How to fix Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?) in APT

The simplest first step is to remove all cached repository metadata and redownload it fresh:

sudo rm -rf /var/lib/apt/lists/*
sudo apt update

This removes all locally cached copies of repository file lists. When you run apt update, APT will redownload the metadata from scratch, which often resolves signature validation issues caused by corrupted cache files.

Verify that your network connection is stable and check if you're behind a proxy that might be modifying traffic:

# Check basic connectivity
ping -c 1 archive.ubuntu.com

# Check current proxy settings (if any)
echo $http_proxy
echo $https_proxy

If you see proxy settings configured, verify they are correct. If behind a corporate proxy that requires authentication, you may need to configure APT to use authenticated proxy settings. Edit or create /etc/apt/apt.conf.d/proxy.conf:

Acquire::http::Proxy "http://username:[email protected]:8080/";
Acquire::https::Proxy "https://username:[email protected]:8080/";

Replace username, password, proxy address, and port with your actual values.

The issue might be specific to your current mirror. Try switching to an alternative mirror:

# Backup your current sources
sudo cp /etc/apt/sources.list /etc/apt/sources.list.backup

# Edit sources.list to use a different mirror
sudo nano /etc/apt/sources.list

Change your current mirror URL (like archive.ubuntu.com or deb.debian.org) to a different one:
- Ubuntu: Try mirror.example.com or regional mirrors
- Debian: Try httpredir.debian.org or regional mirrors

After changing mirrors, clear cache and update:

sudo rm -rf /var/lib/apt/lists/*
sudo apt update

GPG signature validation depends on accurate time. If your system clock is significantly off, signature verification can fail:

# Check current time
date

# Sync time with NTP servers
sudo ntpdate -s time.nist.gov
# Or use timedatectl on modern systems:
sudo timedatectl set-ntp true

An incorrect system clock can cause GPG signature verification to fail because signatures have time-validity windows. Ensure your time is synchronized with NTP servers.

Occasionally, the repository GPG keys themselves need updating. Update the keyring package:

# For Ubuntu/Debian
sudo apt install --reinstall ubuntu-keyring
# or
sudo apt install --reinstall debian-archive-keyring

# Update the local GPG keyring
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys YOUR_KEY_ID

However, note that apt-key is deprecated in newer Debian/Ubuntu versions. For modern systems, verify keys are properly configured in /etc/apt/trusted.gpg.d/ or use the signed-by option in your sources.list entries.

The mirror itself might be temporarily corrupt or undergoing maintenance. Test connectivity and mirror health:

# Test connectivity to the mirror
curl -v https://archive.ubuntu.com/ubuntu/dists/jammy/InRelease 2>&1 | head -20

# Check if you're being redirected through a proxy
curl -I https://archive.ubuntu.com/ubuntu/

If the mirror appears to be down or returning errors, wait a few minutes and retry. You can also switch to a different mirror temporarily while the main mirror recovers.

In some environments with problematic proxies, the HTTPS connection itself is being intercepted. For testing purposes, try switching to HTTP to isolate the issue:

# Edit sources.list
sudo nano /etc/apt/sources.list

# Change https:// to http:// for one line to test
# Example:
# deb http://archive.ubuntu.com/ubuntu jammy main

sudo apt update

Note: HTTP is less secure. If this resolves the issue, it confirms your problem is HTTPS-related, likely from a proxy. Switch to HTTP only as a permanent solution if absolutely necessary, or configure your proxy correctly. Then switch back to HTTPS once the proxy is properly configured.

Sometimes the error originates from a third-party PPA rather than official repositories. Identify and disable problematic sources:

# List all repository sources
cat /etc/apt/sources.list
ls -la /etc/apt/sources.list.d/

# Temporarily disable third-party PPAs to test
sudo mv /etc/apt/sources.list.d/ppa-name.list /etc/apt/sources.list.d/ppa-name.list.bak

# Clear cache and update
sudo rm -rf /var/lib/apt/lists/*
sudo apt update

If update works after disabling a PPA, that PPA is the source of the issue. You can either remove it permanently or fix its configuration.

As a last resort, restore a clean APT configuration and perform a full cleanup:

# Restore sources.list from backup if available
sudo cp /etc/apt/sources.list.backup /etc/apt/sources.list

# Remove all cached data
sudo rm -rf /var/lib/apt/lists/*
sudo apt clean
sudo apt autoclean

# Clear the package cache
sudo rm -f /var/lib/apt/lists/partial/*

# Rebuild the cache from scratch
sudo apt update

This removes all potentially corrupted files and rebuilds the package database from scratch. This approach resolves nearly all clearsigned validation issues.