56 lines
1.2 KiB
NASM
56 lines
1.2 KiB
NASM
* Copyright (c) 2023 Stephen Heumann
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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* Direct page locations
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i gequ 0
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j gequ 1
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S gequ 2
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* Do one step of the RC4 pseudo-random generation algorithm.
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* Call with DP = RC4 context structure, and short M/X.
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RC4_PRGA start
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;i := (i + 1) mod 256
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inc i
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;j := (j + S[i]) mod 256
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ldx i
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clc
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lda S,x
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tay
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adc j
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sta j
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;swap values of S[i] and S[j]
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tax
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lda S,x
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ldx i
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sta S,x
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ldx j
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tya
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sta S,x
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;temp := (S[i] + S[j]) mod 256
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ldx i
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clc
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adc S,x
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;return S[temp]
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tax
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lda S,x
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rtl
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end
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