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Here’s a handy comparison table that breaks down the most common certificate formats: .crt, .key, .pem, .p12, and others like .cer, .der, .pfx, etc

📦 Certificate Format Cheat Sheet

Format File Extension(s) Contains Encoded As Encrypted? Use Case
CRT .crt, .cer Public certificate only Usually PEM ❌ No Web servers, clients, trust chains
KEY .key Private key only PEM ❌ / ✅ Paired with certs, server/client auth
PEM .pem Can contain cert, key, or both PEM (Base64) ❌ / ✅ Flexible format (OpenSSL default)
P12 .p12, .pfx Cert + private key + CA chain Binary ✅ Yes Used on Android, Windows, macOS, etc.
PFX .pfx Same as .p12 Binary ✅ Yes Windows legacy (same format as .p12)
DER .der, .cer Single cert (binary version of CRT) Binary ❌ No Java-based apps (Tomcat, etc.)
CSR .csr Certificate Signing Request PEM ❌ No Sent to CA to get a signed cert
JKS .jks Java KeyStore (multiple entries) Binary ✅ Yes Java apps like Tomcat/Jetty
PKCS#8 .key, .pem Private key (newer format) PEM / DER ✅ or ❌ Often used in modern systems
PKCS#7 .p7b, .p7c Cert chain (no private key) Binary/PEM ❌ No Used for bundling CA chains

🔍 What Are We Using in Our Setup

File Contains Format Use In Your Flow
*.crt Public cert PEM (CRT) Shared with NPM or clients
*.key Private key PEM Needed for .p12 or server usage
*.p12 Cert + key + CA chain Binary 📱 For Android, iOS, etc.
root_ca.crt Root certificate PEM (CRT) Used by NPM for mTLS trust

💡 Notes

  • .pem, .crt, .cer, .key are often interchangeable as long as content matches
  • .p12/.pfx is the best format to bundle everything securely
  • Use openssl to convert between all of these