david wong

Hey ! I'm David, a security consultant at Cryptography Services, the crypto team of NCC Group . This is my blog about cryptography and security and other related topics that I find interesting.

One GCM implementation pitfall 3 weeks ago

If you look at Go's implementation of GCM, in particular this, you can see that the counter is set to nonce||1:

if len(nonce) == gcmStandardNonceSize {
    // Init counter to nonce||1
    copy(counter[:], nonce)
    counter[gcmBlockSize-1] = 1
} 

It needs to be. Without it, the first block of keystream is the encryption of 0 if the nonce is 0 (which can happen if nonces are generated from a counter). The encryption of 0 is also... the authentication key!

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