ssh-keygen fails to read key generated by itself

Question

Using OpenSSH_8.9p1 Ubuntu-3ubuntu0.7 on WSL, ssh-keygen refuses a key it previously generated, outputting error:

do_convert_from_ssh2: parse key: invalid format

Reproduction

$ cd /tmp/some_directory
$ ssh-keygen -f id_rsa -m RFC4716 -N ''
[... nominal output ...]

$ ssh-keygen -i -f id_rsa -m RFC4716                                                                                             
do_convert_from_ssh2: parse key: invalid format

$ ssh-keygen -i -f id_rsa                                                                                         
do_convert_from_ssh2: parse key: invalid format

As this is a test/trashed private key, I can share it with you:

-----BEGIN OPENSSH PRIVATE KEY-----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-----END OPENSSH PRIVATE KEY-----

(note: lines are correctly separated by singles 0x0a, and the file correctly shows a final 0x0a)

My end goal is to convert this key for use with openssl pkeyutl -decrypt -inkey ${converted_key_file}. What am I doing wrong?

Answer 1

Contrary to what its documentation says, OpenSSH's ssh-keygen can only import "RFC4716" (SSH.COM) private keys but not write them, so the key it did write it is in the OpenSSH format – not in the RFC4716 format that you're now asking it to read.

Header	Format
-----BEGIN OPENSSH PRIVATE KEY-----	OpenSSH
---- BEGIN SSH2 ENCRYPTED PRIVATE KEY ----	"RFC4716" (SSH.COM/Tectia/SecureCRT)
-----BEGIN PRIVATE KEY-----	PKCS#8 (standard for TLS certs)
-----BEGIN RSA PRIVATE KEY-----	"PEM" (PKCS#1), old TLS & old OpenSSH

(As a side note, RFC 4716 only specifies the format for a public key, not for a private key. Although a corresponding private key format exists, it was not published as an IETF standard, which might be one of the reasons OpenSSH does not write such keys.)

Most importantly, though, this format will be useless as far as your goal to use it with OpenSSL tools. This is not the "PEM" key format that OpenSSL works with – RFC4716 is basically the "Tectia SSH / VanDyke SecureCRT / MultiNet SSH" key type, and despite looking roughly similar on the outside, it's a completely different format internally.

PuTTY does support writing private keys in the "RFC4716" format (and more accurately calls it the "ssh.com" format), so if you needed to use your keypair with SecureCRT or Tectia SSH, you could use puttygen -O private-sshcom.

But since you want to use the key with OpenSSL, you should be converting it to PKCS8 or PEM formats instead. (Both are natively supported by OpenSSL – PKCS8 is the more modern one that OpenSSL 1.1/3.x generates, while PEM is the "legacy" format that OpenSSL ≤1.0 used to generate; it also happens to be the format that ssh-keygen used to generate.)

• Generate new key in PKCS#8 format:

  ssh-keygen -f id_rsa -t rsa -m PKCS8
  

• Convert an existing key to PKCS#8 format:

  ssh-keygen -f id_rsa -p -m PKCS8
  

Alternatively, you would get exactly the same result if you generated the key using OpenSSL (since, after all, you have the openssl tool available):

• Newer syntax:

  openssl genpkey -algorithm rsa -pkeyopt rsa_keygen_bits:2048 -out id_rsa
  openssl genpkey -algorithm ec -pkeyopt ec_paramgen_curve:P-256 -out id_ecdsa
  

• Older syntax:

  openssl genrsa 2048 -out id_rsa
  

  (Will output a PKCS#8 key by default; use -traditional to output a PEM key.)

If you already have an "RFC4716" SSH.COM private key, you can convert it to an OpenSSL compatible format using:

• OpenSSH using the -i option:

  umask 077; ssh-keygen -i -f id_rsa.priv > id_rsa.pem
  

  (Always produces PEM – they forgot to add options for new formats – but you can pipe it through openssl pkey to get PKCS#8.)

• PuTTYgen (which calls PEM the "OpenSSH format" as it used to be the only format OpenSSH had – not to be confused with "OpenSSH format (new)" which is the actual OpenSSH format):

  puttygen id_rsa.priv -o private-openssh -O id_rsa.pem