From 44e95dc49dc6b8afa4bb49c6dd1cb2002866b0ca Mon Sep 17 00:00:00 2001
From: Malfurious <m@lfurio.us>
Date: Fri, 17 Feb 2023 14:54:13 -0500
Subject: Writeup LACTF 2023 / Switcheroo
Signed-off-by: Malfurious <m@lfurio.us>
---
docs/writeups/2023/lactf/rev/switcheroo.txt | 296 ++++++++++++++++++++++++++++
1 file changed, 296 insertions(+)
create mode 100644 docs/writeups/2023/lactf/rev/switcheroo.txt
diff --git a/docs/writeups/2023/lactf/rev/switcheroo.txt b/docs/writeups/2023/lactf/rev/switcheroo.txt
new file mode 100644
index 0000000..c7cd4e5
--- /dev/null
+++ b/docs/writeups/2023/lactf/rev/switcheroo.txt
@@ -0,0 +1,296 @@
+Never fear, the flag shall not appear (in memory)! I've switched it out for a
+more secure system!
+
+Note: The actual flag consists purely of non-whitespace printable characters
+(aka ascii values 0x21 through 0x7e).
+
+
+
+
+RE
+--
+The switcheroo binary is compiled from assembly language, rather than C or
+anything else higher-level. However, it is still very useful to use Ghidra to
+analyze strings and add additional comments and markup in the code listing view.
+
+Given the above, the file is actually quite small and simple. We have just a
+couple boilerplate sections in the ELF, .data contains 3 strings and an array of
+64-bit integers, and all of the logic lies sequentially under .text.
+
+The program is a flag checker. When run, it first asks you to input the flag,
+then asserts that the size of input == 64, so we know the length of the expected
+flag. To validate the content of the proposed flag, we enter a loop to iterate
+over each input character (this is denoted by the `loop_start` label below).
+
+For each input character, we start by using the value of that character (ascii
+as an int) as an index into the 64-bit integer array found in the program. The
+value we lookup will encode what position(s) the character index may correctly
+occupy in the real flag. We enter an inner loop to decode the 64-bit value.
+
+At each step, an 8-bit left bit rotation is performed on the value, we then look
+at the resulting lowest-order byte in the 64-bit value. The first value we
+extract is a count value.
+
+The code asserts that the value of the current flag character position (as a
+byte) exists in the looked-up value within the next `count` byte rotations.
+This area is denoted by the `inner_loop_start` label below. If we hit a
+matching position value, the outer loop advances. However, if we exhaust
+`count` many rotations before that occurs, we increment another register that
+records how many failed characters were given, and the outer loop continues
+regardless.
+
+If the outer loop finishes without incrementing the failure register, the
+correct flag was given. See the full .text logic here, with some markup from
+me:
+
+```
+ undefined __stdcall entry(void)
+ undefined AL:1 <RETURN>
+ undefined1 Stack[0x0]:1 local_res0 XREF[1]: 00401040(*)
+ entry XREF[4]: Entry Point(*), 00400018(*),
+ 00400088(*),
+ _elfSectionHeaders::00000090(*)
+00401000 48 8d 34 LEA RSI,[s_Give_me_the_flag:_00402000] = "Give me the flag: "
+ 25 00 20
+ 40 00
+00401008 bf 01 00 MOV EDI,0x1
+ 00 00
+0040100d ba 12 00 MOV EDX,0x12
+ 00 00
+00401012 b8 01 00 MOV EAX,0x1
+ 00 00
+ write(stdout, "Give me the flag", size)
+00401017 0f 05 SYSCALL
+00401019 31 ff XOR EDI,EDI
+0040101b 48 89 e6 MOV RSI,RSP
+0040101e ba 64 00 MOV EDX,0x64
+ 00 00
+00401023 31 c0 XOR EAX,EAX
+ read(stdin, &stack, 100)
+00401025 0f 05 SYSCALL
+00401027 48 83 f8 40 CMP RAX,0x40
+ if read() != 64: fail
+0040102b 75 5e JNZ fail
+0040102d 4d 31 e4 XOR R12,R12
+00401030 4c 8d 1c LEA R11,[static_data]
+ 25 3c 20
+ 40 00
+00401038 4d 31 d2 XOR R10,R10
+ loop_start XREF[1]: 00401071(j)
+0040103b 49 39 c2 CMP R10,RAX
+0040103e 7d 33 JGE loop_break
+ R9 = stack[R10] // get current char
+00401040 4e 0f b6 MOVZX R9,byte ptr [RSP + R10*0x1]=>local_res0
+ 0c 14
+ R8 = static_data[R9] // lookup long by current char
+00401045 4e 8b 04 MOV R8,qword ptr [static_data + R9*0x8]
+ cd 3c 20
+ 40 00
+ rotate left by 8-bits (1-byte)
+0040104d 49 c1 c0 08 ROL R8,0x8
+ R13 = (char)(static_data[stack[i]] >> 56)
+00401051 4d 0f b6 e8 MOVZX R13,R8B
+00401055 4d 31 f6 XOR R14,R14
+ while R14 < R13
+ inner_loop_start XREF[1]: 00401069(j)
+00401058 4d 39 ee CMP R14,R13
+0040105b 7d 0e JGE inner_loop_break_bad
+0040105d 49 c1 c0 08 ROL R8,0x8
+00401061 45 38 d0 CMP R8B,R10B
+00401064 74 08 JZ inner_loop_break_good
+00401066 49 ff c6 INC R14
+00401069 eb ed JMP inner_loop_start
+ inner_loop_break_bad XREF[1]: 0040105b(j)
+0040106b 49 ff c4 INC R12
+ inner_loop_break_good XREF[1]: 00401064(j)
+0040106e 49 ff c2 INC R10
+00401071 eb c8 JMP loop_start
+ loop_break XREF[1]: 0040103e(j)
+00401073 4d 85 e4 TEST R12,R12
+00401076 75 13 JNZ fail
+00401078 eb 02 JMP success
+0040107a eb ?? EBh
+0040107b 00 ?? 00h
+ success XREF[1]: 00401078(j)
+0040107c 48 8d 34 LEA RSI,[s_That_was_the_flag!_0040202a] = "That was the flag!"
+ 25 2a 20
+ 40 00
+00401084 ba 12 00 MOV EDX,0x12
+ 00 00
+00401089 eb 0f JMP exit
+ fail XREF[2]: 0040102b(j), 00401076(j)
+0040108b 48 8d 34 LEA RSI,[s_That_was_not_the_flag_:(_00402012] = "That was not the flag :("
+ 25 12 20
+ 40 00
+00401093 ba 18 00 MOV EDX,0x18
+ 00 00
+00401098 eb 00 JMP exit
+ exit XREF[2]: 00401089(j), 00401098(j)
+0040109a bf 01 00 MOV EDI,0x1
+ 00 00
+0040109f b8 01 00 MOV EAX,0x1
+ 00 00
+ write(stdout, "That was [not] the flag", size)
+004010a4 0f 05 SYSCALL
+004010a6 6a 0a PUSH 0xa
+004010a8 48 89 e6 MOV RSI,RSP
+004010ab ba 01 00 MOV EDX,0x1
+ 00 00
+004010b0 b8 01 00 MOV EAX,0x1
+ 00 00
+ write(stdout, "\n", 1)
+004010b5 0f 05 SYSCALL
+004010b7 31 ff XOR EDI,EDI
+004010b9 48 c7 c0 MOV RAX,0x3c
+ 3c 00 00 00
+ exit(0)
+004010c0 0f 05 SYSCALL
+```
+
+
+
+
+Solution
+--------
+To extract the flag, I extracted the 64-bit integer array to attempt to fully
+decode it. With the rules we learned during RE, we know that each entry can be
+used to validate whether or not it resides at any given position in the flag
+string.
+
+I wrote a C program to iterate over a flag string (aka position from 0 -> 64)
+and print any character value (aka array entry at an ascii index) that _would_
+pass validation. This initially gave some ambiguous results, however when my
+search was restricted per the problem description (0x21 -> 0x7e), a unique flag
+appears.
+
+```
+#include <stdio.h>
+
+static const unsigned char A[] = {
+ 0x06, 0x05, 0x04, 0x03, 0x02, 0x01, 0x00, 0x07, 0x7e, 0xfc, 0x3c, 0x78,
+ 0xfb, 0xf4, 0x04, 0x3e, 0x03, 0x87, 0x30, 0x04, 0x3d, 0x81, 0x70, 0x9d,
+ 0x4d, 0xa7, 0x3d, 0x09, 0xf8, 0xa1, 0x51, 0x4a, 0xfc, 0x02, 0x8b, 0xa8,
+ 0xb2, 0x50, 0x49, 0x0c, 0x4b, 0xfd, 0xf0, 0x62, 0x92, 0x1d, 0xd2, 0x02,
+ 0xa9, 0x46, 0xbe, 0xaa, 0xbe, 0x81, 0xe7, 0x88, 0x94, 0x19, 0xbb, 0x85,
+ 0x0e, 0x0f, 0x09, 0x77, 0xfb, 0x49, 0x3f, 0x35, 0x24, 0xb8, 0x4e, 0x56,
+ 0x47, 0x9b, 0x62, 0xeb, 0x9d, 0x7a, 0x0b, 0x7f, 0xd1, 0x18, 0x6e, 0x14,
+ 0x30, 0xb1, 0x3f, 0x01, 0xed, 0x65, 0xd4, 0x4d, 0x7e, 0xe1, 0x50, 0xe6,
+ 0xe7, 0xab, 0x3d, 0x80, 0x04, 0x67, 0xd3, 0x81, 0x7a, 0x8c, 0x71, 0x73,
+ 0xa3, 0x80, 0xce, 0x02, 0x73, 0x48, 0x25, 0x30, 0x09, 0x51, 0x2c, 0x82,
+ 0x4e, 0x68, 0x70, 0x50, 0x47, 0x37, 0x08, 0x57, 0xb5, 0x14, 0x10, 0x32,
+ 0xec, 0x34, 0xd7, 0x3b, 0xb3, 0x98, 0xdd, 0x7e, 0xe3, 0x1e, 0x5e, 0x97,
+ 0xfd, 0x4c, 0x0d, 0xb6, 0xfe, 0x44, 0x81, 0x15, 0x91, 0x42, 0x15, 0xac,
+ 0x6c, 0x7c, 0x40, 0x88, 0x1b, 0xfc, 0x0d, 0x92, 0x2b, 0x19, 0xae, 0x0f,
+ 0x80, 0x26, 0x76, 0x7c, 0x6e, 0x67, 0x0c, 0xfc, 0x27, 0xef, 0x94, 0x6d,
+ 0xd3, 0xb8, 0x50, 0x48, 0xa2, 0x4d, 0x88, 0x0d, 0x72, 0x66, 0x64, 0x08,
+ 0xdc, 0x48, 0x4f, 0xc1, 0x53, 0x04, 0x7e, 0xac, 0xd8, 0xdf, 0x61, 0x24,
+ 0xad, 0x68, 0x25, 0x7c, 0x08, 0x1f, 0x91, 0xde, 0x5d, 0x06, 0x3b, 0x47,
+ 0x71, 0x58, 0xe6, 0x42, 0xd7, 0x1f, 0xed, 0x1e, 0x48, 0x25, 0xb8, 0xc1,
+ 0xc3, 0xed, 0x81, 0x6c, 0xff, 0x02, 0x18, 0xb6, 0x71, 0x9f, 0x16, 0x03,
+ 0xe0, 0x3f, 0xae, 0x92, 0x67, 0x39, 0xa7, 0xe4, 0xca, 0xb7, 0x10, 0x6b,
+ 0xac, 0x08, 0xcf, 0x90, 0xa0, 0x81, 0xf0, 0x31, 0x09, 0xa1, 0x4c, 0x00,
+ 0x3b, 0x68, 0x89, 0x8f, 0xf1, 0x3d, 0xf5, 0x00, 0x49, 0x8f, 0xde, 0xb1,
+ 0xbc, 0x42, 0x42, 0x00, 0xca, 0x84, 0xac, 0xce, 0x00, 0x59, 0x7e, 0x00,
+ 0xa4, 0x0e, 0x11, 0xe8, 0x0f, 0x6f, 0x70, 0x00, 0x4e, 0x50, 0x62, 0x20,
+ 0xb4, 0x78, 0x22, 0x00, 0x9c, 0x2b, 0xa5, 0x5d, 0xa9, 0x05, 0x2a, 0x00,
+ 0x49, 0x0e, 0xdf, 0xb9, 0xd0, 0xc5, 0x4d, 0x00, 0xc4, 0xf0, 0x6c, 0xe0,
+ 0x41, 0x36, 0x6c, 0x00, 0x6e, 0x4a, 0x37, 0x8e, 0xa6, 0x47, 0x79, 0x00,
+ 0x02, 0x68, 0x33, 0x2b, 0x63, 0xd2, 0xd8, 0x00, 0x2e, 0x02, 0xb5, 0x2f,
+ 0xc4, 0x56, 0x38, 0x01, 0x18, 0x39, 0x08, 0x1a, 0xdb, 0x14, 0x3b, 0x00,
+ 0xbb, 0xe1, 0x2f, 0xb1, 0x3a, 0x67, 0xf8, 0x00, 0x24, 0x12, 0xed, 0x14,
+ 0xb0, 0x08, 0xa1, 0x00, 0x04, 0x3b, 0xef, 0x6a, 0x2e, 0x89, 0xb2, 0x00,
+ 0xd6, 0x24, 0x00, 0x18, 0xd3, 0x3c, 0x2c, 0x02, 0x34, 0x5f, 0xfc, 0xd0,
+ 0xaa, 0x11, 0x39, 0x02, 0xe6, 0x4e, 0x4b, 0x71, 0xc9, 0x07, 0x08, 0x02,
+ 0x27, 0x23, 0x30, 0x09, 0x1c, 0x33, 0x1a, 0x07, 0x45, 0xd2, 0xa9, 0x21,
+ 0x29, 0x06, 0x18, 0x04, 0x53, 0x92, 0x0f, 0x2b, 0x34, 0x16, 0x1f, 0x05,
+ 0x7c, 0xe9, 0x0a, 0xda, 0xbf, 0x19, 0x3b, 0x01, 0x1e, 0x2f, 0x19, 0x35,
+ 0x25, 0x17, 0x12, 0x07, 0x95, 0xd2, 0x10, 0xc9, 0x0b, 0x2a, 0x32, 0x03,
+ 0x94, 0x63, 0x19, 0xfa, 0xff, 0x70, 0xc0, 0x00, 0xdd, 0x99, 0x49, 0xa8,
+ 0x72, 0xb0, 0x9a, 0x00, 0xac, 0xaa, 0xa1, 0x74, 0xe1, 0x82, 0x4e, 0x00,
+ 0xb9, 0x84, 0x95, 0x41, 0x05, 0x52, 0xaa, 0x00, 0xd5, 0xe3, 0x1e, 0x8c,
+ 0x2b, 0x8a, 0x67, 0x00, 0xd0, 0xe7, 0xd8, 0xc9, 0xac, 0xbc, 0x45, 0x00,
+ 0xf9, 0xd4, 0xb0, 0x84, 0xc3, 0x19, 0x3a, 0x00, 0x1b, 0xc5, 0x82, 0xa5,
+ 0x60, 0x19, 0x8b, 0x00, 0xcd, 0x2a, 0x2a, 0x6c, 0x50, 0xc4, 0x4e, 0x00,
+ 0xe1, 0x86, 0x57, 0x60, 0xc8, 0x73, 0x74, 0x00, 0x7f, 0xb3, 0xba, 0x6b,
+ 0x53, 0x16, 0x13, 0x01, 0xef, 0xde, 0xdb, 0x7f, 0x24, 0x0d, 0x0f, 0x00,
+ 0x76, 0x1e, 0xde, 0xa9, 0x08, 0xf6, 0xad, 0x00, 0xe3, 0x82, 0xcf, 0x68,
+ 0x08, 0x54, 0x9f, 0x00, 0x1d, 0x80, 0x2b, 0x31, 0xfc, 0xf5, 0x14, 0x00,
+ 0xfe, 0x24, 0x32, 0xc4, 0x14, 0x6f, 0x10, 0x00, 0xc5, 0x37, 0x8b, 0x5d,
+ 0xad, 0x5b, 0xb8, 0x00, 0x0d, 0x99, 0x8c, 0x84, 0x17, 0xce, 0xed, 0x00,
+ 0xc4, 0xbc, 0x11, 0xdf, 0x13, 0xdc, 0xcc, 0x00, 0x9b, 0x55, 0xa0, 0x76,
+ 0x67, 0x2d, 0x0c, 0x02, 0xf7, 0x21, 0x75, 0xea, 0x64, 0x1b, 0x0a, 0x02,
+ 0x82, 0xbe, 0x90, 0xd9, 0x24, 0x1d, 0x39, 0x00, 0xa8, 0x3c, 0xd7, 0x9d,
+ 0x71, 0x00, 0x75, 0x00, 0x37, 0x2a, 0xa8, 0xb2, 0xad, 0x2e, 0xef, 0x00,
+ 0x60, 0x14, 0x96, 0x40, 0x0a, 0xf1, 0x8e, 0x00, 0xa5, 0x67, 0x3f, 0xfe,
+ 0x2c, 0x9a, 0x33, 0x00, 0x9d, 0x87, 0x65, 0x63, 0x30, 0x71, 0xe0, 0x00,
+ 0xf0, 0xdc, 0x75, 0x2a, 0x3d, 0x6e, 0x6e, 0x00, 0xf0, 0x73, 0x82, 0xc1,
+ 0x54, 0x72, 0x2e, 0x01, 0x7f, 0x66, 0x9b, 0xcd, 0xad, 0x08, 0x48, 0x00,
+ 0x95, 0x63, 0xea, 0x30, 0x12, 0x80, 0x10, 0x01, 0x38, 0xfc, 0xf1, 0xf1,
+ 0xc7, 0x98, 0xfa, 0x00, 0xd9, 0x7b, 0xfe, 0xe6, 0x0f, 0x40, 0x0d, 0x01,
+ 0x4d, 0xb9, 0x08, 0x00, 0x15, 0x9e, 0x47, 0x00, 0x95, 0x48, 0x13, 0x38,
+ 0xcf, 0x80, 0xf9, 0x00, 0xac, 0x5d, 0xcb, 0x7b, 0xfe, 0x0f, 0x2a, 0x00,
+ 0x99, 0x9b, 0xce, 0x0f, 0x4b, 0x0b, 0x75, 0x00, 0x58, 0x77, 0xb3, 0x79,
+ 0xfe, 0x6e, 0xbd, 0x00, 0x28, 0x36, 0x15, 0x31, 0x24, 0x0e, 0x20, 0x07,
+ 0x02, 0xbc, 0x32, 0x40, 0xf1, 0xaa, 0x7c, 0x00, 0xbb, 0x95, 0x83, 0x61,
+ 0x1f, 0x01, 0x3d, 0x02, 0x9e, 0x6f, 0xfe, 0x34, 0x78, 0x80, 0x53, 0x00,
+ 0xc8, 0x70, 0x86, 0xe8, 0x3d, 0x30, 0x02, 0x01, 0xf8, 0xd8, 0x08, 0xc7,
+ 0x3a, 0x0c, 0xf5, 0x00, 0xbc, 0x75, 0x9d, 0xa9, 0xf8, 0x5e, 0xff, 0x00,
+ 0x7e, 0x51, 0x0e, 0x10, 0x12, 0x3a, 0x04, 0x02, 0x06, 0xd7, 0x49, 0xd0,
+ 0x19, 0x2d, 0xe8, 0x00, 0x14, 0x42, 0x62, 0xdb, 0x31, 0x26, 0x14, 0x02,
+ 0x0b, 0xa6, 0xff, 0xb0, 0x63, 0x13, 0xa9, 0x00, 0xf9, 0xb2, 0x7d, 0x13,
+ 0xa1, 0x8e, 0x85, 0x00, 0xec, 0x18, 0xb0, 0x06, 0xfa, 0x4f, 0x6f, 0x00,
+ 0x34, 0xf0, 0x80, 0x76, 0xe7, 0x57, 0x00, 0x01, 0x72, 0x72, 0x8c, 0x8b,
+ 0xb8, 0x5a, 0x5a, 0x00, 0x82, 0x1d, 0x08, 0x34, 0x68, 0x19, 0x1d, 0x01,
+ 0xac, 0xea, 0xfa, 0x56, 0x07, 0xee, 0x6d, 0x00, 0x78, 0x00, 0x16, 0x0c,
+ 0xe8, 0x61, 0x16, 0x00, 0x13, 0xc3, 0x58, 0x1a, 0x7a, 0xa6, 0xcc, 0x00,
+ 0xe8, 0x13, 0x1b, 0xb8, 0x83, 0x36, 0x22, 0x01, 0x8d, 0xd6, 0x45, 0x03,
+ 0xc2, 0xca, 0x65, 0x00, 0xdc, 0xec, 0x50, 0x60, 0x1a, 0x1c, 0x03, 0x01,
+ 0x05, 0x19, 0x2c, 0x97, 0xec, 0x04, 0x37, 0x01, 0x25, 0x0b, 0x6c, 0x8b,
+ 0xe4, 0xc9, 0x4e, 0x00, 0xdd, 0xe1, 0xa5, 0x12, 0x0d, 0xc4, 0x55, 0x00,
+ 0x0e, 0xe4, 0x20, 0xc4, 0xfc, 0xd4, 0xf0, 0x00, 0xf3, 0x27, 0x8b, 0x99,
+ 0xc1, 0xb8, 0x9b, 0x00, 0xf6, 0x1a, 0x04, 0x04, 0x5d, 0xc8, 0xca, 0x00,
+ 0x05, 0xba, 0x8e, 0x89, 0xaf, 0x41, 0x05, 0x02, 0x95, 0x10, 0x67, 0x3d,
+ 0x26, 0xb0, 0x2c, 0x00, 0x43, 0x36, 0xac, 0xe7, 0xff, 0x42, 0x3e, 0x02,
+ 0x87, 0x60, 0x09, 0x4e, 0x62, 0xf3, 0x19, 0x00, 0xec, 0x88, 0xa7, 0x5a,
+ 0x99, 0xd3, 0xf8, 0x26 };
+
+static const unsigned long *B = (unsigned long *)A;
+
+static void rol(long *v) {
+ long tmp = *v << 8;
+ tmp |= *v >> 56;
+ *v = tmp;
+}
+
+int check_char_in_pos(int c, int pos) {
+ long value = B[c];
+ rol(&value);
+
+ for (int count = value & 0xff; count > 0; count--) {
+ rol(&value);
+
+ if ((value & 0xff) == pos)
+ return 1;
+ }
+
+ return 0;
+}
+
+int main(void) {
+ for (int pos = 0; pos < 0x40; pos++) {
+ for (int c = 0x21; c <= 0x7e; c++) {
+ if (check_char_in_pos(c, pos)) {
+ printf("%c", (char)c);
+ break;
+ }
+ }
+ }
+
+ printf("\n");
+ return 0;
+}
+```
+
+lactf{4223M8LY_5W17Ch_57473M3n75_4r3_7h3_4850LU73_8357_u+1f60a}
+
+I still don't know where the switch statement was...
--
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