DevToolkit Pro

What Are SSL/TLS Certificates

SSL/TLS certificates are digital certificates used to establish encrypted connections on networks. They contain the server's public key, identity information, and a digital signature from a Certificate Authority (CA).

Certificate Structure

Certificate
├── Version
├── Serial Number
├── Signature Algorithm
├── Issuer
├── Validity
│   ├── Not Before
│   └── Not After
├── Subject
│   ├── Common Name (CN)
│   ├── Organization (O)
│   └── Country (C)
├── Public Key Info
├── Extensions
│   ├── Subject Alternative Names (SAN)
│   ├── Key Usage
│   └── Basic Constraints
└── Signature

Why You Need Certificate Decoding

1. Security audit: Check if server certificate configuration is correct
2. Troubleshooting: Diagnose SSL/TLS connection issues
3. Certificate verification: Confirm the certificate hasn't been tampered with
4. Expiry monitoring: Check certificate expiration dates
5. Compliance checks: Verify certificates meet security standards
6. Development debugging: Test self-signed certificate configurations

Key Certificate Fields

| Field | Description | Example | |-------|-------------|---------| | Subject (CN) | Domain name | www.example.com | | Issuer | Certificate authority | Let's Encrypt Authority X3 | | Not Before | Valid from | 2026-01-01 00:00:00 UTC | | Not After | Valid until | 2026-04-01 23:59:59 UTC | | SAN | Supported domain list | example.com, *.example.com | | Serial Number | Certificate serial | 03:AB:CD:EF:... | | Signature Algorithm | Signing algorithm | SHA256withRSA |

How to Use an Online Tool

Using DevToolkit Pro's Certificate Decoder:

1. Paste PEM-formatted certificate text (starts with -----BEGIN CERTIFICATE-----)
2. Or enter a domain name to automatically fetch and parse the certificate
3. View complete certificate information:
   • Issuing authority
   • Validity period
   • Subject information
   • SAN domain list
   • Key information

Viewing Certificates with OpenSSL

# View detailed certificate info
openssl x509 -in certificate.pem -text -noout

# View validity dates
openssl x509 -in certificate.pem -dates -noout

# View subject
openssl x509 -in certificate.pem -subject -noout

# View issuer
openssl x509 -in certificate.pem -issuer -noout

# Get certificate from remote server
openssl s_client -connect example.com:443 -showcerts

FAQ

What's the Difference Between PEM and DER?

PEM is Base64-encoded text format (starts with -----BEGIN CERTIFICATE-----). DER is binary format. PEM is more common because it can be transmitted and viewed as text.

What If a Certificate Has Expired?

1. Verify it's actually expired (sometimes it's a client clock issue)
2. Contact your Certificate Authority for renewal
3. Check auto-renewal configuration (e.g., certbot)
4. Temporary: use self-signed certificates (dev environment only)

How to Check a Remote Server's Certificate?

# Using OpenSSL
openssl s_client -connect example.com:443 -servername example.com

# Using curl
curl -vI https://example.com 2>&1 | grep -A 6 "Server certificate"

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