Un peu fatiguant de ne pas pouvoir configurer ce genre de chose sur l'ELB directement...
RewriteEngine On
RewriteCond %{HTTP:X-Forwarded-Proto} ^http$
RewriteRule . https://%{HTTP:Host}%{REQUEST_URI} [L,R=permanent]
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https://aws.amazon.com/premiumsupport/knowledge-center/redirect-http-https-elb/