AngstromCTF_2018: Intro To RSA

Category: Crypto Points: 50 Description:

One common method of public key encryption is the RSA algorithm. Given p, q, e, and c, see if you can recover the message and find the flag!

Write-up

Using Python, we can decrypt c with a script!

# Calculate n
n = p * q

# Calculate decryption keys
phi = (p-1) * (q-1)
d = gmpy2.invert(e, phi)

# Decrypt flag
m = int(pow(c, d, n))
print(binascii.unhexlify(hex(m)[2:]).decode())

With the script, we can now get our flag.

# ./solve.py 
actf{rsa_is_reallllly_fun!!!!!!}

Therefore, the flag is actf{rsa_is_reallllly_fun!!!!!!}.

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