no padding no problem

no_padding_no_problem/sol.py

@@ -0,0 +1,53 @@
+#!/home/maxime/.pyvenv/bin/python3
+from pwn import *
+import binascii
+
+def egcd(a, b):
+    x0, x1, y0, y1 = 0, 1, 1, 0
+    while a != 0:
+        (q, a), b = divmod(b, a), a
+        y0, y1 = y1, y0 - q * y1
+        x0, x1 = x1, x0 - q * x1
+    return b, x0, x1
+
+# https://en.wikibooks.org/wiki/Algorithm_Implementation/Mathematics/Extended_Euclidean_algorithm
+def mod_inv(x, modulus):
+    g, x, _ = egcd(x, modulus)
+    if g != 1:
+        raise Exception("gcd(d, lambda_n != !")
+    return x % modulus 
+
+# -------------------------------------------------------------------
+
+conn = remote("mercury.picoctf.net", 60368)
+conn.recvuntil(b':')
+n = conn.recvline().strip()
+conn.recvuntil(b':')
+e = conn.recvline().strip()
+conn.recvuntil(b':')
+c = conn.recvline().strip()
+conn.recvuntil(b':')
+
+n = int(n)
+e = int(e)
+c = int(c)
+
+print("n:", n)
+print("e:", e)
+print("c:", c)
+
+r = 11
+r_inv = mod_inv(r, n)
+c_ = (c*pow(r, e, n))%n
+
+conn.sendline(str(c_).encode('utf-8'))
+conn.recvuntil(b':')
+c_dec = conn.recvline().strip()
+c_dec = int(c_dec)
+
+flag = (c_dec*r_inv)%n
+
+flag = binascii.unhexlify('{:x}'.format(flag))
+
+print(flag)
+