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Frequently asked questions
Are my files or text uploaded anywhere?
No. Every tool runs inside your browser on your own device. Nothing you enter or open is sent to a server.
Do I need to create an account or log in?
No login is required. There are no accounts — open any tool and start using it straight away.
Are the tools really free?
Yes. All tools are free, with no limits or watermarks. The site is supported by ads.
Does it work on my phone?
Yes. The tools work in any modern browser on phones, tablets and computers.
Can I use the tools offline?
Once a tool has loaded it does its work on your device, so it keeps working even if your connection drops.
Which file types are supported?
PDF, Word, Excel, PowerPoint, OpenDocument and RTF documents, JPG, PNG, WebP, HEIC, AVIF, GIF and BMP images, MP4, MOV, WebM and MKV videos, MP3, WAV, M4A, OGG and FLAC audio, ZIP, 7z, RAR, TAR and GZIP archives, CSV, JSON, XML and plain text. Each tool lists exactly what it opens.
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Certificate Decoder
Decode SSL/TLS certificates, chains, CSRs and public keys in PEM or DER: subject, issuer, expiry, SANs, key size, extensions and fingerprints.
Paste a certificate in PEM form, the text that starts with -----BEGIN CERTIFICATE-----, or open a .crt, .cer, .pem or .der file, and the decoder shows what is inside: who the certificate is for, who issued it, when it expires and how many days are left, the domain names it covers, the key type and size, the signature algorithm, all extensions and the SHA-256 and SHA-1 fingerprints.
Paste a whole chain to check that the certificates are in the right order, each one issued by the next. Certificate signing requests (CSRs) and public keys are decoded too, so you can check a CSR before sending it to a certificate authority. Everything is decoded in your browser by the page's own code.
How to use Certificate Decoder
- Paste one or more PEM blocks, or open a certificate file.
- Read the status, validity dates and the names covered.
- Check the key, signature and extensions.
- For a chain, check that the order message is green.
Features
- Certificates, chains, CSRs and public keys
- PEM text or binary DER (.der, .cer) files
- Expiry status with days left
- Subject alternative names, key usage and policies (DV, OV, EV)
- SHA-256 and SHA-1 fingerprints
- Chain order check; private keys are refused
What the fields mean
A TLS certificate binds a public key to names, and is signed by a certificate authority (CA) that browsers trust.
| Field | Meaning |
|---|---|
| Subject | Who the certificate is for; CN is the common name |
| Issuer | The CA that signed it; the same as the subject when self-signed |
| Valid from / until | Browsers reject the certificate outside these dates |
| Alternative names (SAN) | The domain names and IP addresses it is valid for |
| Key | Public key type and size, such as RSA 2048-bit or EC P-256 |
| Signature | How the CA signed it, such as SHA-256 with RSA |
| Type | Root CA, intermediate CA, or end-entity (DV, OV or EV) |
| Fingerprint | A hash of the whole certificate, used to identify it exactly |
Names: why the SAN list matters
Browsers ignore the common name and only check the subject alternative names. A certificate for example.com does not cover www.example.com unless both are listed, and a wildcard such as *.example.com covers one level of subdomains but not example.com itself. If visitors see a name mismatch error, compare the address they use with the SAN list here.
Chains and intermediates
Servers must send their own certificate followed by the intermediate certificates that link it to a root CA in the browser. A missing or misordered intermediate works in some browsers and fails in others, especially on Android and in command-line tools. Paste the full chain from your server or CA bundle: the decoder checks that each certificate's issuer is the next certificate's subject. Roots are usually not included, because browsers already have them.
Keys, algorithms and lifetimes
RSA keys should be at least 2048 bits; elliptic-curve keys on P-256 or P-384 are smaller and faster with the same security. SHA-1 and MD5 signatures are no longer trusted. Public TLS certificates are limited in how long they may be valid, and the industry is shortening the maximum lifetime in steps over the coming years, so automate renewal, for example with an ACME client, and watch the days-left figure for certificates you renew by hand.
Private keys stay private
Certificates, CSRs and public keys are public information and safe to decode anywhere. Private keys are not: if you paste one, the decoder only says what it is and does not decode it. If a private key has been shared, emailed or posted by mistake, treat it as compromised: create a new key and certificate and revoke the old one.
Frequently asked questions
How do I check when my SSL certificate expires?
Paste the certificate or open the .crt file; the status line shows the expiry date and the days left.
What is the difference between .crt, .cer, .pem and .der?
PEM is base64 text between BEGIN and END lines; DER is the same data in binary. .crt and .cer files can be either, and the decoder reads both.
Can it decode a CSR?
Yes. Paste the -----BEGIN CERTIFICATE REQUEST----- block to see the subject, key and requested names before sending it to a CA.
How do I check a certificate chain?
Paste all the certificates in the order your server sends them. The decoder says whether each one is issued by the next.
Is it safe to paste my certificate?
Yes. Certificates are public, and decoding happens in your browser. Do not paste private keys anywhere.
What do DV, OV and EV mean?
Domain, organisation and extended validation: how much the CA checked before issuing. The browser padlock looks the same for all three.
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