Initial commit

3 files changed, 83 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..4e97213
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+present
+.*.sw?
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..1f31cb7
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,17 @@
+
+obj = present
+PORT = 1337
+
+WFLAGS   = -Wall -Wextra
+LDFLAGS  = -Wl,--section-start=.text=0x1000
+LDFLAGS += -Wl,--section-start=.data=0x2000
+
+$(obj): $(obj).S
+	arm-none-eabi-gcc -g $(WFLAGS) -nostdlib $< -o $@ $(LDFLAGS)
+
+run-$(obj): $(obj)
+	qemu-arm -g $(PORT) $(obj) &
+	arm-none-eabi-gdb -q -ex 'tar rem localhost:$(PORT)' $(obj)
+
+edit-$(obj).S: $(obj).S
+	@vim +"let g:syntastic_asm_compiler='arm-none-eabi-gcc'" $<
diff --git a/present.S b/present.S
new file mode 100644
index 0000000..dd9401d
--- /dev/null
+++ b/present.S
@@ -0,0 +1,64 @@
+.global _start
+
+.data
+
+input:
+.byte 5, 1,  2,  3,  4,  5,  6,  7
+.byte 8, 9, 10, 11, 12, 13, 14, 15
+
+sbox:
+.byte 0xC, 0x5, 0x6, 0xB, 0x9, 0x0, 0xA, 0xD
+.byte 0x3, 0xE, 0xF, 0x8, 0x4, 0x7, 0x1, 0x2
+
+.text
+
+// INPUT:  64 bits block (in r0), sbox (in r1)
+// OUTPUT: 64 bits block (in r0)
+// Processing: subtitute every nibble using the sbox
+sub_block:
+/*
+    for (i = 0; i < 8; i++) {
+        uint8_t c = input[i];
+        uint8_t c_lo = c & 15;  // extract (keep) low nibble
+        uint8_t c_hi = c >> 4;  // extract high nibble
+        output[i]  = sbox[c_lo];        // store in low nibble
+        output[i] |= sbox[c_hi] << 4;   // store in high nibble
+    }
+*/
+    // loop through 8 bytes (64 bits) (use r3 as counter)
+    // i = 0
+    eor r3, r3
+sub_block_loop:
+    cmp r3, #8
+    bge sub_block_loop_end
+
+    // c = input[i] (r4 is low, r5 is high)
+    ldrb r4, [r0, r3]
+    mov r5, r4
+    and r4, #15
+    lsr r5, #4
+
+    // sbox[c_lo]
+    ldrb r4, [r1, r4]
+    // sbox[c_hi]
+    ldrb r5, [r1, r5]
+
+    // output[i] = (hi << 4) | lo
+    orr r4, r5
+    strb r4, [r0, r3]
+
+    add r3, #1
+    b   sub_block_loop
+sub_block_loop_end:
+
+    // return
+    mov pc, lr
+
+_start:
+    ldr r0, =input
+    ldr r1, =sbox
+    bl  sub_block
+    b   forever
+
+forever:
+    b   forever