mkj/dropbear

386-class CPUs

If compiling for a 386-class CPU, you must add CFLAGS=-DLTC_NO_BSWAP to prevent libtomcrypt from using 486+ instructions.

PTY Allocation

Dropbear attempts to detect the best PTY allocation method automatically. If openpty() (where HAVE_OPENPTY is defined in config.h) fails, you can use --disable-openpty, but be aware this may require manual device creation for /dev/pty?? and /dev/tty??.

The Dropbear distribution includes a standalone version of OpenSSH's scp.

Compilation steps:

• Run make scp to build the binary.
• Customizing the SSH path: If you need to change the path to the ssh binary, modify _PATH_SSH_PROGRAM in options.h before compiling.
• Enabling the progress meter: By default, the progress meter is disabled to save space. To enable it, include SCPPROGRESS=1 in your make command.

Dropbear supports public key authentication using the ~/.ssh/authorized_keys file, following the same convention as OpenSSH.

Requirements:

• Key entries must be in the standard format: ssh-rsa <KEY_DATA> <COMMENT>.
• Permissions must be strictly enforced: ~/.ssh and the key files must be writable only by the user.
• Warning: Avoid using text editors that automatically split long key strings into multiple lines, as this will break the key.

For advanced options for authorized_keys entries, refer to the local manpage.

ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIEAwVa6M6cGVmUcLl2cFzkxEoJd06Ub4bVDsYrWvXhvUV+ZAM9uGuewZBDoAqNKJxoIn0Hyd0NkyU99UVv6NWV/5YSHtnf35LKds56j7cuzoQpFIdjNwdxAN0PCET/MG8qyskG/2IE2DPNIaJ3Wy+Ws4IZEgdJgPlTYUBWWtCWOGc= someone@hostname

The project is built using make. Standard targets include:

• make: Build the library.
• make clean: Remove build artifacts.
• make install: Install the library to the system.

Note: For detailed instructions on building specifically for GNU Linux, refer to the "Building LibTomMath" section in the doc/bn.pdf documentation.

Tests are located in the demo/ directory and can be executed in two different modes:

1. Standalone Tests: Creates a single binary that runs several internal test routines.

   • Command: make test

2. Opponent Testing (mtest): A specialized mode where libtommath (the 'opponent') is tested against test vectors generated by an alternative MPI library (mtest).

   • Build the opponent: make mtest_opponent
   • Build the generator: make mtest
   • Execute the test: ./mtest/mtest | ./mtest_opponent

# Standalone test
make test

# Opponent test against mtest
make mtest
make mtest_opponent
./mtest/mtest | ./mtest_opponent

When using Dropbear as a client (dbclient), you cannot use OpenSSH-style private keys directly. You must convert them to the Dropbear format using dropbearconvert.

To convert an OpenSSH private key and then connect to a host:

1. Convert the key using dropbearconvert openssh dropbear <input_path> <output_path>.
2. Use the converted key with dbclient -i.

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

Dropbear allows fine-grained configuration of cryptographic algorithms, SSH features, and file paths through header files.

• localoptions.h: Located in the build directory. This is a local file (not tracked by git) where you can define custom build options. Use this to remove unneeded ciphers and algorithms to reduce binary size.
• src/default_options.h: Contains the default available options.
• src/distrooptions.h: Used by distributions to set specific configuration options if the file exists.

By default, if OpenSSH is installed, Dropbear is configured not to start to avoid port conflicts on port 22. To run both services simultaneously, you must modify the Dropbear configuration file to allow it to start and assign it a different port than OpenSSH.

1. Open /etc/default/dropbear in an editor.
2. Set NO_START to 0.
3. Change the port number assigned to Dropbear to something other than 22.