baby modulus — Small RSA

Platform: CUSTOM | Category: Crypto | Type: Challenge | Difficulty: Easy | OS: N/A | **Author: XESXOR Techniques: rsa_small_modulus_factorization

Summary

Walkthrough of the CUSTOM Crypto baby modulus — Small RSA. Task: RSA with small 128-bit modulus. Solution: Factor n via factordb.com and compute private key d to decrypt.

Description

RSA with small 128-bit modulus.

• n = 127515183131859395739078505442605447613
• e = 31337
• c = 33403920388615840326926465556447601348

Analysis

The modulus n is only 128 bits — easily factorizable via factordb.com or yafu.

Solution

 python read-only • copy allowed Copy
#!/usr/bin/env python3
from Crypto.Util.number import long_to_bytes

n = 127515183131859395739078505442605447613
e = 31337
c = 33403920388615840326926465556447601348

# Factorization via factordb.com:
p = 11192001222936284809
q = 11393421122090001557
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assert p * q == n

# Calculate phi(n) and d
phi = (p - 1) * (q - 1)
d = pow(e, -1, phi)

# Decrypt
m = pow(c, d, n)
flag = long_to_bytes(m)
print(flag.decode())
# Output: spbctf{REDACTED}

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Key Takeaways / Lessons

• Mastering rsa_small_modulus_factorization was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

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Crafted by XESXOR Research • 2026-03-08 • CUSTOM • baby modulus — Small RSA • Difficulty: Easy