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New Intermediate Certificates

On Wednesday, March 13, 2024, Let’s Encrypt generated 10 new Intermediate CA Key Pairs, and issued 15 new Intermediate CA Certificates containing the new public keys. These new intermediate certificates provide smaller and more efficient certificate chains to Let’s Encrypt Subscribers, enhancing the overall online experience in terms of speed, security, and accessibility. First, a bit of history. In September, 2020, Let’s Encrypt issued a new root and collection of intermediate certificates. Those certificates helped us improve the privacy and efficiency of Web security by making ECDSA end-entity certificates widely available. However, those intermediates are approaching their expiration dates, so it is time to replace them. Our new batch of intermediates are very similar to the ones we issued in 2020, with a few small changes. We’re going to go over what those changes are and why we made them. The New Certificates We created 5 new 2048-bit RSA intermediate certificates named in sequence from R10 through R14. These are issued by ISRG Root X1. You can think of them as direct replacements for our existing R3 and R4 intermediates. We also created 5 new P-384 ECDSA intermediate certificates named in sequence from E5 through E9. Each of these is represented by two certificates: one issued by ISRG Root X2 (exactly like our existing E1 and E2), and one issued (or cross-signed) by ISRG Root X1. You can see details of all of the certificates on our updated hierarchy page. Rotating Issuance Rotating the set of intermediates we issue from helps keep the Internet agile and more secure. It encourages automation and efficiency, and discourages outdated practices like key pinning. “Key Pinning” is a practice in which clients — either ACME clients getting certificates for their site, or apps connecting to their own backend servers — decide to trust only a single issuing intermediate certificate rather than delegating trust to the system trust store. Updating pinned keys is a manual process, which leads to an increased risk of errors and potential business continuity failures. Intermediates usually change only every five years, so this joint is exercised infrequently and client software keeps making the same mistakes. Shortening the lifetime from five years to three years means we will be conducting another ceremony in just two years, ahead of the expiration date on these recently created certificates. This ensures we exercise the joint more frequently than in the past. We also issued more intermediates this time around. Historically, we’ve had two of each key type (RSA and ECDSA): one for active issuance, and one held as a backup for emergencies. Moving forward we will have five: two conducting active issuance, two waiting in the wings to be introduced in about one year, and one for emergency backup. Randomizing the selected issuer for a given key type means it will be impossible to predict which intermediate a certificate will be issued from. We are very hopeful that these steps will prevent intermediate key pinning altogether, and help the WebPKI remain agile moving forward. These shorter intermediate lifetimes and randomized intermediate issuance shouldn’t impact the online experience of the general Internet user. Subscribers may be impacted if they are pinning one of our intermediates, though this should be incredibly rare. Providing Smaller Chains When we issued ISRG Root X2 in 2020, we decided to cross-sign it from ISRG Root X1 so that it would be trusted even by systems that didn’t yet have ISRG Root X2 in their trust store. This meant that Subscribers who wanted issuance from our ECDSA intermediates would have a choice: they could either have a very short, ECDSA-only, but low-compatibility chain terminating at ISRG Root X2, or they could have a longer, high-compatibility chain terminating at ISRG Root X1. At the time, this tradeoff (TLS handshake size vs compatibility)

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