What is a CSR and what does it contain?

A PKCS#10 certificate signing request: your public key, the identity you claim (subject and SANs), and a self-signature made with your private key proving you hold it. The CA verifies it, then issues the certificate — the private key never leaves your server.

Why do the subject alternative names matter more than the CN?

Browsers validate hostnames against the SAN list exclusively; the CN is legacy decoration. A CSR without SANs yields a certificate no browser accepts for any domain — the single most common CSR mistake, and the first thing to check here.

What does the signature verification prove?

That the request is internally consistent: the embedded public key verifies the CSR's own signature, so the bytes are exactly what the private-key holder produced. A failure almost always means the paste was truncated or altered — not a cryptographic attack.

What key size and algorithm should a CSR use in 2026?

RSA 2048 is the enforced minimum and RSA 3072 or EC P-256 the comfortable choice; SHA-256 signatures are standard. Public CAs reject sub-2048 RSA and SHA-1 outright — the decoder flags both before you waste a submission.

Is it safe to paste a CSR into a browser tool?

A CSR contains only public information, so yes in principle — and this decoder parses it on your device without any upload. The private key generated alongside it is a different matter: it goes nowhere, ever.

CSR Decoder

Decode a PKCS#10 certificate signing request before it goes to the CA — subject, requested SANs, key size, algorithm and a signature check.

The CSR Decoder runs entirely in your browser. Requests are decoded on your device and never uploaded.

Open the X.509 Certificate Decoder

About CSR Decoder

CSR Decoder shows exactly what a certificate signing request asks for before you commit it to a certificate authority. Paste the PEM block and read the subject distinguished name field by field, the requested subject alternative names — the part that actually determines which hostnames the certificate will secure, since browsers ignore the CN — the public key type and size, and the signature algorithm. The decoder also verifies the CSR's own signature, which is the fastest way to catch the classic failure: a request truncated or mangled somewhere between the server that generated it and the form it was pasted into. Warnings fire on the things CAs bounce: RSA below 2048 bits and a missing SAN extension. It recognises a certificate or a private key pasted by mistake and says so, rather than producing nonsense.

Features

Full subject DN split into labelled fields
Requested subject alternative names — DNS, IP, email
Key type and size: RSA, EC with curve, Ed25519
Signature algorithm named, not just an OID
Self-signature verification catches truncated pastes
Warns on sub-2048 RSA and missing SANs before the CA does
Detects certificates and private keys pasted by mistake
Everything decoded on your device

How to use the CSR Decoder

Paste the CSR PEM (or load the sample)
Check the subject and especially the SAN list
Confirm key size and algorithm meet the CA's bar
Green signature badge means the paste survived intact

Example

Input

CSR for example.com

Output

CN=example.com · SANs: example.com, www.example.com · RSA 2048 · SHA256withRSA · signature verifies

The SAN list is what the certificate will actually secure.

Common errors & troubleshooting

The signature fails verification. — The bytes changed since generation — usually a truncated copy, an editor that rewrapped lines, or smart quotes from a document. Re-copy directly from the original file on the generating machine.
No SANs appear even though the CN names the domain. — Browsers have ignored the CN for years; a certificate issued from a SAN-less CSR secures nothing. Regenerate with a subjectAltName extension listing every hostname, www variant included.
The decoder says the paste is a certificate, not a CSR. — BEGIN CERTIFICATE and BEGIN CERTIFICATE REQUEST are different objects. Use an X.509 decoder for issued certificates; this tool reads the request that precedes issuance.
A tool elsewhere asked for the private key to check the CSR. — Decline. A CSR is self-contained and public by design — decoding it never requires the key. Any service requesting the private key for “verification” should not be trusted with it.

Frequently asked questions

What is a CSR and what does it contain?: A PKCS#10 certificate signing request: your public key, the identity you claim (subject and SANs), and a self-signature made with your private key proving you hold it. The CA verifies it, then issues the certificate — the private key never leaves your server.
Why do the subject alternative names matter more than the CN?: Browsers validate hostnames against the SAN list exclusively; the CN is legacy decoration. A CSR without SANs yields a certificate no browser accepts for any domain — the single most common CSR mistake, and the first thing to check here.
What does the signature verification prove?: That the request is internally consistent: the embedded public key verifies the CSR's own signature, so the bytes are exactly what the private-key holder produced. A failure almost always means the paste was truncated or altered — not a cryptographic attack.
What key size and algorithm should a CSR use in 2026?: RSA 2048 is the enforced minimum and RSA 3072 or EC P-256 the comfortable choice; SHA-256 signatures are standard. Public CAs reject sub-2048 RSA and SHA-1 outright — the decoder flags both before you waste a submission.
Is it safe to paste a CSR into a browser tool?: A CSR contains only public information, so yes in principle — and this decoder parses it on your device without any upload. The private key generated alongside it is a different matter: it goes nowhere, ever.

Related tools

X.509 Certificate Decoder — Decode a PEM or DER certificate to view subject, issuer, validity, SANs and fingerprints.
Certificate & Private Key Matcher — Confirm an X.509 certificate or CSR and a private key belong together (the classic modulus mismatch check) — locally in your browser.
SSH Key Generator — Generate Ed25519, RSA or ECDSA SSH keypairs in your browser and download the public and private keys.
PFX to PEM Extractor — Open a password-protected .pfx/.p12 file and split it into certificate, chain, and private-key PEM blocks.
PPK ↔ PEM Converter — Convert SSH private keys between PuTTY .ppk and OpenSSH/PEM (RSA & Ed25519) — locally in your browser.
Hash Generator — SHA-256 / SHA-1 / SHA-384 / SHA-512 via the Web Crypto API.