QUIC uses ChaCha20 for its header protection: https://tools.ietf.org/html/draft-ietf-quic-tls-25#section-5.4.4. In order to calculate the header protection mask, a QUIC implementation needs to be able to set the counter based on the ciphertext:

The first 4 bytes of the sampled ciphertext are the block counter.

Unfortunately, x/crypto/chacha20 currently doesn't provide an API to manually set the counter, and I therefore have to maintain a fork for my quic-go.

I already brought this up in #24485 (comment), and @FiloSottile suggested adding a SetCounter method, but it looks like this issue was closed without exposing this method.