CODEWITHSUNDEEP
javacryptographyaes-gcm
Pete
Pete

Reputation: 93

GCM mode for AAD in javax.crypto library

have a question about this library and GCM in general.

I understand GCM mode can both encrypt and authenticate data, or merely just authenticate it. With the javax library, if I want to authenticate a message only I can do:

cipher.updateAAD(AAD)

My question is, when I compute

cipher.doFinal(buffer)

What does buffer become? Is it merely the ciphertext blocks + the GCM auth tag? Or is AAD itself actually included in buffer now?

It's really not clear to me from the docs: https://docs.oracle.com/javase/7/docs/api/javax/crypto/Cipher.html

Thanks!

Upvotes: 0

Views: 659

Answers (2)

Gary
Gary

Reputation: 81

Do you have a specific reason you are trying to use AAD values?

Not sure your experience level or specific use case, but here is a blog article I wrote about AE if you have any questions.

Upvotes: 0

gusto2
gusto2

Reputation: 12075

If you will use the AES/GCM/NoPadding, the doFinal method produces ciphertext + tag when encrypting. The same is expected when decrypting the AES/GCM mode.

If you updateAAD, the MAC (tag) is updated, but the AAD itself value is not included (that would even break the purpose of extra AAD).

Upvotes: 1

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