Open SSH key formats

Open SSH key formats

Open SSH has had a number of key formats over the years. As of version 7.0 some of the older keys are no longer working.

We can start by looking in the .ssh/authorized_keys file. This is the ssh-dss format:

ssh-dss AAAAB3NzaC1kc3MAAACBA...x= userid

The AAAAB3NzaC1kc3M simply encodes "7 characters ssh-dss" in base 64

The RSA1 format has key length, a prime and the key as digits:

 1024 37 1234567......1234 userid

This format was used in the original ssh version 1.

The current default format (as of Aug 2015) is an ssh-rsa key

ssh-rsa AAAAB3NzaC1yc2EAAAABIwA...x= userid

The AAAAB3NzaC1yc2E encodes "ssh-rsa"

The Ed25519 Elliptic curve keys:

ssh-ed25519 userid@host

This is a newer type of key and isn't supported on older versions of SSH.

The NIST p-256 (FIPS 186 standard) Elliptic curve keys:

ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAA....8= userid@host

There are reports that the NIST standard was weakened by the NSA.

Elliptic curve ECDH has the same weakness has the DSA if the random number generator is bad.

To answer the question of which type of key to use, there are several factors. DSA keys used to work on all versions of ssh but they have known weaknesses and are being phased out. There is a known weakness if the hosts random number generator is broken. DSA keys must be 1024 bits which is considered the minium size for that type of key today. Best practice at this point is to use a 2048 or 4096 bit RSA key. Note that other key sizes work too and SSH is happy with a 2049 bit RSA key.

OpenSSH Version 7.0 migration

For the ssh client:
ssh -oPubkeyAcceptedKeyTypes=+ssh-dss -i some-dss-key hostname

For the sshd server:

/home/src/archive/openssh-7.0p1/sshd -oPubkeyAcceptedKeyTypes=+ssh-dss

Then look in the logs:

grep sshd sshlog | grep Accepted | grep DSA | sed s/.*\ for\ // | sed s/\ from\ .*// | sort -u

What size are the keys?

These are approximate and were from the wc program.
Type Size Private size
in bytes
Public size
in bytes
RSA 1024 887 241
RSA 2048 1679 409
RSA 4096 3243 753

Bad common and well known DH primes

Old mod_ssl from apache 1.3 days uses hard coded well known primes. The built in primes can be replaced because ssl_engine_dh.c happens to be both C source and a perl program that knows how to update its self.

For example, gave this wrning:
Uses common DH primes Yes Replace with custom DH parameters if possible

The solution is simple:
$ cd apache_1.3.42/src/modules/ssl
perl ssl_engine_dh.c
cd ../../.. && make && make install

With other code, look for the function "static DH *get_dh1024()" and see if it was generated with "openssl dh -noout -text -in dh1024.pem; openssl dh -noout -C -in dh1024.pem"

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