Verifying the integrity of downloaded ISO images is crucial for a secure Linux installation. This process ensures the downloaded file is an exact copy of the original, free from corruption or malicious alterations. This guide details how to verify ISO images in Linux, focusing on checksum verification and digital signature checks.

Table of Contents

• Why Verify Linux ISO Images?
• Verifying ISO Image Integrity
• Streamlined Checksum Comparison with sha256sum
• Authenticating the sha256sum.txt File
• Verifying ISOs with Linux Mint's Built-in Tools
• Conclusion

Why Verify Linux ISO Images?

Verifying an ISO image serves two key purposes:

• Integrity Check: This confirms the downloaded file is a perfect replica of the original, preventing installation problems caused by download errors.
• Authenticity Check: This verifies the image's signature from the official distributor (like Linux Mint), guarding against modified or malicious copies.

Verifying ISO Image Integrity

1. Download the ISO and Checksum: Download the ISO from the official website, along with its checksum file (e.g., .sha256, .md5).

2. Calculate the Checksum: Use sha256sum (or md5sum) to compute the hash of your downloaded ISO. For example:

   sha256sum openSUSE-Leap-15.6-DVD-x86_64-Media.iso

   

3. Compare Checksums: Compare the calculated hash with the one in the checksum file. A match confirms the ISO's integrity.

   

Streamlined Checksum Comparison with sha256sum

Manually comparing checksums is prone to errors. The -c option in sha256sum automates this:

1. Download Files: Obtain the ISO and checksum file.

2. Verify with -c:

   sha256sum -c openSUSE-Leap-15.6-DVD-x86_64-Media.iso.sha256

   

   An "OK" output indicates a successful verification. A "FAILED" message means the ISO is corrupted or tampered with. This method also works with sha256sum.txt files provided by some distributions.

Authenticating the sha256sum.txt File

A compromised sha256sum.txt file could lead to accepting a malicious ISO. To verify its authenticity:

1. Import the Signing Key: Import the distribution's GPG key (e.g., for Linux Mint: gpg --keyserver hkp://keys.openpgp.org:80 --recv-key 27DEB15644C6B3CF3BD7D291300F846BA25BAE09).

2. Verify the Signature: Use gpg to verify the signature of sha256sum.txt.gpg:

   gpg --verify sha256sum.txt.gpg sha256sum.txt

   A successful verification confirms the checksum file's authenticity.

Verifying ISOs with Linux Mint's Built-in Tools

Linux Mint offers a convenient graphical tool and a command-line utility:

• Graphical Tool: Right-click the ISO and select "Verify".
• Command-line: Use mint-iso-verify linux-mint.iso.

Conclusion

Always verify ISO images before installation. Checksum and signature verification are essential steps for ensuring a secure and reliable Linux system. The methods outlined above provide a comprehensive approach to this critical security practice.

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