James Lee
51 lines
1.7 KiB
NASM
51 lines
1.7 KiB
NASM
;-----------------------------------------------------------------------------;
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; Author: Michael Schierl (schierlm[at]gmx[dot]de)
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; Version: 1.0 (29 December 2012)
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;-----------------------------------------------------------------------------;
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[BITS 32]
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; Input: EBP - Data to decode
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; ECX - Data length
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; ESI - Key (16 bytes for simplicity)
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; EDI - pointer to 0x100 bytes scratch space for S-box
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; Direction flag has to be cleared
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; Output: None. Data is decoded in place.
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; Clobbers: EAX, EBX, ECX, EDX, EBP (stack is not used)
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; Initialize S-box
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xor eax, eax ; Start with 0
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init:
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stosb ; Store next S-Box byte S[i] = i
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inc al ; increase byte to write (EDI is increased automatically)
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jnz init ; loop until we wrap around
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sub edi, 0x100 ; restore EDI
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; permute S-box according to key
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xor ebx, ebx ; Clear EBX (EAX is already cleared)
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permute:
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add bl, [edi+eax] ; BL += S[AL] + KEY[AL % 16]
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mov edx, eax
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and dl, 0xF
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add bl, [esi+edx]
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mov dl, [edi+eax] ; swap S[AL] and S[BL]
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xchg dl, [edi+ebx]
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mov [edi+eax], dl
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inc al ; AL += 1 until we wrap around
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jnz permute
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; decryption loop
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xor ebx, ebx ; Clear EBX (EAX is already cleared)
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decrypt:
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inc al ; AL += 1
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add bl, [edi+eax] ; BL += S[AL]
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mov dl, [edi+eax] ; swap S[AL] and S[BL]
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xchg dl, [edi+ebx]
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mov [edi+eax], dl
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add dl, [edi+ebx] ; DL = S[AL]+S[BL]
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mov dl, [edi+edx] ; DL = S[DL]
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xor [ebp], dl ; [EBP] ^= DL
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inc ebp ; advance data pointer
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dec ecx ; reduce counter
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jnz decrypt ; until finished
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