Insufficiently random values in golang.org/x/crypto/salsa20

XORKeyStream generates incorrect and insecure output for very large inputs.

If more than 256 GiB of keystream is generated, or if the counter otherwise grows greater than 32 bits, the amd64 implementation will first generate incorrect output, and then cycle back to previously generated keystream. Repeated keystream bytes can lead to loss of confidentiality in encryption applications, or to predictability in CSPRNG applications.

The issue might affect uses of golang.org/x/crypto/nacl with extremely large messages.

Architectures other than amd64 and uses that generate less than 256 GiB of keystream for a single salsa20.XORKeyStream invocation are unaffected.