Introduction to SSH Key Conversion

Understanding OpenSSH Private Keys

OpenSSH private keys serve as digital fingerprints for secure server access, typically stored in the ~/.ssh/ directory. These keys use the PEM (Privacy-Enhanced Mail) format by default, recognizable by the -----BEGIN OPENSSH PRIVATE KEY----- header. Modern OpenSSH keys often include additional metadata like key comments and encryption parameters, making them versatile for newer systems but occasionally incompatible with legacy tools.

A typical OpenSSH private key combines both public and private components in a single file, secured with passphrase-based encryption. These keys work seamlessly with SSH protocols but may struggle in environments expecting traditional RSA/PEM formatting.

What is RSA Format and Why Convert?

The RSA key format, standardized under PKCS#1, predates OpenSSH’s custom format and remains widely used in legacy systems, IoT devices, and certain cloud platforms. Converting an OpenSSH private key to RSA format becomes necessary when integrating with older APIs, Java-based applications, or services that explicitly require -----BEGIN RSA PRIVATE KEY----- headers.

RSA keys prioritize backward compatibility and simpler structure, stripping away OpenSSH-specific extensions. Developers often convert keys to RSA format for use with tools like OpenSSL, older Git clients, or enterprise systems that haven’t adopted modern SSH key standards.

Key Differences Between OpenSSH and RSA Formats

Structurally, OpenSSH keys embed algorithm identifiers and versioning details, while RSA keys focus purely on modulus/exponent pairs. Encryption methods differ too: OpenSSH defaults to AES-256-CBC for passphrase protection, whereas traditional RSA keys often use PKCS#8 encoding with varied cipher options.

File extensions can be misleading—both formats might use .pem or no extension. The critical distinction lies in the header/footer syntax and internal ASN.1 encoding. Compatibility issues arise when software expects specific key framing, making format conversion essential for cross-platform workflows.

Preparing for Key Conversion

Installing and Configuring OpenSSL

OpenSSL is our essential toolkit for this conversion journey. Most Linux distributions and macOS systems come with OpenSSL pre-installed - we can check its presence by typing openssl version in the terminal. For Windows users, installing Git Bash or directly downloading OpenSSL binaries from the official site gives us the necessary functionality.

I always recommend updating OpenSSL to the latest stable release before conversion work. On Ubuntu-based systems, running sudo apt update && sudo apt upgrade openssl does this cleanly. For security-focused conversions, we might enable specific cryptographic modules through the configuration file located at /etc/ssl/openssl.cnf, though default settings usually suffice for RSA key transformations.

Creating Secure Backups Before Conversion

Protecting our original keys is non-negotiable - one accidental overwrite could lock us out of critical systems. Before attempting any format changes, I create encrypted backups of the OpenSSH private key using symmetric encryption. This command creates a password-protected backup: openssl enc -aes256 -pbkdf2 -in id_rsa -out id_rsa.backup.enc.

These backups get stored in isolated locations - never in the same directory as active keys. I often use external encrypted USB drives or secured cloud storage with client-side encryption. Digital preservation doesn't stop at creation - periodically validating backup integrity through checksum verification ensures our recovery options remain viable when needed most.

Ensuring Proper Permissions and File Safety

SSH keys demand fortress-like protection. Our private keys should always have 600 permissions (chmod 600 filename), locking out all other users on the system. I double-check ownership too - keys must belong to the current user, not root or other accounts. Stray permissions can trigger "Permission denied" errors during conversion that halt the entire process.

During file handling, I avoid temporary directories and unencrypted transfers. When moving keys between systems, SCP with SSH connection multiplexing keeps data off insecure channels. For conversion work, I operate in a dedicated, non-networked directory to prevent accidental exposure. These precautions create essential security boundaries around our cryptographic assets.

Step-by-Step Conversion Using OpenSSL

Basic Command Syntax for Conversion

The core conversion command follows a straightforward pattern: openssl rsa -in <input_key> -out <output_key>. What makes this powerful is its ability to handle various encoding formats through additional flags. When working with legacy systems, adding the -traditional flag ensures compatibility with older RSA key parsers.

I often include the -des3 parameter for password protection during conversion (openssl rsa -in id_ed25519 -out id_rsa -des3), though this depends on the target system's requirements. For keys needing PEM format explicitly, the -text flag outputs human-readable details that help verify structure during debugging sessions.

Detailed Walkthrough: Converting .pem to RSA

Let's convert a typical OpenSSH private key stored in PEM format. Starting with id_openssh as our input file, we run:
openssl rsa -in id_openssh -traditional -out id_rsa_converted

If the original key has a passphrase, OpenSSL prompts for it before proceeding. I immediately set strict permissions on the new file with chmod 600 id_rsa_converted. For systems requiring RFC4716 formatting instead of PKCS#1, adding -RFC4716 modifies the output structure accordingly. This process maintains cryptographic integrity while transforming the container format.

Handling Key Formats and Encoding Options

Modern OpenSSH keys often use PKCS#8 format, which we can convert to traditional PEM using -topk8 parameter inversion. The command openssl pkcs8 -in pkcs8_key -out traditional_rsa -nocrypt achieves this without re-encryption.

When dealing with binary DER format, the -inform DER flag becomes crucial. For ASCII-armored output required by some web interfaces, adding -a creates Base64 PEM encoding. I frequently test these variations by converting back and forth between formats using different encoding flags to verify consistency.

Verifying the Converted RSA Key

Validation occurs through two methods: structural checks and functional testing. Running openssl rsa -check -in id_rsa_converted confirms key integrity and displays critical parameters like modulus bits. For paired key verification, comparing the public key modulus from both original and converted keys using openssl rsa -noout -modulus ensures mathematical consistency.

I always perform live authentication tests using the converted key. Executing ssh -i id_rsa_converted user@hostname validates practical usability. When the connection establishes successfully, we've confirmed both technical correctness and functional compatibility.

!/bin/bash

for key in ~/legacy_keys/.pem; do openssl rsa -in "$key" -out "${key%.}_converted.rsa" chmod 600 "${key%.*}_converted.rsa" done

Troubleshooting Common Conversion Errors

Identifying and Resolving Permission Issues

I often see "Permission denied" errors during conversions. This usually happens when OpenSSL can't read the original key file due to incorrect permissions. I fix this by running chmod 600 my_key on the source file before conversion. SSH agents are strict about permissions - they'll reject keys if group or others have any access. After generating new RSA keys, I immediately set restrictive permissions using chmod 400 converted_key.rsa.

Ownership conflicts cause similar headaches. When working in Docker containers or across users, I verify file ownership with ls -l. Transferring keys between systems sometimes resets permissions. I solve this by recreating the key's directory structure manually before copying files.

Fixing Format Mismatches and Invalid Key Errors

"Invalid format" messages make me check the key's actual structure. Modern OpenSSH keys start with -----BEGIN OPENSSH PRIVATE KEY-----, while traditional RSA keys use -----BEGIN RSA PRIVATE KEY-----. If OpenSSL rejects a key, I first confirm whether it's actually PKCS#8 format. When tools complain about "bad magic number", I convert the key to PKCS#8 explicitly using ssh-keygen -p -m PEM -f keyfile.

For legacy systems requiring PKCS#1, I extract the RSA key from PKCS#8 containers with openssl rsa -in pkcs8_key -out traditional_rsa_key. PEM versus DER encoding issues get resolved by adding -inform DER or -outform PEM flags to OpenSSL commands based on error logs.

Debugging with OpenSSL Verbose Output

When conversions fail silently, I activate OpenSSL's diagnostic mode. Adding -text -noout flags reveals the key's internal structure: openssl rsa -in problem_key -text -noout. This shows whether the key contains unexpected parameters or truncated data. For passphrase-related failures, -passin stdin lets me verify input accuracy by typing manually instead of using scripted inputs.

ASN.1 parse errors require deeper inspection. I pipe OpenSSL's output through hexdump -C to examine binary structures when text outputs show gibberish. Comparing byte patterns against known-good keys often reveals corruption points.

Handling Passphrase-Related Problems

Passphrase mismatches halt conversions immediately. If I forget a passphrase, conversion tools won't proceed - there's no bypass besides regenerating keys. When certain applications reject converted keys, I check for trailing newline issues in passphrases. Converting using echo -n "pass" | openssl... prevents added line breaks.

Different tools handle encryption standards differently. OpenSSL defaults to AES-256-CBC while ssh-keygen uses newer formats. I specify compatibility explicitly with -aes128 or -aes256 flags. For systems requiring insecure legacy encryption, -des3 provides backward compatibility at security cost.

Best Practices and Conclusion

When to Use RSA Format and Compatibility Tips

I recommend using RSA format primarily when interacting with legacy systems or protocols that don't support modern OpenSSH keys. Many IoT devices and enterprise hardware from the 2010s still require traditional RSA keys for SSH authentication. When working with Java applications using JSch library or older versions of Git clients, RSA keys often prove more compatible than OpenSSH's default format.

Always check documentation for target systems. Cloud platforms like AWS EC2 switched to OpenSSH as default but still accept RSA keys through their APIs. For cross-platform compatibility, I maintain both formats using ssh-keygen -t rsa -b 4096 -m PEM to generate native RSA keys. When transferring keys between Windows and Unix systems, I confirm line endings (LF vs CRLF) using dos2unix to prevent parsing errors.

Be mindful of PKCS#1 vs PKCS#8 distinctions. Tools like PuTTY require PPK conversions from RSA keys. I keep a dedicated folder with RSA keys in PKCS#1 format labeled "Legacy_Compatibility" to avoid format confusion during deployments.

Security Recommendations for Key Management

Through hard-earned experience, I enforce three non-negotiable rules: rotate keys quarterly, use 4096-bit encryption minimum, and never store decrypted keys. For passphrases, I combine diceware phrases with special characters (e.g., "Turtle$Bounce72!Vault") instead of complex gibberish that gets written down.

Storage security demands multiple layers. I encrypt key directories with VeraCrypt containers and use separate physical drives for backups. Cloud storage gets AES-256 encrypted keys only, with names like "finance_prod_rsa.aes" to indicate protection status. Audit trails matter - I track key usage through SSH logs and HashiCorp Vault's access history.

Revocation preparedness saves headaches. Every key pair I create gets an expiration date in metadata (openssl rsa -in key -text | grep 'Not After'). For teams, I implement mandatory key signing ceremonies using GPG to prevent unauthorized modifications.

Summary of Key Takeaways and Final Advice

The conversion journey teaches that key formats aren't interchangeable magic. Modern OpenSSH keys offer better security defaults, but RSA remains essential for specific use cases. Mastery comes from understanding when to convert versus when to generate native keys.

Three principles guide my workflow: validate keys immediately after conversion (ssh-keygen -lf converted_key), maintain clear versioned backups, and document every key's purpose in a README file. For those managing large infrastructures, automate conversions with Ansible playbooks but keep manual verification steps.

Final wisdom: Treat SSH keys like physical master keys. Never share them across environments, always assume networks are compromised, and remember that the safest key is one that's properly retired after use. When in doubt, regenerate rather than reuse - disk space is cheaper than security breaches.