Presidential Policy Directive 21 (PPD-21) Presidential Policy Directive 21 Presidential policy directive from 2013 Presidential Policy Directive 21 Presidential Policy Directive 21 Presidential Policy Directive 21, PPD 21 - Critical Infrastructure Security and Resilience Presidential Policy Directive 21 PDD-21 heightened cybersecurity protection OWH archive Presidential Policy Directive 21, Presidential Policy Directive 21 PPD-21 PPD-21 Presidential Policy Directive 21 (PPD-21). presidential directive Presidential Policy Directive 21, Presidential Policy Directive 21 heightened cybersecurity protection Presidential Policy Directive 21 United States Presidential Policy Directive 21 OWH archive Presidential Policy Directive 21 Presidential Policy Directive 21 Presidential Policy Directive 21 PDD-21 Presidential Policy Directive 21 16 disparate U.S. critical infrastructure sectors Presidential Policy Directive-21 Presidential Policy Directive 21 directive Presidential Policy Directive 21 Presidential Policy Directive 21 Presidential Policy Directive 21 critical infrastructure heightened cybersecurity protection PDD-21 Presidential Policy Directive 21 nations critical infrastructure and economy Presidential Policy Directive 21 (2013)