glacier ctf 25

2025/glacier/rev/c2/dec_rc5_chatgpt.py

@@ -0,0 +1,138 @@
+import hashlib
+import struct
+
+
+# ----------------------------
+#  RC5 core parameters
+# ----------------------------
+W = 32          # word size in bits
+R = 12          # rounds
+B = 16          # key bytes (RC5-32/12/16)
+WORD_MASK = 0xFFFFFFFF
+BLOCK_SIZE = 8  # 2 * 32-bit words
+
+
+# ----------------------------
+#  Utility: Rotate left/right
+# ----------------------------
+def rotl(x, n):
+    return ((x << n) & WORD_MASK) | (x >> (32 - n))
+
+
+def rotr(x, n):
+    return ((x >> n) | (x << (32 - n))) & WORD_MASK
+
+
+# ----------------------------
+#  Key Expansion
+# ----------------------------
+def expand_key(key: bytes):
+    # Constants for RC5 with w=32
+    P32 = 0xB7E15163
+    Q32 = 0x9E3779B9
+
+    # Convert key into c 32-bit words L[0..c-1]
+    c = max(1, B // 4)
+    L = list(struct.unpack("<" + "I" * c, key))
+
+    # Initialize S-array
+    t = 2 * (R + 1)
+    S = [0] * t
+    S[0] = P32
+    for i in range(1, t):
+        S[i] = (S[i - 1] + Q32) & WORD_MASK
+
+    # Mix key into S
+    i = j = 0
+    A = Bv = 0
+    for _ in range(3 * max(t, c)):
+        A = S[i] = rotl((S[i] + A + Bv) & WORD_MASK, 3)
+        Bv = L[j] = rotl((L[j] + A + Bv) & WORD_MASK, (A + Bv) & 31)
+        i = (i + 1) % t
+        j = (j + 1) % c
+
+    return S
+
+
+# ----------------------------
+#  Block Encrypt/Decrypt
+# ----------------------------
+def rc5_encrypt_block(block: bytes, S):
+    A, B = struct.unpack("<II", block)
+    A = (A + S[0]) & WORD_MASK
+    B = (B + S[1]) & WORD_MASK
+
+    for i in range(1, R + 1):
+        A = (rotl(A ^ B, B & 31) + S[2 * i]) & WORD_MASK
+        B = (rotl(B ^ A, A & 31) + S[2 * i + 1]) & WORD_MASK
+
+    return struct.pack("<II", A, B)
+
+
+def rc5_decrypt_block(block: bytes, S):
+    A, B = struct.unpack("<II", block)
+
+    for i in range(R, 0, -1):
+        B = rotr((B - S[2 * i + 1]) & WORD_MASK, A & 31) ^ A
+        A = rotr((A - S[2 * i]) & WORD_MASK, B & 31) ^ B
+
+    B = (B - S[1]) & WORD_MASK
+    A = (A - S[0]) & WORD_MASK
+
+    return struct.pack("<II", A, B)
+
+
+# ----------------------------
+#  CBC mode + PKCS#7
+# ----------------------------
+
+def rc5_cbc_encrypt(key: bytes, iv: bytes, plaintext: bytes) -> bytes:
+    if len(iv) != BLOCK_SIZE:
+        raise ValueError("IV must be 8 bytes")
+
+    S = expand_key(key)
+
+    blocks = []
+    prev = iv
+
+    for i in range(0, len(plaintext), BLOCK_SIZE):
+        block = plaintext[i:i + BLOCK_SIZE]
+        x = bytes(a ^ b for a, b in zip(block, prev))
+        enc = rc5_encrypt_block(x, S)
+        blocks.append(enc)
+        prev = enc
+
+    return b"".join(blocks)
+
+
+def rc5_cbc_decrypt(key: bytes, iv: bytes, ciphertext: bytes) -> bytes:
+    if len(iv) != BLOCK_SIZE:
+        raise ValueError("IV must be 8 bytes")
+    if len(ciphertext) % BLOCK_SIZE != 0:
+        raise ValueError("Ciphertext not aligned to block size")
+
+    S = expand_key(key)
+    blocks = []
+    prev = iv
+
+    for i in range(0, len(ciphertext), BLOCK_SIZE):
+        block = ciphertext[i:i + BLOCK_SIZE]
+        dec = rc5_decrypt_block(block, S)
+        x = bytes(a ^ b for a, b in zip(dec, prev))
+        blocks.append(x)
+        prev = block
+
+    return b"".join(blocks)
+
+KEY = bytes.fromhex("99 90 9c 9d d5 8c 81 82 91 98 94 95 dd 84 89 9a")
+ITERATIONS = 85454
+key_bytes = hashlib.pbkdf2_hmac("sha256", KEY, b"", ITERATIONS, dklen=16)
+    
+print(f"[+] Key: {key_bytes.hex()}")
+iv  = bytes.fromhex("bb7cdd2e104c3410") 
+with open("./output/85454_bb7cdd2e104c3410.enc", "rb") as f:
+    ct = f.read()
+
+dec = rc5_cbc_decrypt(key_bytes, iv, ct)
+print("decrypted:", dec)
+