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

# One GCM implementation pitfall posted March 2017

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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