Securing your servers and accessing them efficiently often involves working with different key formats. One common task is to convert PEM key to SSH-RSA format, a process crucial for compatibility with various systems and tools. PEM (Privacy Enhanced Mail) is a widely used container format that can store cryptographic keys, including private and public keys, as well as certificates. SSH-RSA, on the other hand, is a specific public key algorithm used for secure shell (SSH) authentication. Understanding how to perform this conversion ensures seamless and secure access to your remote servers, preventing potential authentication issues and enhancing overall security posture. This process is especially important when dealing with cloud services and different operating systems that might prefer one format over the other. Misunderstanding key formats can lead to frustrating access denials and security vulnerabilities.

Understanding PEM and SSH-RSA Key Formats

Before diving into the conversion process, it’s essential to understand what PEM and SSH-RSA formats represent. PEM is a container format that can hold various types of data, encoded in Base64, making it easily transferable across different systems. It typically starts with “—–BEGIN [TYPE]—–” and ends with “—–END [TYPE]—–”, where [TYPE] can be “PRIVATE KEY,” “PUBLIC KEY,” or “CERTIFICATE.” The PEM format is versatile but not directly usable by all SSH clients.

SSH-RSA, specifically, refers to an RSA public key formatted for use with SSH. This format is primarily used for public keys and is often stored in the ~/.ssh/authorized_keys file on a server to grant SSH access. Unlike the PEM format, SSH-RSA is a specific key type recognized by SSH clients and servers. The key is represented as a single line containing algorithm identifier (“ssh-rsa”), the Base64 encoded key, and an optional comment. Many older systems still rely on SSH-RSA, though newer, more secure algorithms like Ed25519 are gaining traction. Knowing how to convert PEM key to SSH-RSA format ensures compatibility with these systems.

The need for conversion arises because some tools or systems might provide keys in PEM format, while the SSH server expects the key in SSH-RSA format. This disparity necessitates a conversion process to ensure seamless authentication. A common scenario is when you generate a key pair using OpenSSL, which defaults to PEM format, and then need to use the public key with an SSH server that only accepts SSH-RSA formatted keys. As reported by the National Institute of Standards and Technology (NIST), proper key management and formatting are vital components of a robust cybersecurity strategy [1].

Converting PEM Key to SSH-RSA Using OpenSSL

OpenSSL is a powerful command-line tool that can perform various cryptographic operations, including key format conversions. The process to convert PEM key to SSH-RSA format using OpenSSL involves extracting the public key from the PEM file and then formatting it according to the SSH-RSA standard. This typically involves two steps: first, extracting the public key, and then formatting it into the correct SSH-RSA string. The following steps provide a clear guide:

1. Extract the Public Key: Use the following command to extract the public key from the PEM file: ``` openssl rsa -in private.pem -pubout -out public.pem

   
    This command reads the private key from private.pem and extracts the corresponding public key, saving it to public.pem.
   

2. Convert to SSH-RSA Format: Use the following command to convert the public key to SSH-RSA format: ``` ssh-keygen -f public.pem -e -m PKCS8

   
    This command reads the public key from public.pem and outputs the SSH-RSA formatted key. Note the specific -m PKCS8 argument to ensure proper formatting.
   

3. Verify the Output: The output will be a single line starting with “ssh-rsa” followed by a Base64 encoded key and an optional comment. This is the SSH-RSA formatted public key.

For example, if your public key is stored in public.pem, after running the commands, you will get an output like: ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQ… user@example.com. This is the exact string you need to add to your ~/.ssh/authorized_keys file on the remote server. Errors in these steps can lead to authentication failures, highlighting the importance of careful execution.

While OpenSSL is the most common and reliable tool, there are alternative methods and tools that can help you convert PEM key to SSH-RSA format. One such method involves using scripting languages like Python with libraries such as cryptography. This offers more flexibility and automation, especially when dealing with large-scale key management. Another approach involves online converters, but these should be used with caution due to security risks associated with uploading private keys to untrusted websites.

Another useful tool is puttygen, which comes with the PuTTY SSH client. Although primarily used for converting keys to PuTTY’s .ppk format, it can also export keys in the OpenSSH format, which is essentially the SSH-RSA format. To use puttygen, load your PEM key, and then select “Conversions” -> “Export OpenSSH key.” Remember, when using alternative tools, ensure they are reputable and trustworthy to avoid compromising your private keys. Always verify the output to ensure it conforms to the SSH-RSA format.

Consider using a dedicated key management system (KMS) if you’re dealing with a large number of keys. KMS solutions provide secure storage, rotation, and access control for your cryptographic keys, reducing the risk of exposure and simplifying key management tasks. According to a report by Cybersecurity Ventures, the global cost of cybercrime is projected to reach $10.5 trillion annually by 2025 [2], highlighting the critical need for robust key management practices.

Best Practices and Security Considerations

When working to convert PEM key to SSH-RSA format, security should be your top priority. Private keys are sensitive data, and their compromise can lead to unauthorized access to your systems. Therefore, it’s crucial to follow best practices to protect your keys throughout the conversion process and beyond. Always store your private keys securely, restrict access to them, and never share them with unauthorized parties. Regularly rotate your keys and use strong passphrases to protect them further.

Here are some key security considerations:

• Secure Storage: Store your private keys in a secure location with restricted access. Avoid storing them in plain text on your computer.
• Access Control: Limit access to your private keys to only those who need them. Use file permissions to restrict access.

The featured snippet-optimized paragraph: To convert a PEM key to SSH-RSA format, extract the public key from the PEM file using OpenSSL and then format it into the SSH-RSA standard using ssh-keygen. This ensures compatibility with SSH servers and clients that require this specific key format. Remember to secure your private key throughout this process.

Additionally, consider using key-based authentication instead of password-based authentication for SSH. Key-based authentication is more secure as it eliminates the risk of password cracking. Also, monitor your systems for suspicious activity and implement intrusion detection systems to detect and respond to security incidents. Implementing multi-factor authentication (MFA) adds an extra layer of security to your SSH connections, further protecting against unauthorized access. A study by Google found that using MFA can block up to 100% of automated bot attacks [3].

• Regular Rotation: Regularly rotate your SSH keys to minimize the impact of a potential compromise.
• Multi-Factor Authentication: Implement multi-factor authentication for SSH to add an extra layer of security.

FAQ

**What is the difference between PEM and SSH-RSA?**

PEM is a container format that can store various cryptographic keys and certificates, while SSH-RSA is a specific public key algorithm and format used for SSH authentication.

**Why do I need to convert PEM to SSH-RSA?**

Some SSH servers and clients require the public key to be in SSH-RSA format for authentication.

**Is it safe to use online PEM to SSH-RSA converters?**

It is generally not recommended to use online converters for private keys due to security risks.

**What if the conversion fails?**

Double-check your commands, ensure you have the correct input files, and verify that OpenSSL and ssh-keygen are properly installed.

**What other key formats are commonly used for SSH?**

Ed25519 is another popular and more secure key format used for SSH.

Question & Answer :
I have a certificate in der format, from it with this command I generate a public key:

openssl x509 -inform der -in ejbcacert.cer -noout -pubkey > pub1key.pub 

Which results in this:

-----BEGIN PUBLIC KEY----- MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC7vbqajDw4o6gJy8UtmIbkcpnk O3Kwc4qsEnSZp/TR+fQi62F79RHWmwKOtFmwteURgLbj7D/WGuNLGOfa/2vse3G2 eHnHl5CB8ruRX9fBl/KgwCVr2JaEuUm66bBQeP5XeBotdR4cvX38uPYivCDdPjJ1 QWPdspTBKcxeFbccDwIDAQAB -----END PUBLIC KEY----- 

How can I obtain a public key like this? Either from certificate or from this public key?

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQC7vbqajDw4o6gJy8UtmIbkcpnkO3Kwc4qsEnSZp/TR+fQi62F79RHWmwKOtFmwteURgLbj7D/WGuNLGOfa/2vse3G2eHnHl5CB8ruRX9fBl/KgwCVr2JaEuUm66bBQeP5XeBotdR4cvX38uPYivCDdPjJ1QWPdspTBKcxeFbccDw== 

This was obtained with this command:

ssh-keygen -y -f private_key1.pem > public_key1.pub 

No need to compile stuff. You can do the same with ssh-keygen:

ssh-keygen -f pub1key.pub -i 

will read the public key in openssl format from pub1key.pub and output it in OpenSSH format.

Note: In some cases you will need to specify the input format:

ssh-keygen -f pub1key.pub -i -m PKCS8 

From the ssh-keygen docs (From man ssh-keygen):

-m key_format Specify a key format for the -i (import) or -e (export) conversion options. The supported key formats are: “RFC4716” (RFC 4716/SSH2 public or private key), “PKCS8” (PEM PKCS8 public key) or “PEM” (PEM public key). The default conversion format is “RFC4716”.