What is .key?

.key files typically contain private cryptographic keys used in TLS/SSL, VPNs, SSH, and other secure communication systems. A private key proves ownership of a certificate and enables decryption and digital signatures. KEY files may be stored as PEM-encoded text or other container formats and must be kept strictly confidential.

This quick guide explains when to use .key files, how to open them on any device, and how to share them instantly with FileXhost.

When to use .key files

You are configuring HTTPS/TLS for a web server, proxy, or load balancer.
You need to store the private key associated with an X.509 certificate.
You are managing keys for VPNs, client certificates, or other PKI-secured services.
You are working with automation tools that reference private keys from files for secure connections.

How to open .key files

KEY files can be opened in text editors if they are PEM-encoded, showing headers like '-----BEGIN PRIVATE KEY-----' and Base64-encoded data. For inspection and conversion, use tools such as OpenSSL, keytool, or platform-specific key management utilities. When you upload KEY files to FileXhost, treat them as highly sensitive secrets and share them only through secure, access-controlled links with trusted recipients.

Algorithm details

KEY files usually contain private keys in formats such as PKCS#1 (RSA), PKCS#8 (generic private key container), or EC private keys. They may be encrypted with a passphrase for additional protection. Internally, keys are encoded using ASN.1 and stored as DER (binary) or PEM (Base64 text) and are paired with corresponding public certificates distributed separately as CRT, CER, or PEM files.

Browser & platform support

Desktop: Browsers do not open KEY files directly. Private keys are typically imported via OS or browser certificate management tools, not handled manually by end users.
Mobile: Mobile platforms rely on system key stores and configuration profiles to manage private keys rather than directly opening KEY files.
OS: Operating systems and server software use KEY files to terminate TLS connections and authenticate services when configured in web or proxy servers.

Format comparison

Feature	Details
KEY vs PEM	PEM refers to a text-based encoding with headers/footers; KEY is a file extension often used for private key material that may itself be PEM-encoded.
KEY vs CRT/CER	KEY files hold the private key, while CRT/CER files hold public certificates. Both are required for full TLS configuration but serve different purposes.
KEY vs PFX/P12	PFX/P12 bundles private keys and certificates together in a password-protected binary container; KEY files usually contain only the private key.
Encryption	KEY files can be encrypted with a passphrase, while many certificate files (CRT/CER) are not; encrypted keys provide an extra layer of security at rest.

How to create key files

OpenSSL: Generate private keys for certificates, CSRs, or VPN profiles using OpenSSL commands.
Certificate Authorities: Some workflows supply or derive KEY files as part of the certificate issuance process.
Server Software: Web and mail server tooling can generate key pairs and store the private key in KEY files.
Automation/DevOps: Infrastructure-as-code and CI/CD pipelines use KEY files to manage TLS for services and ingress controllers.

How to convert key files

FileXhost: Store encrypted KEY files on FileXhost for secure, controlled distribution to operations teams.
OpenSSL CLI: Convert private keys between formats (PKCS#1, PKCS#8), encrypt or decrypt keys, and bundle them with certificates into PFX/P12.
Key Management Tools: Use keytool, certutil, and cloud key management services to import/export keys in different containers.
Hardware/Cloud HSMs: Migrate keys into hardware security modules or cloud key vaults for stronger protection.

Advantages & disadvantages

Advantages

Essential for establishing secure TLS connections and digital signatures
Can be stored in standard, well-understood PEM and PKCS formats
Works across many servers, proxies, and infrastructure tools

Disadvantages

Highly sensitive; exposure of a private key compromises associated certificates
Requires strict access control, rotation, and auditing practices
Mismanagement can lead to outages or security incidents

Tools & software

CLI Tools

OpenSSL, keytool, certutil, ssh-keygen (for some key types)

Servers/Proxies

Nginx, Apache HTTP Server, HAProxy, Envoy, Traefik

Key Management

Cloud KMS (AWS KMS, GCP KMS, Azure Key Vault), HSMs, enterprise key managers

Frequently asked questions

Should I ever share a .key file?

No. Private key files must never be shared with untrusted parties or stored in public repositories. Only authorized operators and systems should have access to KEY files, and they should be protected with strong access controls and, where possible, encryption.

Where should I store private key files?

Store KEY files on secure servers, in encrypted storage, or in dedicated key management systems or HSMs. Avoid storing them in source control, logs, or unsecured file shares.

Can I recover a lost KEY file?

If a private key is lost and no backup exists, it cannot be recovered. You must generate a new key pair and obtain new certificates. Always keep secure backups of critical private keys.

How do I know which certificate matches a KEY file?

Use OpenSSL or similar tools to compare the modulus or fingerprint of the KEY file and certificate (CRT/CER/PEM). Matching pairs will share the same public key parameters.

Technical specs

File type: Security
Extension: .key
MIME type: application/x-pem-file, application/octet-stream
Compression: Uncompressed
Max file size on FileXhost: Up to 25 MB per file on the free plan and up to 1 GB on Pro FileXhost accounts.

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