43 lines
1.2 KiB
Go
43 lines
1.2 KiB
Go
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// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
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// SPDX-License-Identifier: MIT
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// The n-fold of RFC 3961 appendix A, ported from the reference
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// implementation of MIT krb5: the input is cycled through the least
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// common multiple of the input and output byte lengths, and the
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// repetitions summed with end-around carry.
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package krb5
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// NFold stretches in to outBytes octets with every input bit weighted
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// equally into every output bit.
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func NFold(in []byte, outBytes int) []byte {
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inBytes := len(in)
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a, b := outBytes, inBytes
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for b != 0 {
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a, b = b, a%b
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}
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lcm := outBytes * inBytes / a
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out := make([]byte, outBytes)
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carry := 0
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for i := lcm - 1; i >= 0; i-- {
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msbit := ((inBytes << 3) - 1 + ((inBytes<<3)+13)*(i/inBytes) +
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((inBytes - i%inBytes) << 3)) % (inBytes << 3)
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hi := (inBytes - 1 - (msbit >> 3)) % inBytes
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lo := (inBytes - (msbit >> 3)) % inBytes
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carry += int(((uint16(in[hi])<<8 | uint16(in[lo])) >> uint((msbit&7)+1)) & 0xff)
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carry += int(out[i%outBytes])
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out[i%outBytes] = byte(carry)
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carry >>= 8
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}
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if carry != 0 {
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for i := outBytes - 1; i >= 0; i-- {
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carry += int(out[i])
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out[i] = byte(carry)
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carry >>= 8
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}
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}
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return out
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}
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