Creating a custom RSA Key

Question

I'm currently using the Crypto module to create my private and public key. The standard way is to use RSA.generate(KEY_LENGTH, random_gen). But this uses values created by a random number generator. I would like the key to be customized to a certain extent.

KEY_LENGTH = 1024
random_gen = Random.new().read
privateKey = RSA.generate(KEY_LENGTH, random_gen)
public_Key = keys.publickey()

Problem now is when I try to print random_gen, I get the following output

bound method RNGFile.read of Crypto.Random._UserFriendlyRNG.RNGFile object at 0x75713c50>>

And if i try to put a fixed value in the random_gen spot, I get a TypeError: randfunc must be callable.

Basically, I don't want a randomized key

KelvTheKid
– KelvTheKid

2017-06-23 08:26:45 +00:00
Commented Jun 23, 2017 at 8:26 — KelvTheKid
– KelvTheKid, Commented Jun 23, 2017 at 8:26

r3mainer · Accepted Answer · 2017-06-23 09:46:40Z

If you take a look at the RSA module documentation, you'll see that it also supports a construct() function that you can use to create RSA keys by supplying your own parameters.

Here's an example:

from Crypto.PublicKey import RSA

n = long('2c1c62f96c8b4a177de41d3df08148eb58bc852edd0fc343faa1de9ada5dccb8e4db708f' \
         'cd71451b4135c7328f1641e79049ab9e88de3d5be28817b898f7ae431a52909fb7c1902e' \
         '694ecda41ff821035a0a45f20871c5a8dbae366189bdd84aac3f0e88541711fe670b4ea2' \
         '2125762dbd39788ebe4ef7fc2d4da5d468f6353c1ab88d2ee5b658bdf195dfd0c7d3fc1f' \
         'bf2511de97d10399259d41d476d38ae18a094ece4ef718d8aef72a33df31737be404cf02' \
         '425db3ee736b279f2be5557b3b4593f02ff8b7853709e791064d0d793063b1ca09747a35' \
         'b2ff98f7c19275224657f879d07b22671a9b37964f45324c9dc6f9de8e65346724b7376f',16)
e = 0x10001L
d = long('b369c2273e964c85c04653a9dacfff073fa5890f5395096335c7ace3abca5924afb4dbe0' \
         '0a7cebd82f23cd52972ab0646b3b3a0f582296ef6434a646ede67ba3f32c5f914296c919' \
         'fcb9c93205d5f670eb895d7553636dd1a1b4b4fb8483e779d19da83e57c87e2782773b1d' \
         'b633e8b9a12c606e37207b05f83e1a801967f10e0c0e1ca7a38feed9d0ab8e1bf3c6fecd' \
         '87cb799afa8140d5437226dd94a9ab835cfeffbfa8c40eb7e13829f995986eeb457072ad' \
         '9dd9539c955bf267dcf4bfd8a9a3423b0f4fb86d1a457490c7df177df339fdfc79c19c06' \
         'fdece56d112660115b485a51737452bb53c4925de7dd111ba72c116aa46e6222ee90661',16)
p = long('6fd004c04efd1b023a50eb9abaf9d701bd4c8294a832bde96426c3de156e6ce861e3abe7' \
         'c712329f86cf0f190f4618a12220f75dcedac64c72c21f76359a4171b7b1ea7012befc8a' \
         'e5e508e8c299a44bf951616109f461fc5372bd3e8e485924837e891180dc646f5028931c' \
         '3f98676ae22d219bf22114acf89d24a7570b',16)
q = long('64fe6f8eda823fadc5335ebb1178973bdabdb7b71299bb05beb5ed631eccc017b8c68a99' \
         '2c958893e019fd4f20b85d0301a1cde4b7f747d0da89780b3027ff5275935f19031965b2' \
         '582cbb5397b309a2357100ae769c50ce5d84abe2a100ac54441251908ac4609bb4342a8f' \
         'c5fced8329b6f0f181aa003dd9ebba6f4fad',16)
u = long('30e5313ae3ce770a951f9c3cc659cb0e79dc2f93b6464ebd5dae0207b38b96e099e383cc' \
         'c48af796ba8587802f8e7bafda432b5e8194841f4535b07f665af110dff573d98a2496c1' \
         '5610b8e78521101ca381ac6cde11c23e60559931db2da08de94205002b0ad1d021e6d9ce' \
         'bd5e3131c76bf57046a289c52ba1ae336bf8',16)

key_params = (n, e, d, p, q, u)
key = RSA.construct(key_params)
print key.exportKey()

Some of the parameters are optional; to create a public key, you only need to specify n and e.

Shmulik Asafi · Accepted Answer · 2017-06-23 08:33:24Z

0

Random.new().read is a function that generates random numbers.

It is used by the RSA.generate method to randomly create a new private key.

If you really want to "see" the random numbers for some reason (perhaps explain what you're trying to acheive with this) you can wrap it with a function that prints the returned values first:

def create_wrapper_rand(random_gen):
   def wrapper_rand(n):
      values = random_gen(n)
      print values
      return values
   return wrapper_rand

wrapper_rand_gen = create_wrapper_rand(random_gen)

Now pass wrapper_rand_gen to RSA.generate instead of random_gen

answered Jun 23, 2017 at 8:33

Shmulik Asafi

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3 Comments

Shmulik Asafi Over a year ago

Instead if really don't want random values, you can just create a method that returns a fixed string of size n

KelvTheKid Over a year ago

I'm trying to create a private key based on custom values such as a passphrase etc. In a way removing the random aspect.

phd Over a year ago

@KelvTheKid that would be a very weak crypto. You need a KDF (key derivation function) to create a secret key; generate a random RSA keypair and use the secret key to encrypt private key. When you need the private key back — ask user for passphrase, use the KDF to derive secret key and decrypt private key. And read a good book on cryptography.

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