Create README.md

imaginary-ctf/round-38/seas-air/README.md

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+# seas air
+
+## problem
+
+![image](https://github.com/quasar098/ctf-writeups/assets/70716985/afef3475-0b5c-4461-9bcd-883bdf3a1145)
+
+```py
+#!/usr/bin/env python3
+
+from random import randint
+from math import sin, pi
+
+def seasair(s, key):
+    return bytes((c + k)%256 for c, k in zip(s, (int(256*sin(key*(i+1)/256*pi)) for i in range(len(s)))))
+
+flag = open("flag.txt", 'rb').read()
+
+key = randint(0, 2**32)
+
+print(seasair(flag, key))
+
+# Output:
+# b"\xca\xae`fg\x94\x88d\xfe'ut|\x1a\x11_\xd4\xc2UmK\xb4\xcf_\x13\x1dy_\x87\x1a\x00_\xd5\xb3MnK\xbe\xd8i\x0c\x14}ns)\x07e\xc0\x8ei"
+```
+
+## solution
+
+it puts the flag into a seasair function which takes a key and applys some weird sin function for each letter
+here, i reversed the function to decrypt the message and then brute forced the key
+
+```py
+#!/usr/bin/env python3
+
+from random import randint
+from math import sin, pi
+
+def seasair(s, key):
+    return bytes(
+        (a + b) % 256 for a, b in zip(
+            s,
+            [int(
+                256 * sin(key * (i+1) / 256 * pi)
+            ) for i in range(len(s))]
+        )
+    )
+
+
+def de_seasair(st, key):
+    return bytes(
+        (a - b) % 256 for a, b in zip(
+            st,
+            [int(
+                256 * sin(key * (i+1) / 256 * pi)
+            ) for i in range(len(st))]
+        )
+    )
+
+flag = b"ii"
+
+key = randint(0, 2**32)
+
+for _ in range(512):
+    print(de_seasair(b"\xca\xae`fg\x94\x88d\xfe'ut|\x1a\x11_\xd4\xc2UmK\xb4\xcf_\x13\x1dy_\x87\x1a\x00_\xd5\xb3MnK\xbe\xd8i\x0c\x14}ns)\x07e\xc0\x8ei", _))
+
+print(seasair(flag, key), key % 512)
+```
+
+![image](https://github.com/quasar098/ctf-writeups/assets/70716985/ee0bfbf4-6806-4292-a5dc-c70dfbf98646)