Crypto Fails

An instance of textbook RSA has three parameters: e, d, and n, where e = d^-1 (mod phi(n)). Encryption of a message m is m^e, and decryption of a ciphertext c is c^d = (m^e)^d = m^ed = m¹ = m.

When you set e = 1, encrypting a message does not change it, since raising any number to the power of 1 does not change it. SaltStack was using 1 as their RSA public key (e), so encryption was doing nothing:

 @@ -47,7 +47,7 @@ def gen_keys(keydir, keyname, keysize, user=None):  
    priv = '{0}.pem'.format(base)  
    pub = '{0}.pub'.format(base)  
 -  gen = RSA.gen_key(keysize, 1, callback=lambda x, y, z: None)  
 +  gen = RSA.gen_key(keysize, 65537, callback=lambda x, y, z: None)  
    cumask = os.umask(191)  
    gen.save_key(priv, None)  
    os.umask(cumask)   

• Have a professional cryptographer review your parameter choices.
• Write unit tests to check that messages can’t be decrypted with a different key (edit: This might not catch the problem, see the comments).
• Before using a cryptography library, it’s a necessary to understand something about what it’s doing behind the scenes.