Merge branch 'master' of github.com:failedxyz/easyctf

lost-seeds/description.md

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+Every time I encrypt a flag with [this program](${encrypt}), it gives me [something different](${flag_out}).

lost-seeds/encrypt.c

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+#include <stdio.h>
+#include <unistd.h>
+#include <string.h>
+
+#define RAND_MAX ((1U << 31) - 1)
+#define MAXLENGTH 80
+
+int realrand() {
+	int n;
+	FILE *fp = fopen("/dev/urandom", "r");
+	fread(&n, 4, 1, fp);
+	fclose(fp);
+	return n % MAXLENGTH;
+}
+
+int seed;
+
+int pseudorand() {
+	return seed = (seed * 19394489 + 132241) & 0xff;
+}
+
+int main() {
+	char input[MAXLENGTH];
+	FILE *fp = fopen("flag.in", "r");
+	fread(&input, 1, MAXLENGTH, fp);
+	fclose(fp);
+
+	fp = fopen("flag.out", "wb");
+	seed = realrand();
+	int r;
+	char c;
+	for (int i = 0, l = strlen(input); i < l; ++i) {
+		c = ((char) input[i] ^ (r = pseudorand())) & 0xff;
+		fwrite(&c, 1, 1, fp);
+	}
+	fclose(fp);
+}

lost-seeds/flag.in

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+easyctf{r3ndom_numb3rs_m3an_n0thing_wh3n_y0u_can_brute_force!}

lost-seeds/flag.out

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+·2˙ÄEsĆJ<EFBFBD>ZA!‚÷WNľľ¤ŻlyjęŞđŹě
ťK±ĎsżĹ˘ä°±ě‰WťˇXbĎÇ…ŮGđ"2

lost-seeds/grader.py

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+def grade(autogen, answer):
+    if answer.find("r3ndom_numb3rs_m3an_n0thing_wh3n_y0u_can_brute_force!") != -1:
+        return True, "Correct!"
+    return False, "Nope, try again."

lost-seeds/problem.yml

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+title: Lost Seed
+author: mzhang
+category: Cryptography
+autogen: false
+programming: false
+value: 150
+files:
+  - encrypt
+  - flag.out
+