Does node.js crypto use fixed tag size with GCM mode?

2
votes

I am implementing a scheme with cipher in GCM mode in node.js.

I have to append/prepend GCM tag to the ciphertext in order to check the integrity. However, I am not sure how big the tag will be!

On crypto++ wiki, I read that the size could vary and that it's actually a parameter of the GCM mode. Citing from the wiki, emphasis mine:

The parameters which must be supplied and used by both parties are:

  • key and key size
  • iv and iv size
  • tag size

However, in node documentation, there is nothing about the tag size. Just that the tag exist. Citing from the documentation:

cipher.getAuthTag()

For authenticated encryption modes (currently supported: GCM), this method returns a Buffer that represents the authentication tag that has been computed from the given data. Should be called after encryption has been completed using the final method!

Should I expect the tag size to vary and just save the tag size together with the ciphertext? Or can I just assume the tag size is always smaller than 128 bits and pad it with zeroes on the left?

1
node.jsencryptionaes-gcm
Node.js uses OpenSSL under the hood, so it might be viable to look there. Padding the tag to 128 bit would be wrong, because then you wouldn't know how many bits have to be compared (not that node.js provides the produced tag during decryption so that you could compare it yourself). - Artjom B.
Hm. I actually want the resulting code to be working in both node and browserify. That complicates things a bit. - Karel Bílek
If the authentication tag is smaller it is just the first x bits of the full sized tag. So I would presume 128 bits are returned. Then the application can decide how many to use. Note though that the security provided by GCM is seriously degraded for smaller tag sizes so you should probably use the full 128 bits. - Maarten Bodewes

1 Answers

1
votes

Both node.js and the browserify-crypto use 128 bits tags.

I haven't found the actual code, but it's obvious by looking at the tests.

Node.js tests here, crypto-browserify tests here.