Re: [netconf] SSH private key format?

[Kent Watsen <[e-mail]>]

After a little digging…

1) `ssh-keygen` default private key format is a proprietary format:
    https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.key?annotate=HEAD <https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.key?annotate=HEAD>

2) `ssh-keygen` can told to save a generated private key to the standard 
     PKCS/CMS format using the “-m PEM” parameter

3) `ssh-keygen` can also convert its proprietary private key format to the 
     PKCS/CMS format (e.g., ssh-keygen -p -N "" -m PEM -f <filename>)

4) OpenSSH can read (but not write) PKCS#8/OneAsymmetricKey
     formats. See the 9th paragraph of the accepted answer here:
     https://superuser.com/questions/1477472/openssh-public-key-file-format <https://superuser.com/questions/1477472/openssh-public-key-file-format>

 5) strangely, for “ecdsa” keys only, creating the “PEM” format via (3) or (4)
     produces different EC keys (different number of fields), though both are
     still parsable via `openssl ec …`

More info here: https://www.thedigitalcatonline.com/blog/2018/04/25/rsa-keys <https://www.thedigitalcatonline.com/blog/2018/04/25/rsa-keys>

Conclusion:

  - It seems that both SSH and TLS private keys use the same PKCS/CMS and
    OneAsymmetricKey formats.

Kent // contributor



> On Feb 12, 2020, at 7:06 AM, Kent Watsen <[e-mail]> wrote:
> 
> Related to the crypto-types, keystore, and truststore drafts...
> 
> Everyone seems to agree that the *public* key format can be specified as:
> 
> 	leaf public-key {
> 	   type binary;
> 	   description
> 	      "The binary public key data for an SSH key, as
> 	       specified by RFC 4253, Section 6.6, i.e.:
> 
> 	         string    certificate or public key format identifier
> 	         byte[n]   key/certificate data.";
> 	   reference
> 	      "RFC 4253: The Secure Shell (SSH) Transport
> 	       Layer Protocol”;
> 	}
> 
> 	BTW, being “binary”, this produces a block a base64,
> 	a la https://tools.ietf.org/html/rfc4716#section-3.6 <https://tools.ietf.org/html/rfc4716#section-3.6>, but
> 	without the headers and footers, or EOL characters.
> 
> 	Also note that “key data” is underspecified.  “certificate
> 	data” is better, as PGP/X.509 certs are well-specified,
> 	though we have to *assume* the encoding is “DER".
> 
> 
> But what is the *private* key format?  (i.e., ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 or ~/.ssh/id_rsa).  
> 
> SSH-KEYGEN(1) says this:
> 
>      -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''.
> 
> FWIW, RFC 4716 does NOT define a private key format.  PKCS8 does define a private key format.  PKCS8 was obsoleted by RFC 5958, which defines the “OneAsymmetricKey" format, which is one of the formats used for TLS private keys.
> 
> That said, without actually trying to parse these private keys files,I suspect that they are actually raw private keys (e.g., RSAPrivateKey, ECPrivateKey, etc.) and not PKCS8/OneAsymmetricKey because, if they were a OneAsymmetricKey, there would be no need for different filenames...
> 
> Can anyone dig into this and confirm what these file formats are?
> 
> 
> Kent // contributor
> 
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