Use `dev/random` Instead Of `/dev/urandom` Due To Changes In Linux

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Introduction

In recent years, Linux has undergone significant changes in its random number generation (RNG) mechanism. The in-kernel random number generator in /dev/random is now considered cryptographically secure and no longer blocks, unlike its predecessor. This shift has significant implications for applications that rely on random number generation, particularly those that require strong randomness. In this article, we will explore the changes in Linux's RNG mechanism and discuss the implications for applications that use /dev/urandom instead of /dev/random.

Background

In the past, /dev/urandom was used as a fallback when /dev/random was not available or was blocking. This was because /dev/urandom was considered pseudo-random and was not suitable for cryptographic purposes. However, with the recent changes in Linux, /dev/random is now considered cryptographically secure and no longer blocks. This means that applications can now rely on /dev/random for strong randomness without worrying about blocking.

The Problem with /dev/urandom

/dev/urandom is a pseudo-random number generator that uses the system's entropy pool to generate random numbers. However, it does not guarantee strong randomness and can be vulnerable to attacks. In contrast, /dev/random uses the system's entropy pool to generate random numbers, but it also blocks when the entropy pool is low, ensuring that the generated numbers are strong and unpredictable.

The Solution: Use dev/random

Given the changes in Linux's RNG mechanism, it is now recommended to use /dev/random instead of /dev/urandom for strong randomness. This is because /dev/random is now considered cryptographically secure and no longer blocks. By using /dev/random, applications can ensure that they are generating strong and unpredictable random numbers, which is essential for cryptographic purposes.

Implementation

To use /dev/random instead of /dev/urandom, you can set the DISABLE_DEV_RANDOM_SEED environment variable to false. This will force the application to use /dev/random for seeding, even if it is not available. You can also override the default behavior by setting the RANDOM_SEED environment variable to /dev/random.

Example Code

Here is an example code snippet that demonstrates how to use /dev/random instead of /dev/urandom:

import os

# Set the DISABLE_DEV_RANDOM_SEED environment variable to false
os.environ['DISABLE_DEV_RANDOM_SEED'] = 'false'

# Set the RANDOM_SEED environment variable to /dev/random
os.environ['RANDOM_SEED'] = '/dev/random'

# Use the RANDOM_SEED environment variable to seed the random number generator
seed = os.environ['RANDOM_SEED']

Conclusion

In conclusion, the recent changes in Linux's RNG mechanism have significant implications for applications that rely on random number generation. With the in-kernel random number generator in /dev/random now considered cryptographically secure and no longer blocking, it is recommended to use /dev/random instead of /dev/urandom for strong randomness. By using /dev/random, applications can ensure that they are generating strong and unpredictable random numbers, which is essential for cryptographic purposes.

Recommendations

Based on the changes in Linux's RNG mechanism, we recommend the following:

  • Use /dev/random instead of /dev/urandom for strong randomness.
  • Set the DISABLE_DEV_RANDOM_SEED environment variable to false to force the application to use /dev/random for seeding.
  • Override the default behavior by setting the RANDOM_SEED environment variable to /dev/random.

References

Environment Info

  • NooBaa Version: 5.19
  • Platform: Kubernetes 1.28.7

Actual Behavior

We implemented the seeding using dev/random (reference: #2721, #4555 and openssl docs).

Back then on VMs and Containers we started encountering issues were the host did not have enough entropy and we could not generate more entropy using input from any of the available devices, so we decided to use /dev/urandom to avoid getting stuck starting our containers (reference #5745 and #5752).

Overall, what this means is that in the default configuration, we do not require our seed to be strongly random, and would accept a pseudo-random seed to prevent getting stuck while starting the containers. You can override the default and force it to wait for a strongly random seed.

Expected Behavior

According to the following references:

On modern Linux systems, the in-kernel random number generator in /dev/random is considered cryptographically secure and, crucially, no longer blocks.

It would mean that we can improve and simplify our existing code.

Steps to Reproduce

  1. none

More Information - Screenshots / Logs / Other Output

Q&A

Q: What is the difference between /dev/random and /dev/urandom?

A: /dev/random is a true random number generator that uses the system's entropy pool to generate random numbers. It blocks when the entropy pool is low, ensuring that the generated numbers are strong and unpredictable. On the other hand, /dev/urandom is a pseudo-random number generator that uses the system's entropy pool to generate random numbers, but it does not guarantee strong randomness and can be vulnerable to attacks.

Q: Why is /dev/random considered cryptographically secure?

A: /dev/random is considered cryptographically secure because it uses the system's entropy pool to generate random numbers, which ensures that the generated numbers are strong and unpredictable. Additionally, /dev/random blocks when the entropy pool is low, which prevents the generation of weak random numbers.

Q: What are the implications of using /dev/urandom instead of /dev/random?

A: Using /dev/urandom instead of /dev/random can lead to weak random numbers being generated, which can compromise the security of cryptographic applications. Additionally, /dev/urandom can be vulnerable to attacks, such as predicting the generated numbers.

Q: How can I ensure that my application uses /dev/random instead of /dev/urandom?

A: You can ensure that your application uses /dev/random instead of /dev/urandom by setting the DISABLE_DEV_RANDOM_SEED environment variable to false. This will force the application to use /dev/random for seeding, even if it is not available. You can also override the default behavior by setting the RANDOM_SEED environment variable to /dev/random.

Q: What are the benefits of using /dev/random instead of /dev/urandom?

A: The benefits of using /dev/random instead of /dev/urandom include:

  • Strong and unpredictable random numbers
  • Cryptographically secure
  • Prevents weak random numbers from being generated
  • Prevents attacks on the generated numbers

Q: Are there any limitations to using /dev/random?

A: Yes, there are limitations to using /dev/random. For example, /dev/random can block when the entropy pool is low, which can cause delays in the application. Additionally, /dev/random may not be available on all systems, which can cause issues.

Q: How can I troubleshoot issues related to /dev/random?

A: You can troubleshoot issues related to /dev/random by checking the system's entropy pool and ensuring that it is sufficient to generate strong random numbers. You can also check the system's logs for any errors related to /dev/random.

Q: What are the best practices for using /dev/random?

A: The best practices for using /dev/random include:

  • Setting the DISABLE_DEV_RANDOM_SEED environment variable to false
  • Setting the RANDOM_SEED environment variable to /dev/random
  • Ensuring that the system's entropy pool is sufficient to generate strong random numbers
  • Checking the system's logs for any errors related to /dev/random

Conclusion

In conclusion, using /dev/random instead of /dev/urandom is recommended for cryptographic applications that require strong and unpredictable random numbers. By following the best practices and troubleshooting tips outlined in this article, you can ensure that your application uses /dev/random and generates strong and unpredictable random numbers.