147

I'm trying to create a private key and having an issue.

When I use ssh-keygen -t rsa -b 4096 -C "[email protected]", I get a private key in the following format.

-----BEGIN OPENSSH PRIVATE KEY-----
uTo43HGophPo5awKC8hoOz4KseENpgHDLxe5UX+amx8YrWvZCvsYRh4/wnwxijYx
...
-----END OPENSSH PRIVATE KEY-----

And this is not being accepted for an application that I'm trying to use.

I'm expecting a key in the following RSA format.

-----BEGIN RSA PRIVATE KEY-----
Proc-Type: 4,ENCRYPTED
DEK-Info: AES-128-CBC,25737CC2C70BFABADB1B4598BD8AB9E9

uTo43HGophPo5awKC8hoOz4KseENpgHDLxe5UX+amx8YrWvZCvsYRh4/wnwxijYx
...
-----END RSA PRIVATE KEY-----

How do I create the correct format? This is weird because every other mac I have creates the correct format, except the one I'm having problem with.

I'm on a fresh installed Mac OS Mojave

6
  • and the other Macs are not on Mojave? I am on Mojave too and get the "new" openssh key format. So I assume other Macs are on lower versions. Check the OpenSSL version used.
    – Zina
    Nov 14, 2018 at 0:21
  • @Zina other Macs are also on Mojave and have the same OpenSSL version.
    – Moon
    Nov 14, 2018 at 1:44
  • RSA should be the default type. What if you omit the -t rsa option altogether?
    – guzzijason
    Nov 14, 2018 at 1:45
  • @guzzijason it's the same. That's what's driving me crazy. Even if I omit the -t rsa on my mac (working one), it generates RSA correctly.
    – Moon
    Nov 14, 2018 at 1:46
  • I've had the same problem. As workaround I've used older version of openssh to generate key. You can test if your generated key is correct with openssl rsa -text -in key_file -passin 'pass:passphrase'. Version 7.4p1-16 works.
    – atype
    Nov 14, 2018 at 7:15

3 Answers 3

150

I faced the same problem recently (after upgrade to mojave 10.14.1), here are 2 possible solutions for this issue.

  • Downgrade your ssh-keygen binary (you can easily get old version from any linux/docker image)

OR

  • Add option -m PEM into your ssh-keygen command. For example, you can run ssh-keygen -m PEM -t rsa -b 4096 -C "[email protected]" to force ssh-keygen to export as PEM format.

It seems like in the current ssh-keygen version in mojave, the default export format is RFC4716 as mentioned here

3
  • 2
    The default export publickey format is indeed rfc4716, but the format in the Q is an internal privatekey format and is OpenSSH's 'new' format, which didn't exist in 1999(!), and is no longer in described in current; try 6.0 to 6.3. Nov 28, 2018 at 9:25
  • 4
    I'm not able to edit the answer but please be clear that 1. and 2. are independent solutions, not sequential steps. You do NOT need to downgrade for a one-off key generation. Just add the -m PEM
    – sdoxsee
    Jan 23, 2019 at 15:55
  • 2
    Adding -m PEM fixed a very frustrating issue I was having with JWT signing. Thank you!! Dec 31, 2020 at 16:48
132

New keys with OpenSSH private key format can be converted using ssh-keygen utility to the old PEM format.

ssh-keygen -p -m PEM -f ~/.ssh/id_rsa

There is no need to downgrade to older OpenSSH just to achieve this result.

3
  • 26
    What's not clear in the accepted answer is that you don't need to create a new key pair. You can take your existing key and convert them with that command. It overwrites the file, so I think it's a good idea to make a backup before, just in case.
    – martintama
    Feb 22, 2019 at 11:15
  • 2
    Thanks for adding instructions on how to convert an existing private key to RSA format. Just saved my bacon! Aug 7, 2019 at 9:50
  • 1
    it does not change the content to me,
    – Whimusical
    May 12, 2020 at 17:09
5

Some elaboration on the above answers to provide a clear path for both the public and private key.

You can directly export (-e) your ssh keys to a pem format:

For your public key:

cd ~/.ssh
ssh-keygen -e -m PEM -f id_rsa > id_rsa.pub.pem

For your private key:

Things are a little tricker as ssh-keygen only allows the private key file to be change 'in-situ'. (i.e. it replaces your key file with the new file).

So you can keep your old file:

Given we are just exporting the file the <new pass phrase> can be identical to your <old pass phrase> (unless you want to change the pass phrase at the same time).

cd ~/.ssh
cp id_rsa id_rsa.bak
ssh-keygen -p -P "<old pass phrase>" -N "<new pass phrase>" -m PEM -f id_rsa 
cp id_rsa id_rsa.priv.pem
cp id_rsa.bak id_rsa

NOTE: it is a bad idea to pass your pass phrase as an argument to a cli app.

The secure method is:

cd ~/.ssh
cp id_rsa id_rsa.bak
ssh-keygen -p  -m PEM -f id_rsa 
cp id_rsa id_rsa.priv.pem
cp id_rsa.bak id_rsa

With this method you will be prompted for your old and new pass phrase.

Note: after converting your private key file to a .pem the file is now in clear text, this is bad.

1
  • 1
    Note that the -e option now requires -f in front of the input filename: ssh-keygen -e -m PEM -f id_rsa > id_rsa.pub.pem Nov 4, 2021 at 0:42

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