonce || 0
  encryptedChunk0 := len(chunk0) || nacl.secretbox(chunk0, fullNonce0, key)
  fullNonce1 := nonce || 1
  encryptedChunk1 := len(chunk1) || nacl.secretbox(chunk1, fullNonce1, key)
  ...
  

  where len(chunk) is a 4-byte little-endian plaintext chunk length.

• The last chunk's fullNonce has the most significant bit set:

  fullNonceN := nonce || setMSB(N)
  encryptedChunkN = len(chunkN) || nacl.secretbox(chunkN, fullNonceN, key)
  

• Encrypted chunks are concatenated to form the encrypted stream.

The API is fairly low-level and should be used as a building block for some high-level API which would deal with actual streams or files.

The module provides two constructor functions (in window.nacl namespace or as a CommonJS module):

Returns a stream encryptor object using 32-byte key and 16-byte nonce (both of Uint8Array type) and the maximum chunk length with the following methods:

Encrypts the given Uint8Array chunk and returns a new Uint8Array array with encrypted chunk.

If encrypting the last chunk of stream, isLast must be set to true.

Zeroes out temporary space. Should be called after encrypting all chunks.

Returns a stream decryptor object using 32-byte key and 16-byte nonce (both of Uint8Array type) and the maximum chunk length with the following methods:

Decrypts the given Uint8Array encrypted chunk and returns a new Uint8Array array with decrypted chunk.

The given encryptedChunk should be in the format created by encryptChunk, i.e. prefixed with original chunk length.

Returns null if the chunk cannot be decrypted. In this case, futher calls to decryptChunk will deliberately fail: callers should stop trying to decrypt this stream and, if possible, discard previously decrypted chunks.

If decrypting the last chunk of stream, isLast must be set to true.

Zeroes out temporary space. Should be called after decrypting all chunks.

Reads four bytes from the given offset (or from the beginning, if no offset is given) of Uint8Array data and returns the chunk length. Length of encryptedChunk passed to decryptChunk should be 4 + 16 + chunkLength bytes.

• nacl-blob