feat: full NFSv4.2 server and client in pure Go

Assisted-by: GLM 5.3 Flash

internal/krb5/nfold.go

@@ -0,0 +1,42 @@
+// 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
+}