CODEWITHSUNDEEP
node.jsencryptionaes-gcm
Karel Bílek
Karel Bílek

Reputation: 37724

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

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?

Upvotes: 2

Views: 1935

Answers (1)

Karel Bílek
Karel Bílek

Reputation: 37724

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.

Upvotes: 1

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