Originally published at authgear.com . tl;dr — An SSL certificate chain is the sequence of digital certificates that links your website's certificate to a root Certificate Authority (CA) browsers trust: your leaf certificate → one or more intermediate certificates → a trusted root certificate . Your server must send the leaf and intermediates; the root is pre-installed in the client. If any intermediate is missing, mobile browsers and API clients fail — even though desktop Chrome may still work due to caching. What Is an SSL Certificate Chain? An SSL certificate chain — also called a certificate trust chain or chain of trust — is a sequence of certificates that connects your website's certificate back to a root Certificate Authority (CA) that browsers inherently trust. Think of it like a chain of vouchers: your site's certificate is vouched for by an intermediate CA, which is in turn vouched for by a root CA. Browsers come pre-installed with a list of trusted root CAs.…