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Why

Per Mozilla’s OpenSSH guidelines for OpenSSH 6.7+, “all Diffie-Hellman moduli in use should be at least 3072-bit-long”. The Diffie-Hellman algorithm is used by SSH to establish a secure connection. The larger the moduli (key size) the stronger the encryption.

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How It Works

SSH uses Diffie-Hellman key exchange to establish secure connections. The strength of this key exchange depends on the size of the moduli used. Smaller moduli (less than 3072 bits) are considered weak and potentially vulnerable to attacks. The /etc/ssh/moduli file contains precomputed Diffie-Hellman groups that SSH can use during key exchange. By removing the shorter, weaker keys from this file, we ensure SSH only uses strong cryptographic parameters.

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Goals

  • Remove all Diffie-Hellman keys that are less than 3072 bits long

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Steps

1

Backup the moduli file

Make a backup of SSH’s moduli file /etc/ssh/moduli:
sudo cp --archive /etc/ssh/moduli /etc/ssh/moduli-COPY-$(date +"%Y%m%d%H%M%S")
2

Remove short moduli

Remove moduli shorter than 3072 bits:
sudo awk '$5 >= 3071' /etc/ssh/moduli | sudo tee /etc/ssh/moduli.tmp
sudo mv /etc/ssh/moduli.tmp /etc/ssh/moduli
This command:
  • Uses awk to filter lines where the 5th field (the moduli size) is >= 3071
  • Writes the filtered content to a temporary file
  • Replaces the original file with the filtered version
3

Verify the changes

Check that only strong moduli remain:
awk '$5 < 3071' /etc/ssh/moduli
This command should return no output, indicating all weak moduli have been removed.
4

Restart SSH (optional)

While SSH will pick up the new moduli file automatically for new connections, you can restart SSH to ensure it’s using the updated file:
sudo service sshd restart

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Understanding the Moduli File

The /etc/ssh/moduli file contains Diffie-Hellman groups in this format: 
Time Type Tests Tries Size Generator Modulus
  • Time: Timestamp when the moduli was generated
  • Type: Type of test performed
  • Tests: Number of tests performed
  • Tries: Number of attempts
  • Size: The size of the moduli in bits (this is what we’re filtering on)
  • Generator: The generator value
  • Modulus: The actual modulus value
The 5th field (Size) is what determines the strength of the key exchange. By filtering to keep only entries where this value is >= 3071, we ensure SSH only uses strong cryptographic parameters.

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Security Impact

Removing short Diffie-Hellman keys:
  • Prevents the use of weak cryptographic parameters
  • Protects against attacks on smaller key sizes
  • Ensures compliance with modern security standards
  • May slightly reduce compatibility with very old SSH clients (though this is unlikely to be an issue)
This is a one-time operation. However, if SSH is updated and a new moduli file is installed, you may need to repeat this process.

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References

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