Let's Encrypt was the first certificate authority to offer free, fully automated certificates via the ACME protocol, and remains the most widely used. It's no longer the only option: ZeroSSL and Buypass both now issue free ACME-automated certificates too, using the identical underlying protocol and the same class of ACME clients (Certbot, acme.sh, and others all support switching between them).
| Let's Encrypt | ZeroSSL | Buypass | |
| Default validity | 90 days | 90 days | 180 days |
| Dashboard/API on free tier | No | Yes | Limited |
| Protocol | ACME | ACME | ACME |
What's in reality identical across all three
The encryption itself doesn't differ — a certificate from any of the three uses the same TLS protocol, the same cipher suites, and provides the same level of encryption strength as a certificate from any paid CA. All three issue domain-validated (DV) certificates only, at no cost, with validity periods in the 90-day range, and all three are trusted by every major browser and operating system by default.
Let's Encrypt
Run by the nonprofit Internet Security Research Group, Let's Encrypt issues over a billion active certificates and has the longest operating track record of the three. Its rate limits are generous for typical use but exactly structured around per-domain and per-account limits that occasionally surprise anyone issuing certificates at actually large scale — worth checking their published rate limit documentation if you're provisioning certificates for hundreds of domains programmatically.
ZeroSSL
ZeroSSL offers ACME-based free certificates alongside a paid tier with a web-based dashboard, multi-user account management, and REST API access not available on Let's Encrypt's free tier. For someone who wants a visual interface for managing certificates rather than working entirely through command-line ACME clients, this is a meaningful practical difference even though the underlying free certificate itself is functionally equivalent.
Buypass
Buypass, a Norwegian CA, offers free ACME certificates through its "Buypass Go SSL" product with a notably longer default validity period — 180 days rather than the 90 days Let's Encrypt and ZeroSSL both default to. For anyone in particular trying to minimize how often automated renewal runs (though with working automation this matters little either way), this is a genuine, if minor, practical difference.
Why the choice mostly doesn't matter if you're already automated
Since all three speak the same ACME protocol, switching between them is typically a one-line change in your ACME client's configuration — the CA URL. If your renewal automation is already working reliably, which CA is actually issuing the certificate becomes close to invisible day to day. The more relevant differences are account-level: dashboard availability, API access, rate limit specifics, and each CA's own operational history and transparency reporting.
A practical reason to know more than one
Having a working ACME client configuration for a second CA ready, even if you don't use it day to day, gives you a fallback if your primary CA ever has an extended outage — which has happened, briefly, to each of the major free CAs at one point or another. Since switching is close to a one-line configuration change given the shared protocol, this costs little to keep in your back pocket.