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ACMEv2 client written in plain C code with minimal dependencies
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README.md

uacme

lightweight client for the RFC8555 ACMEv2 protocol, written in plain C code with minimal dependencies (libcurl and one of GnuTLS, OpenSSL or mbedTLS). The ACMEv2 protocol allows a Certificate Authority (https://letsencrypt.org is a popular one) and an applicant to automate the process of verification and certificate issuance. The protocol also provides facilities for other certificate management functions, such as certificate revocation.

Features

  • Written in C - It runs on any unix machine, including Linux, BSD, ...
  • Minimal dependencies - Other than the standard C library, uacme depends only on libcurl and one of GnuTLS, OpenSSL or mbedTLS. It does all the network communications and crypto without spawning external processes. Particularly when using mbedTLS, it is small enough to run on embedded systems with severe RAM and program memory restrictions (such as OpenWRT routers, for example). This is in contrast to solutions based on python or shell scripts, which may well be a few hundred lines but are ugly, brittle and also require many other large applications such as python or openssl to work.
  • Native ECC support - Elliptic Curve keys and certificates can be generated with a commmand line option (-t EC)
  • Easily extensible - It optionally calls an external hook program with the tokens required for domain authorization by the server. The hook program can be an executable, shell script, perl script, python script, or any file that the operating system can execute.
  • ACME challenge agnostic - It provides the user or hook program with all tokens and information required to complete any challenge type (including http-01, dns-01 and others) but leaves the task of setting up and cleaning up the challenge environment to the user or hook. An example shell script to handle http-01 challenges is provided.
  • Can run as a cron job - to renew certificates automatically when needed, even for remote machines
  • Robust - It checks every operation, retrying or failing gracefully as appropriate
  • Detailed error reporting - By default totally quiet when everything works ok, it reports precise and detailed error information on stderr when something goes wrong. Optionally it can also print debug information by specifying the --verbose flag once or more.

Installation

mkdir uacme
wget -O - https://github.com/ndilieto/uacme/archive/upstream/latest.tar.gz | tar zx -C uacme --strip-components=1
cd uacme
./configure --disable-maintainer-mode
make install

You'll also find the latest release in the git repository:

git clone -b upstream/latest https://github.com/ndilieto/uacme

Getting started

Once you have obtained uacme (see Installation above), the next step is to use

uacme -v -c /path/to/uacme.d new

to create an ACME account. This will create the configuration folder and account private key:

/path/to/uacme.d/private/key.pem

You can then issue a certificate for your domain by doing

uacme -v -c /path/to/uacme.d issue www.your.domain.com

If everything goes well, uacme will ask you to set up a challenge, for example

uacme: challenge=http-01 ident=www.your.domain.com token=kZjqYgAss_sl4XXDfFq-jeQV1_lqsE76v2BoCGegFk4
key_auth=kZjqYgAss_sl4XXDfFq-jeQV1_lqsE76v2BoCGegFk4.2evcXalKLhAybRuxxE-HkSUihdzQ7ZDAKA9EZYrTXwU

Note the challenge type in the example is http-01 which means you should set up your web server to serve a URL based on the token:

http://www.your.domain.com/.well-known/acme-challenge/kZjqYgAss_sl4XXDfFq-jeQV1_lqsE76v2BoCGegFk4

The URL must return a text file containing a single line with the key authorization:

kZjqYgAss_sl4XXDfFq-jeQV1_lqsE76v2BoCGegFk4.2evcXalKLhAybRuxxE-HkSUihdzQ7ZDAKA9EZYrTXwU

once you set up the above, you can then type 'y' followed by a newline on uacme's input and it will proceed with the challenge. If everything goes well, the following will be created:

/path/to/uacme.d/www.your.domain.com/cert.pem
/path/to/uacme.d/private/www.your.domain.com/key.pem

Note other types of challenges are possible. If you type anything other than 'y', uacme will skip the challenge and propose a different one. The easiest is http-01 but any other type can be dealt with. Keep in mind that challenge types may be served in random order by the server. Do not make any assumptions and read uacme's output carefully.

Automating updates

Use the -h flag:

uacme -v -c /path/to/uacme.d -h /usr/share/uacme/uacme.sh issue www.your.domain.com

or (depending on your installation)

uacme -v -c /path/to/uacme.d -h /usr/local/share/uacme/uacme.sh issue www.your.domain.com

This will use the example uacme.sh script included in the distribution to set up http-01 challenges. You might need to edit the script to match your webserver's environment.

Once everything works correctly you can also set up cron, for example

6 15 * * * /usr/bin/uacme -c /path/to/uacme.d -h /usr/share/uacme/uacme.sh issue www.your.domain.com 

The cron job will automatically update the certificate when needed. Note the absence of -v flag, this makes uacme only produce output upon errors.

Note also that you will need to restart or reload any service that uses the certificate, to make sure it uses the renewed one. This is system and installation dependent. I normally put the necessary instructions in another script (for example /usr/share/uacme/reload.sh) that is executed when uacme returns 0 (indicating the certificate has been reissued).

6 15 * * * /usr/bin/uacme -c /path/to/uacme.d -h /usr/share/uacme/uacme.sh issue www.your.domain.com && /usr/share/uacme/reload.sh

Documentation

There is a regular unix man page in the distribution, also available here

Bugs and suggestions

If you believe you have found a bug, please log it at https://github.com/ndilieto/uacme/issues

If you have any suggestions for improvements, pull requests are welcome.

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