Convert PEM-Encoded Public Key to OpenSSH Format using ssh‑keygen

When working with public keys, you'll often encounter different encoding formats depending on the tool or system in use. Two common formats are PEM (a base64-encoded, human-readable representation) and the OpenSSH format (used in authorized_keys files and by the ssh client). This tutorial explains how to convert PEM-encoded public key to OpenSSH format using ssh-keygen.

Suppose you have a public key saved in a file named test.pub with the following content:

-----BEGIN PUBLIC KEY-----
MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAN/ovAmhoNOnE2VoJeUunpEB+CwTyPbK
I8VnQ+rLa75riBtmfYrGdUD8Cv/p+ioL86xwSvX+HYXnkPffEXc7IlUCAwEAAQ==
-----END PUBLIC KEY-----

To convert this PEM-encoded public key into a OpenSSH format, run the following ssh-keygen command:

ssh-keygen -i -m PKCS8 -f test.pub > id_rsa.pub

Explanation of options:

• -i - instructs to import a key from another format.
• -m PKCS8 - specifies the input format as PKCS#8.
• -f test.pub - points to the file containing the input key.
• > id_rsa.pub - writes converted key to the output file.

Output:

ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAQQDf6LwJoaDTpxNlaCXlLp6RAfgsE8j2yiPFZ0Pqy2u+a4gbZn2KxnVA/Ar/6foqC/OscEr1/h2F55D33xF3OyJV