var crypto/elliptic.mask
2 uses
crypto/elliptic (current package)
elliptic.go#L328: var mask = []byte{0xff, 0x1, 0x3, 0x7, 0xf, 0x1f, 0x3f, 0x7f}
elliptic.go#L345: priv[0] &= mask[bitSize%8]
The pages are generated with Golds v0.4.9. (GOOS=linux GOARCH=amd64)