paragonie/random_compat · error · Exception

Error reading from source device

Error message

Error reading from source device

What it means

Sentinel failure of the random_compat polyfill's random_bytes() on the /dev/urandom backend: every read attempt from the entropy source failed or returned a buffer whose length did not match the requested $bytes count. It means the OS entropy device was unreadable or returned short/failed reads (e.g. /dev/urandom missing or restricted in a chroot/container), so no cryptographically secure entropy could be produced.

Solutions

1. Verify /dev/urandom exists and is readable by the PHP process (check open_basedir, chroot, and container device mounts)
2. Ensure the PHP build supports reading the device (ext/openssl or ext/mcrypt alternatives are tried before this throw; enabling them avoids the urandom path)
3. Upgrade PHP to 7.0+ where native random_bytes() exists and bypasses the polyfill entirely
4. Log the environment details and retry the operation; a transient read failure may succeed on a subsequent call

When it happens

Trigger: Thrown at lib/random_bytes_dev_urandom.php:186 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of paragonie/random_compat@b5d188cc9d (2026-09-13). Data as JSON: /api/errors/96b84598f95938ac. Report an issue: GitHub.

Appendix: source

Thrown at lib/random_bytes_dev_urandom.php:186

            /**
             * Is our result valid?
             * @var string|bool $buf
             */
            if (is_string($buf)) {
                if (RandomCompat_strlen($buf) === $bytes) {
                    /**
                     * Return our random entropy buffer here:
                     */
                    return $buf;
                }
            }
        }

        /**
         * If we reach here, PHP has failed us.
         */
        throw new Exception(
            'Error reading from source device'
        );
    }
}

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