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| ;(C)2005 wek http://www.efton.sk ;free for personal use ;for commercial use, please contact wek@efton.sk ;implementation of XTEA algorithm by Needham&Wheeler for AVR ; ;6347 cycles excl. ret ;$67 words = $CE bytes = 206 bytes of code FLASH, including 16 bytes of key ;uses 8 bytes of SRAM (input/output data) and 10 registers ;key can be easily placed into SRAM - replace lpm xxx,Z+ by ld xxx,Z+ ;for devices which don't support lpm xxx,Z+ replace it by lpm; mov xxx,r0 ;the 4 registers used by sum0..sum3 can be spared by moving sum to SRAM with 3 modifications: ; - when getting the pointer to key, load the appropriate byte from sum into tmp4 ; - the increment of sum should be done moving bytes from and to SRAM (e.g. in tmp4) ; - when adding up sum to key, an additional register is necessary (so the spare is 3 registers only...) .dseg ydata: .byte 4 zdata: .byte 4 ;register definitions .def ZL = r30 .def ZH = r31 .def tmp0 = r16 .def tmp1 = r17 .def tmp2 = r18 .def tmp3 = r19 .def tmp4 = r20 .def sum0 = r21 .def sum1 = r22 .def sum2 = r23 .def sum3 = r24 .def RoundCnt = r25 .cseg Tea: ldi RoundCnt,2*32 clr sum0 clr sum1 clr sum2 clr sum3 ldi ZL,LOW(zdata) ldi ZH,HIGH(zdata) ldd tmp0,Z+0 ldd tmp1,Z+1 ldd tmp2,Z+2 ldd tmp3,Z+3 TeaRound: clr tmp4 ;z>>5 but as (z<<3)>>8 (simultaneously preparing for z<<4) lsl tmp0 rol tmp1 rol tmp2 rol tmp3 rol tmp4 lsl tmp0 rol tmp1 rol tmp2 rol tmp3 rol tmp4 lsl tmp0 rol tmp1 rol tmp2 rol tmp3 rol tmp4 lsl tmp0 ;the last step for z<<4 and simultaneously the xor eor tmp0,tmp1 rol tmp1 eor tmp1,tmp2 rol tmp2 eor tmp2,tmp3 rol tmp3 eor tmp3,tmp4 ldd tmp4,Z+0 ;now add z add tmp0,tmp4 ldd tmp4,Z+1 adc tmp1,tmp4 ldd tmp4,Z+2 adc tmp2,tmp4 ldd tmp4,Z+3 adc tmp3,tmp4 ldi ZL,LOW(Key*2) ;now calculate key pointer depending on which subround is it ldi ZH,HIGH(Key*2) sbrc RoundCnt,0 rjmp TeaRoundX1 sbrc sum0,0 adiw ZL,4 sbrc sum0,1 adiw ZL,8 rjmp TeaRoundX2 TeaRoundX1: sbrc sum1,3 ;sum>>11 = (sum>>8)>>3 adiw ZL,4 sbrc sum1,4 adiw ZL,8 TeaRoundX2: lpm tmp4,Z+ ;(tmp) xor (sum+k[ptr]) add tmp4,sum0 eor tmp0,tmp4 lpm tmp4,Z+ adc tmp4,sum1 eor tmp1,tmp4 lpm tmp4,Z+ adc tmp4,sum2 eor tmp2,tmp4 lpm tmp4,Z+ adc tmp4,sum3 eor tmp3,tmp4 sbrc RoundCnt,0 ;in the first subround increment sum (AVR: decrement by its neg) rjmp TeaRoundX3 subi sum0,$47 sbci sum1,$86 sbci sum2,$C8 sbci sum3,$61 ldi ZL,LOW(ydata) ;and set pointer to ydata to add the calculated value in tmp (and it remains valid for the next subround, too) ldi ZH,HIGH(ydata) rjmp TeaRoundX4 TeaRoundX3: ldi ZL,LOW(zdata) ;in the second subround only set pointer to zdata ldi ZH,HIGH(zdata) TeaRoundX4: ldd tmp4,Z+0 ;add the calculated value to y or z add tmp0,tmp4 std Z+0,tmp0 ldd tmp4,Z+1 adc tmp1,tmp4 std Z+1,tmp1 ldd tmp4,Z+2 adc tmp2,tmp4 std Z+2,tmp2 ldd tmp4,Z+3 adc tmp3,tmp4 std Z+3,tmp3 dec RoundCnt brne TeaRoundA ret TeaRoundA: rjmp TeaRound Key: .db $4b,$5a,$69,$78 .db $0f,$1e,$2d,$3c .db $c3,$d2,$e1,$f0 .db $87,$96,$a5,$b4 ; 0x78695a4b 0x3c2d1e0f 0xf0e1d2c3 0xb4a59687 </PRE></BODY></HTML> |