This is Dropbear, a smallish SSH server and client.
https://matt.ucc.asn.au/dropbear/dropbear.html

INSTALL has compilation instructions.

MULTI has instructions on making a multi-purpose binary (ie a single binary
which performs multiple tasks, to save disk space)

SMALL has some tips on creating small binaries.

A mirror of the Dropbear website and tarballs is available at https://dropbear.nl/mirror/

Please contact me if you have any questions/bugs found/features/ideas/comments etc :)
There is also a mailing list http://lists.ucc.gu.uwa.edu.au/mailman/listinfo/dropbear

Matt Johnston
matt@ucc.asn.au


In the absence of detailed documentation, some notes follow:
============================================================================

Server public key auth:

You can use ~/.ssh/authorized_keys in the same way as with OpenSSH, just put
the key entries in that file. They should be of the form:

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAwVa6M6cGVmUcLl2cFzkxEoJd06Ub4bVDsYrWvXhvUV+ZAM9uGuewZBDoAqNKJxoIn0Hyd0Nk/yU99UVv6NWV/5YSHtnf35LKds56j7cuzoQpFIdjNwdxAN0PCET/MG8qyskG/2IE2DPNIaJ3Wy+Ws4IZEgdJgPlTYUBWWtCWOGc= someone@hostname

You must make sure that ~/.ssh, and the key file, are only writable by the
user. Beware of editors that split the key into multiple lines.

Dropbear supports some options for authorized_keys entries, see the manpage.

============================================================================

Client public key auth:

Dropbear can do public key auth as a client, but you will have to convert
OpenSSH style keys to Dropbear format, or use dropbearkey to create them.

If you have an OpenSSH-style private key ~/.ssh/id_rsa, you need to do:

dropbearconvert openssh dropbear ~/.ssh/id_rsa  ~/.ssh/id_rsa.db
dbclient -i ~/.ssh/id_rsa.db <hostname>

Dropbear does not support encrypted hostkeys though can connect to ssh-agent.

============================================================================

If you want to get the public-key portion of a Dropbear private key, look at
dropbearkey's '-y' option.

============================================================================

To run the server, you need to generate server keys, this is one-off:
./dropbearkey -t rsa -f dropbear_rsa_host_key
./dropbearkey -t dss -f dropbear_dss_host_key
./dropbearkey -t ecdsa -f dropbear_ecdsa_host_key
./dropbearkey -t ed25519 -f dropbear_ed25519_host_key

or alternatively convert OpenSSH keys to Dropbear:
./dropbearconvert openssh dropbear /etc/ssh/ssh_host_dsa_key dropbear_dss_host_key

You can also get Dropbear to create keys when the first connection is made -
this is preferable to generating keys when the system boots. Make sure 
/etc/dropbear/ exists and then pass '-R' to the dropbear server.

============================================================================

If the server is run as non-root, you most likely won't be able to allocate a
pty, and you cannot login as any user other than that running the daemon
(obviously). Shadow passwords will also be unusable as non-root.

============================================================================

The Dropbear distribution includes a standalone version of OpenSSH's scp
program. You can compile it with "make scp", you may want to change the path
of the ssh binary, specified by _PATH_SSH_PROGRAM in options.h . By default
the progress meter isn't compiled in to save space, you can enable it by 
adding 'SCPPROGRESS=1' to the make commandline.