nfs/internal/krb5/nfold.go

// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

// The n-fold of RFC 3961 appendix A, ported from the reference
// implementation of MIT krb5: the input is cycled through the least
// common multiple of the input and output byte lengths, and the
// repetitions summed with end-around carry.

package krb5

// NFold stretches in to outBytes octets with every input bit weighted
// equally into every output bit.
func NFold(in []byte, outBytes int) []byte {
	inBytes := len(in)

	a, b := outBytes, inBytes
	for b != 0 {
		a, b = b, a%b
	}
	lcm := outBytes * inBytes / a

	out := make([]byte, outBytes)
	carry := 0
	for i := lcm - 1; i >= 0; i-- {
		msbit := ((inBytes << 3) - 1 + ((inBytes<<3)+13)*(i/inBytes) +
			((inBytes - i%inBytes) << 3)) % (inBytes << 3)
		hi := (inBytes - 1 - (msbit >> 3)) % inBytes
		lo := (inBytes - (msbit >> 3)) % inBytes
		carry += int(((uint16(in[hi])<<8 | uint16(in[lo])) >> uint((msbit&7)+1)) & 0xff)
		carry += int(out[i%outBytes])
		out[i%outBytes] = byte(carry)
		carry >>= 8
	}
	if carry != 0 {
		for i := outBytes - 1; i >= 0; i-- {
			carry += int(out[i])
			out[i] = byte(carry)
			carry >>= 8
		}
	}
	return out
}