// https://cs.opensource.google/go/go/+/refs/tags/go1.18.3:src/crypto/rsa/rsa.go

// Copyright (c) 2009 The Go Authors. All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//    * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//    * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//    * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

package pss

import (
	"hash"
)

// incCounter increments a four byte, big-endian counter.
func incCounter(c *[4]byte) {
	if c[3]++; c[3] != 0 {
		return
	}
	if c[2]++; c[2] != 0 {
		return
	}
	if c[1]++; c[1] != 0 {
		return
	}
	c[0]++
}

// mgf1XOR XORs the bytes in out with a mask generated using the MGF1 function
// specified in PKCS #1 v2.1.
func mgf1XOR(out []byte, hash hash.Hash, seed []byte) {
	var counter [4]byte
	var digest []byte

	done := 0
	for done < len(out) {
		hash.Write(seed)
		hash.Write(counter[0:4])
		digest = hash.Sum(digest[:0])
		hash.Reset()

		for i := 0; i < len(digest) && done < len(out); i++ {
			out[done] ^= digest[i]
			done++
		}
		incCounter(&counter)
	}
}