CSR / Certificate Decoder

Decode a PEM-encoded X.509 certificate or Certificate Signing Request (CSR) to see its subject, issuer, validity dates, and signature algorithm, entirely in your browser.

PEM Certificate or CSR

How was this tool?

Your feedback helps us improve CSR / Certificate Decoder.

Why use CSR / Certificate Decoder?

This tool tells certificates and CSRs apart by checking the PEM header text, then hands the block to the jsrsasign library to parse. Paste a full X.509 certificate and it pulls out the subject, issuer, serial number, "not before" and "not after" dates, and the signature algorithm. Paste a Certificate Signing Request instead and it shows the subject, the public key type and size such as RSA 2048-bit, and the signature algorithm used to sign the request. If the pasted text lacks "CERTIFICATE" or "CERTIFICATE REQUEST" in its header, the tool says so rather than guessing. All parsing happens in your browser using jsrsasign, so the PEM text you paste is never sent to a server. That fits how CSRs and certificates work anyway: both hold only public information, a public key and subject details, never the private key that pairs with them, so there is nothing secret to protect in the first place. Use it to sanity check a CSR before submitting it to a certificate authority, or to confirm the expiry and subject details on a certificate you already have.

Key benefits

Decodes both certificates and CSRs from the same input box
Shows subject, issuer, validity, and algorithm details at a glance
jsrsasign parses the PEM block in your browser, no upload required
Automatically detects whether you pasted a certificate or a CSR
Free with no signup, useful for a quick check before a CA submission

Features

X.509 certificate parsing (subject, issuer, serial number, validity dates, signature algorithm)
CSR parsing (subject, public key type and size, signature algorithm)
Automatic type detection from the PEM header text
Clear error message when the pasted text is not a recognizable PEM block
Client-side parsing with the jsrsasign library, nothing sent to a server

Popular use cases

Verifying a CSR before submitting it to a certificate authority
Checking a certificate's expiration date
Confirming the subject details on an issued certificate
Double-checking the public key size and signature algorithm on a CSR you generated

How to use CSR / Certificate Decoder

1
Paste your PEM data
Paste a full PEM block, starting with "-----BEGIN CERTIFICATE-----" or "-----BEGIN CERTIFICATE REQUEST-----".
2
Decode
Click Decode to parse the certificate or CSR.
3
Review the details
See the subject, issuer (for certificates), validity dates, and algorithm fields.
4
Fix and retry if needed
If the block is incomplete or not recognized, you'll get a message explaining why; paste the full PEM text and decode again.

Frequently asked questions

What's the difference between a certificate and a CSR?

A CSR (Certificate Signing Request) is what you submit to a certificate authority to request a certificate; the CA then issues back a signed X.509 certificate, which is what gets installed on your server.

Is my private key needed or exposed?

No. Both certificates and CSRs contain only public information (a public key and subject details), no private key is ever needed or transmitted.

Is my certificate data uploaded anywhere?

No. The PEM block is parsed by the jsrsasign library running in your browser. Note that a CSR contains only your public key, never the private key, so there is nothing secret in it to expose.

What fields does the tool show for a CSR versus a certificate?

For a CSR you get the subject, public key type and size, and signature algorithm. For a certificate you additionally get the issuer, serial number, and "not before" and "not after" validity dates.

What happens if I paste something that is not a valid PEM block?

The tool checks the header text for "CERTIFICATE" or "CERTIFICATE REQUEST" and, if neither is found, tells you the text does not look like a PEM certificate or CSR rather than showing incorrect results.

Last reviewed: July 2026

How was this tool?

Related tools