code: func (c *Cipher) SetCounter(ctr uint64) {
  	if c.noncesize == INonceSize {
  		binary.LittleEndian.PutUint32(c.state[48:], uint32(ctr))
  	} else {
  		binary.LittleEndian.PutUint64(c.state[48:], ctr)
  	}
  	c.off = 0
  }
docs: // SetCounter skips ctr * 64 byte blocks. SetCounter(0) resets the cipher.
  // This function always skips the unused keystream of the current 64 byte block.