Tag Archives: SSH

RSA/DSA ssh(d) keys, a synthetic guide

There is a lot of useless and cryptic information in regard to any type of encryption, typical as per USA’s FUD standards. I’ll post here a synthesis of the steps necessary to wave plain text password logins goodbye.

I’ll assume you already have the private/public key couple by now, if not you can use puttygen. This topic is well covered, although they have a tendency to suggest a low level of encryption. Isn’t it strange how for apparently “anybody” an 8 letter password, or a 2048 bit key, is enough for everyone? For the record, I used a 4096 bit DSA key.

I will also assume that you’re setting up a server on Linux, so ymmv.

Coming back to the topic at hand, you have a private key, that you use to login from your computer, and a public key that you will deploy to one or more PCs/servers. The public key will probably look like this:

This won’t work in most cases, as SSHD expects a certain format. You will then have to convert that key into this:

The beginning of the line is ssh-dss for DSA keys, ssh-rsa for RSA keys. With this line of text in hand, you can open ~/.ssh/authenticated_keys on your servers, copy the key data into it, and save.

The last thing to do is to reconfigure the sshd.

While checking the configuration I noticed that the ephemeral key size (ServerKeyBits) was defaulted to 1 kilobit. ONE FREAKING KILOBIT. To give you a comparison, in 2002 on IRC channels we used DH with 2048 bits of encryption. That’s 13 years ago. For chat. You might want to turn it up several notches.

For the server to actually use the key you provided, you will need to uncomment the AuthorizedKeysFile, keep in mind that the path may differ. It could be .ssh/authorized_keys on CentOS, %h/.ssh/authorized_keys on Ubuntu, so on and so forth.

AFTER you made sure you can actually log in with your DSA/RSA key, you will disable plain text authentication by uncommenting AuthorizedKeysFile and setting it to no.

This is all the black magic involved in it, without the convoluted mess that always surrounds OpenSSL/SSHD documentations.