Re: Importing hostkey

[Anderson Sasaki <ansasaki@xxxxxxxxxx>]

----- Original Message -----
> From: "Michal Vaško" <mvasko@xxxxxxxxx>
> To: libssh@xxxxxxxxxx
> Sent: Thursday, January 23, 2020 10:47:49 AM
> Subject: Importing hostkey
> 
> Hello,
> we have recently found an issue with importing RSA host key of length 2048
> but presumably the problem may occur also with other key types/lengths. It
> seems to work fine when libssh is compiled with OpenSSL but not with gcrypt.
> The problem is tracked on GitHub [1]. My query is whether anyone can tell me
> more about this/had the same problem and fixed it somehow. It would be best
> if there was a format compatible with both backend libraries. Thanks for any
> input.
> 
> Some more details regarding the problem, the key is generated by:
> openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -outform PEM
> 
> Then, the header and footer is removed only to be added later [2] and stored
> back into a file. This file is then imported [3], which fails.
> 
> Regards,
> Michal
> 
> [1] https://github.com/CESNET/Netopeer2/issues/534
> [2]
> https://github.com/CESNET/libnetconf2/blob/devel/src/session_server_ssh.c#L43
> [3]
> https://github.com/CESNET/libnetconf2/blob/devel/src/session_server_ssh.c#L1291
> 
> 

Hello,

This happens because OpenSSL uses the PKCS#8 format when writing RSA keys to PEM files by default. When compiled with OpenSSL we use its parser, which supports PKCS#8 format.

Unfortunately, AFAIK, libgcrypt does not have a PEM file parser; what we use is an internally implemented parser which only supports the legacy OpenSSL format (also called "SSLeay format" or "traditional format").

You can convert the PKCS#8 PEM file to the legacy format by running:

$ openssl rsa -in pkcs8-file.pem -out legacy.pem

Then such key in legacy format can be used both with OpenSSL and libgcrypt.

Regards,
Anderson