aes256ecb.asm: better comments (as in aes192.asm)

1 files changed, 188 insertions, 71 deletions

diff --git a/src/aes256ecb.asm b/src/aes256ecb.asm
index f67f4bd..68430a1 100644
--- a/src/aes256ecb.asm
+++ b/src/aes256ecb.asm
@@ -37,83 +37,200 @@ inverse_key_schedule oword 15 dup(0)
 @raw_aes256ecb_encrypt@48 endp
 
 expand_keys_256ecb proc
-    lea edx, [key_schedule + 20h]
-    movdqa [key_schedule], xmm1
-    movdqa [key_schedule + 10h], xmm2
-
-    aeskeygenassist xmm7, xmm2, 1h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 2h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 4h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 8h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 10h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 20h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 0
-    pshufd xmm7, xmm7, 0AAh
-    call gen_round_key
-
-    aeskeygenassist xmm7, xmm2, 40h
-    pshufd xmm7, xmm7, 0FFh
-    call gen_round_key
+    ; A "word" (in terms of the FIPS 187 standard) is a 32-bit block.
+    ; Words are denoted by `w[N]`.
+    ;
+    ; A key schedule is composed of 14 "regular" keys and a dumb key for
+    ; the "whitening" step.
+    ; It's stored in `key_schedule`.
+    ;
+    ; A key schedule is thus composed of 60 "words".
+    ; The FIPS standard includes an algorithm to calculate these words via
+    ; a simple loop:
+    ;
+    ; i = 8
+    ; while i < 60:
+    ;     temp = w[i - 1]
+    ;     if i % 8 == 0:
+    ;         temp = SubWord(RotWord(temp))^Rcon
+    ;     elif i % 8 == 4:
+    ;         temp = SubWord(temp)
+    ;     w[i] = w[i - 8]^temp
+    ;     i = i + 1
+    ;
+    ; The loop above may be unrolled like this:
+    ;
+    ; w[8] = SubWord(RotWord(w[7]))^Rcon^w[0]
+    ; w[9] = w[8]^w[1]
+    ;      = SubWord(RotWord(w[7]))^Rcon^w[1]^w[0]
+    ; w[10] = w[9]^w[2]
+    ;       = SubWord(RotWord(w[7]))^Rcon^w[2]^w[1]^w[0]
+    ; w[11] = w[10]^w[3]
+    ;       = SubWord(RotWord(w[7]))^Rcon^w[3]^w[2]^w[1]^w[0]
+    ; w[12] = SubWord(w[11])^w[4]
+    ; w[13] = w[12]^w[5]
+    ;       = SubWord(w[11])^w[5]^w[4]
+    ; w[14] = w[13]^w[6]
+    ;       = SubWord(w[11])^w[6]^w[5]^w[4]
+    ; w[15] = w[14]^w[7]
+    ;       = SubWord(w[11])^w[7]^w[6]^w[5]^w[4]
+    ; w[16] = SubWord(RotWord(w[15]))^Rcon^w[8]
+    ; w[17] = w[16]^w[9]
+    ;       = SubWord(RotWord(w[15]))^Rcon^w[9]^w[8]
+    ; w[18] = w[17]^w[10]
+    ;       = SubWord(RotWord(w[15]))^Rcon^w[10]^w[9]^w[8]
+    ; w[19] = w[18]^w[11]
+    ;       = SubWord(RotWord(w[15]))^Rcon^w[11]^w[10]^w[9]^w[8]
+    ; w[20] = SubWord(w[19])^w[12]
+    ; w[21] = w[20]^w[13]
+    ;       = SubWord(w[19])^w[13]^w[12]
+    ; w[22] = w[21]^w[14]
+    ;       = SubWord(w[19])^w[14]^w[13]^w[12]
+    ; w[23] = w[22]^w[15]
+    ;       = SubWord(w[19])^w[15]^w[14]^w[13]^w[12]
+    ;
+    ; ... and so on.
+    ;
+    ; The Intel AES-NI instruction set facilitates calculating SubWord
+    ; and RotWord using `aeskeygenassist`, which is used in this routine.
+    ;
+    ; Preconditions:
+    ; * xmm2[127:96] == w[7],
+    ; * xmm2[95:64]  == w[6],
+    ; * xmm2[63:32]  == w[5],
+    ; * xmm2[31:0]   == w[4],
+    ; * xmm1[127:96] == w[3],
+    ; * xmm1[95:64]  == w[2],
+    ; * xmm1[63:32]  == w[1],
+    ; * xmm1[31:0]   == w[0].
+
+    movdqa [key_schedule], xmm1          ; sets w[0], w[1], w[2], w[3]
+    movdqa [key_schedule + 10h], xmm2    ; sets w[4], w[5], w[6], w[7]
+
+    lea edx, [key_schedule + 20h]        ; ecx = &w[8]
+
+    aeskeygenassist xmm7, xmm2, 1h       ; xmm7[127:96] = RotWord(SubWord(w[7]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[8], w[9], w[10], w[11]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[11])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[12], w[13], w[14], w[15]
+
+    aeskeygenassist xmm7, xmm2, 2h       ; xmm7[127:96] = RotWord(SubWord(w[15]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[16], w[17], w[18], w[19]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[19])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[20], w[21], w[22], w[23]
+
+    aeskeygenassist xmm7, xmm2, 4h       ; xmm7[127:96] = RotWord(SubWord(w[23]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[24], w[25], w[26], w[27]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[27])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[28], w[29], w[30], w[31]
+
+    aeskeygenassist xmm7, xmm2, 8h       ; xmm7[127:96] = RotWord(SubWord(w[31]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[32], w[33], w[34], w[35]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[35])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[36], w[37], w[38], w[39]
+
+    aeskeygenassist xmm7, xmm2, 10h      ; xmm7[127:96] = RotWord(SubWord(w[39]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[40], w[41], w[42], w[43]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[43])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[44], w[45], w[46], w[47]
+
+    aeskeygenassist xmm7, xmm2, 20h      ; xmm7[127:96] = RotWord(SubWord(w[47]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[48], w[49], w[50], w[51]
+
+    aeskeygenassist xmm7, xmm2, 0        ; xmm7[95:64] = SubWord(w[51])
+    pshufd xmm7, xmm7, 0AAh              ; xmm7[127:96] = xmm7[63:32] = xmm7[31:0] = xmm7[95:64]
+    call gen_round_key                   ; sets w[52], w[53], w[54], w[55]
+
+    aeskeygenassist xmm7, xmm2, 40h      ; xmm7[127:96] = RotWord(SubWord(w[55]))^Rcon
+    pshufd xmm7, xmm7, 0FFh              ; xmm7[95:64] = xmm7[63:32] = xmm7[31:0] = xmm7[127:96]
+    call gen_round_key                   ; sets w[56], w[57], w[58], w[59]
 
     call invert_key_schedule
     ret
 
 gen_round_key:
-    movdqa xmm6, xmm1
-
-    pslldq xmm6, 4
-    pxor xmm1, xmm6
-    pslldq xmm6, 4
-    pxor xmm1, xmm6
-    pslldq xmm6, 4
-    pxor xmm1, xmm6
-
-    pxor xmm1, xmm7
-
-    movdqa [edx], xmm1
-    add edx, 10h
+    ; Preconditions:
+    ; * xmm2[127:96] == w[i+7],
+    ; * xmm2[95:64]  == w[i+6],
+    ; * xmm2[63:32]  == w[i+5],
+    ; * xmm2[31:0]   == w[i+4],
+    ; * xmm1[127:96] == w[i+3],
+    ; * xmm1[95:64]  == w[i+2],
+    ; * xmm1[63:32]  == w[i+1],
+    ; * xmm1[31:0]   == w[i],
+    ; * xmm7[127:96] == xmm7[95:64] == xmm7[63:32] == xmm7[31:0] == HWGEN,
+    ;   where HWGEN is either RotWord(SubWord(w[i+7]))^Rcon or SubWord(w[i+7]),
+    ;   depending on the number of the round being processed,
+    ; * ecx == &w[i+8].
+    ;
+    ; Postconditions:
+    ; * xmm2[127:96] == w[i+11] == HWGEN^w[i+3]^w[i+2]^w[i+1]^w[i],
+    ; * xmm2[95:64]  == w[i+10] == HWGEN^w[i+2]^w[i+1]^w[i],
+    ; * xmm2[63:32]  == w[i+9]  == HWGEN^w[i+1]^w[i],
+    ; * xmm2[31:0]   == w[i+8]  == HWGEN^w[i],
+    ; * xmm1[127:96] == w[i+7],
+    ; * xmm1[95:64]  == w[i+6],
+    ; * xmm1[63:32]  == w[i+5],
+    ; * xmm1[31:0]   == w[i+4],
+    ; * ecx == &w[i+12],
+    ; * the value in xmm6 is also modified.
+
+    ; Calculate
+    ;     w[i+3]^w[i+2]^w[i+1]^w[i],
+    ;     w[i+2]^w[i+1]^w[i],
+    ;     w[i+1]^w[i] and
+    ;     w[i].
+    movdqa xmm6, xmm1    ; xmm6 = xmm1
+    pslldq xmm6, 4       ; xmm6 <<= 32
+    pxor xmm1, xmm6      ; xmm1 ^= xmm6
+    pslldq xmm6, 4       ; xmm6 <<= 32
+    pxor xmm1, xmm6      ; xmm1 ^= xmm6
+    pslldq xmm6, 4       ; xmm6 <<= 32
+    pxor xmm1, xmm6      ; xmm1 ^= xmm6
+                         ; xmm1[127:96] == w[i+3]^w[i+2]^w[i+1]^w[i]
+                         ; xmm1[95:64]  == w[i+2]^w[i+1]^w[i]
+                         ; xmm1[63:32]  == w[i+1]^w[i]
+                         ; xmm1[31:0]   == w[i]
+
+    ; Calculate
+    ;     HWGEN^w[i+3]^w[i+2]^w[i+1]^w[i],
+    ;     HWGEN^w[i+2]^w[i+1]^w[i],
+    ;     HWGEN^w[i+1]^w[i] and
+    ;     HWGEN^w[i].
+    pxor xmm1, xmm7    ; xmm1 ^= xmm7
+                       ; xmm1[127:96] == w[i+11] == HWGEN^w[i+3]^w[i+2]^w[i+1]^w[i]
+                       ; xmm1[95:64]  == w[i+10] == HWGEN^w[i+2]^w[i+1]^w[i]
+                       ; xmm1[63:32]  == w[i+9]  == HWGEN^w[i+1]^w[i]
+                       ; xmm1[31:0]   == w[i+8]  == HWGEN^w[i]
+
+    ; Set w[i+8], w[i+9], w[i+10] and w[i+11].
+    movdqa [edx], xmm1    ; w[i+8]  = HWGEN^w[i]
+                          ; w[i+9]  = HWGEN^w[i+1]^w[i]
+                          ; w[i+10] = HWGEN^w[i+2]^w[i+1]^w[i]
+                          ; w[i+11] = HWGEN^w[i+3]^w[i+2]^w[i+1]^w[i]
+    add edx, 10h          ; ecx = &w[i+12]
+
+    ; Swap the values in xmm1 and xmm2.
+    pxor xmm1, xmm2
+    pxor xmm2, xmm1
+    pxor xmm1, xmm2
 
-    movdqa xmm7, xmm1
-    movdqa xmm1, xmm2
-    movdqa xmm2, xmm7
     ret
 
 invert_key_schedule: