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examples/single-char.c

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+// single-char.c
+// -------------
+//
+// The binary reads some chars from stdin and checks it against a hard-coded flag.
+// If the entered flag is correct, a corresponding message will be printed out.
+//
+// Compile with
+//  gcc -o single-char single-char.c
+//
+// Quick binary analysis
+//  - load into gdb
+//  - start, so the binary is mapped to the final position
+//  - execute "disas main"
+// Look at 0x00005555555551c7 <+130>. It moves 0 to rbp-0x4, that's the foundFlag = 0 below.
+// This is the perfect address for --negative-addr
+// Finding the win function is even easier. We just need to search for the point where puts("yay, ...") is called.
+// And that is at 0x00005555555551ec. It is not important if you choose the instruction moving the string into
+// memory, or the instruction calling puts(), as long as it is inside the correct part of the if() block ;)
+//
+// With the addresses identified above, we call barf with:
+//  ./barf.sh --negative-addr 0x5555555551c7 --win-addr 0x5555555551ec ./single-char
+// 
+// Please note that your addresses will likely differ, e.g. if you edit the source file below.
+
+#include <stdio.h>
+#include <string.h>
+
+#define BUFSIZE 32
+
+int main(int argc ,char* argv[]) {
+	char buf[BUFSIZE];
+	char flag[BUFSIZE] = "CTF{F00_b4R_B4z_fL4g!}\n";
+	int foundFlag = 1;
+
+	// read flag
+	fgets(buf, BUFSIZE, stdin);
+
+	// walk flag
+	int i = 0;
+	while(buf[i] != '\0' && i < BUFSIZE) {
+		if(buf[i] != flag[i]) {
+			foundFlag = 0;
+		}
+		i++;
+	}
+
+	// check flag
+	if(foundFlag) {
+		puts("yay, that's the flag! :)");
+	} else {
+		puts("nay, that's not the flag! :(");
+	}
+}
+