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DDC-2024/quarter-finals/crypto/dist-Independence/chal.py

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+from Crypto.Util.number import *
+
+FLAG = open('flag.txt', 'rb').read()
+
+def encrypt(m, pubkey, privkey):
+    g, p = pubkey
+    x, _ = privkey
+    h = pow(g, x, p)
+    C = []
+    while m:
+        y = getRandomRange(2, p)
+        c1 = pow(g, y, p)
+        y = (y<<1) | (m & 1)
+        c2 = pow(h, y, p)
+        C += [(c1, c2)]
+        m >>= 1
+    return C
+
+def keygen(nbits=512):
+    p = getStrongPrime(nbits)
+    g = getRandomRange(2, p)
+    x = getRandomRange(2, p)
+    pub = (g, p)
+    priv = (x, p)
+    return pub, priv
+
+pub, priv = keygen()
+m = bytes_to_long(FLAG)
+print(m)
+c = encrypt(m, pub, priv)
+
+with open('out1.txt', 'w') as f:
+    f.write(f'{pub = }\n')
+    f.write(f'out = {str(c)}')

DDC-2024/quarter-finals/crypto/dist-Independence/solve.py

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+from Crypto.Util.number import long_to_bytes
+
+with open('out.txt', 'r') as f:
+    data = f.read()
+
+
+pub = (5464549774190809852923763408523051958716400587576327799474715226373287205246183801056700913652415087121976663782311766735601091617825037804761387911068511, 9581257592556018473305786754018994054986440370491067910997313283399579058244765977967617476919486211692103485121526918608638896652486174462300514168144287)
+g, q = pub
+# Extract the encrypted message
+bin_flag=[]
+out_start = data.find('out = ') + len('out = ')
+c = eval(data[out_start:])
+for cipher in c:
+    c1, c2 = cipher
+    if pow(c2,(q-1)//2, q) == 1:
+        bin_flag.append(0)
+    else:
+        bin_flag.append(1)
+
+m = 0
+for bit in reversed(bin_flag):
+    m = (m << 1) | bit
+
+flag = long_to_bytes(m)
+print(flag.decode('utf-8'))