hmac-stream

A streaming hmac authenticator, the idea being for use in an 'Encrypt-then-MAC' (EtM) approach meaning you want a flow something like. Other similar ideas.

var hmacStream = require('hmac-stream')
var authenticator = new hmacStream.Authenticate('key');
var verifier = new hmacStream.Verify('key');
dataSource.pipe(cipher).pipe(authenticator).pipe(outStream);
//then
inStream.pipe(verifier).pipe(decipher).pipe(doStuff);

cipher and decipher can be any stream that needs to be authenticated.

procedure:

• take a key, aad, and MAX CHUNK SIZE
  • if aad is undefined default to an empty buffer
  • if key is less then 128 bits (aka 16 bytes) throw an error.
  • if MAX CHUNK SIZE is undefined default it to Infinity
• create an 4 byte block of the aad length as a big endian 32 bit unsigned integers.
• create an hmac for the aad and the previous block and emit it
• increment the key by 1
• for each chunk
  • if chunk is larger then MAX CHUNK SIZE split into chunks of MAX CHUNK SIZE or less
  • generate a header of 4 bytes
  • write the byte size of the chunk to the header
  • generate and emit an hmac for the header
  • emit the header
  • increment the key by 1
  • generate and emit an hmac for the data
  • emit the data
  • increment the key by 1
• when done
  • emit an hmac and header for a chunk of size zero

To verify:

• accept 2 arguments a key and an aad buffer.
  • if key is less then 128 bits (aka 16 bytes) throw an error.
  • if aad is undefined default to an empty buffer.
• accept initial information
  • first 32 bytes of stream will be hmac for aad
  • create hmac of aad, add aad length as 32 bit BE buffer
  • verify against aad received
• then
  • wait for 32 + 4 bytes this is the hmac plus chunk len
  • verify chunk length
    • if verifies set chunk length
    • else throw error
  • if chunk length is zero stream is ended
  • wait for 32 + chunk length
  • verify chunk
    • if verified emit chunk
    • else throw error

It should throw errors if

• any data is modified
• any data is omitted (even at the end)
• pieces of data are swapped
• pieces of data are duplicated
• the aad is incorrect

It should allow streaming with no buffering and without an adversary being able to do a DDOS against you.

API

var hmacStream = require('hmac-stream');

hmacStream.Authenticate(key, aad, maxSize);
hmacStream.Verify(key, aad);

• password: key must be at least 128 bits (16 bytes)
• aad: any additional data to authenticate
• maxSize: maximum size of a chunk, defaults to Infinity.

Versions

• 0.0.0: first version
• 0.1.0: lowered default chunk size.
• 1.0.0: add end tag to guard against the last block being dropped
• 2.0.0: added variable block sizes
• 3.0.0: takes a key instead of a password, supports aad, treats end tags differently.
• 4.0.0: chunk length is mac'd separately.
• 4.0.2: fix dependencies to work better in the browser