CryptoServer

CryptoServer provides a technique for delegating cryptographic hmacing to a server. The author feels this has advantages for many applications.

It is specifically designed for when a secret is non-rotatable after creation. We solve this problem by using a very large array of secrets. Even if millions of requests are processed, the probability of using the same secret twice is close to 0.

• Usage
• Building
• Implementation Notes
• Environmet Variables
• Tips and Tricks

Usage

To demonstrate this we will start with standard python code and refactor it to use the cryptoserver.

import hmac
from hashlib import sha256


# A secret key, hardcoded or stored in AWS secrets
# or read from the file system etc.
SECRET_KEY = bytes.fromhex(
    "13dd9f2fed7d8c61a1782d450ee3505e"
)


def compute(msg):
	"""
	compute will compute the hmac of the given msg
	msg should be of type bytes.
	"""
	hasher = hmac.new(key=SECRET_KEY, digestmod=sha256)
	hasher.update(msg)
	return hasher.digest()


if __name__ == '__main__':
	# Compute some digests
	print(compute(b"hello world"))
	print(compute(b"apples and pears"))

Now if we refactor this code to use the cryptoserver. We make use of the third party requests library.

from requests import post


CRYPTO_SERVER_URL = "http://localhost:8080/hmac"


def compute(msg):
	res = post(CRYPTO_SERVER_URL, data=msg)
	if res.status_code != 200:
		raise RuntimeError("couldn't hmac data")

	return res.content


if __name__ == '__main__':
	# Compute some digests
	print(compute(b"hello world"))
	print(compute(b"apples and pears"))

We see our hmac call and the required secrets are delegated to the cryptoserver.

Building

This requires the Rust programming language to be installed. Following this.

   $ cargo build --release
   $ cp target/release/crytposerver ${HOME}/.local/bin

NOTE: Any directory in $PATH may be used, below the author assumes cryptoserver is installed into one of the directories.

Implementation

The default behaviour of cryptoserver is to read a single secret from the filesystem. This is expected to be a single file of length 32 at the position /secrets/secret.

Creating the default mode

To run on the default with a single secret follow these steps:

   $ mkdir /secrets
   $ dd if=/dev/urandom of=/secrets/secret bs=32 count=1
   $ cryptoserver

When running this way, cryptoserver simply hmac's the data it is sent as one would expect. There is no special behaviour.

Mode16 and Mode32

The biggest feature of cryptoserver is the ability to use a large number of secrets. These are set with the environment variable CRYPTOSERVER_MODE.

To run in Mode16 we need to generate (2 ^ 16 = 65536) secrets.

   $ mkdir /secrets
   $ dd if=/dev/urandom of=/secrets/0000 bs=32 count=65536
   $ CRYPTOSERVER_MODE=MODE16 cryptoserver

When running in Mode16, request data is hashed down to a 16 bit unsigned integer. This is how cryptoserver decides which secret to use for the hmacing.

Mode32

Mode32 extends Mode16 to use (2 ^ 32 = 4294967296) secrets. We make use of python to permutate over all possible 4 char hexstrings.

NOTE: If running on AWS etc. this requires approx 160GB of HDD space.

   $ mkdir /secrets
   $ python
   >>> HEXCHARS = "0123456789abcdef"
   >>> from os import urandom
   >>> from itertools import product
   >>> def hexchars():
   ...     for a in HEXCHARS:
   ...         for b in HEXCHARS:
   ...             yield a + b
   ...
   >>> for name in product(hexchars(), hexchars()):
   ...     name = "".join(name)
   ...     with open (f"/secrets/{name}", 'wb') as file:
   ...         file.write(urandom(32 * 65536))
   ...
   >>>
   $ CRYPTOSERVER_MODE=Mode32 cryptoserver

2 ^ 32 is a huge number of secrets and will almost certainly mean you never need to rotate the secrets unless the HDD which is storing them is somehow compromised.

Environment Variables

CRYPTOSERVER_MODE

This can be one of three values: MODE0, MODE16, MODE32. The default is Mode0 (see Implementation section above).

CRYPTOSERVER_SECRETDIR

The directory to search for the secret(s). The application will default to /secrets.

RUST_LOG

Set the logging level, NOTE: by default there will be no logging output.

The lowest level of logging of this application is DEBUG.

   $ RUST_LOG=INFO cryptoserver
   2021-01-03T11:18:58Z INFO  cryptoserver cryptoserver started

CRYPTOSERVER_BIND

The bind path given to the server. The default is "0.0.0.0:8080".

Tips and Tricks

If one sets up an EC2 instance with a HDD. After generating the secrets, one can "lose" the SSH key - this keeping the secrets secret forever!