Chapter 4. Hardening Your System with Tools and Services
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/chap-hardening_your_system_with_tools_and_services
In information theory, entropy is the level of uncertainty associated with a random variable and is presented in bits. The higher the entropy value, the more secure the password is. According to NIST SP 800-63-1, passwords that are not present in a dictionary comprised of 50000 commonly selected passwords should have at least 10 bits of entropy.
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