Isolated creation of Let's Encrypt's TLS certificates

This post explains why UI Drafter issues its TLS certificates from an orchestration server, as opposed to from each server. Also, the chosen frequency, diversity, and key strengths.

Challenge type

DNS challenges are preferred because they require fewer privileges than the HTTP ones. For instance, they only need to create TXT records that start with an _acme-challenge subdomain. For example:

_acme-challenge.example.com.     120 IN TXT "rAnd0mK"
_acme-challenge.foo.example.com. 120 IN TXT "43aZ09i"

On the other hand, HTTP challenges need a program on a live server; write permissions on a servable directory; and 'pass out' firewall exceptions without IP scope:

Renewal frequency

UI Drafter renews certificates at least once a month per server.

Let's Encrypt certificates last for 90 days, and can be renewed up to 5 times a week. Josh Aas, from the Internet Security Research Group (ISRG), recommends renewing them every 60 days in his 2015 post:

Diversity

Each server has a different certificate. As UI Drafter doesn't need stateful load balancing, this way mitigates the decryption of all the traffic of a particular user on a given period. In other words, an adversary hoarding encrypted traffic has to break more certificates.

Requisites

• acme.sh
• a DNS provider on this list

Although this guide uses Cloudflare®, that list shows how to adapt other providers. If your provider is not on that list, or if its API isn't compatible with your security polices, you could use a different domain than the one the certificate is going to be issued for. For that, see challenging with an alias.

Cloudflare token

Create a custom API Token with these permissions:

Zone | Zone | Read
Zone | DNS  | Edit

Cloudflare doesn't let you limit the privileges to "Only edit TXT records", neither allow-listing subdomains. But other providers might, as suggested by Joona Hoikkala in his article.

Issuing a certificate

Cloudflare-specific

export CF_Token=TheToken
export CF_Account_ID=TheAccountId

./acme.sh --issue --dns dns_cf -k ec-256 --force \
  -d     example.com \
  -d foo.example.com

Key

ec-256 is stronger than the default key (RSA 2048 bits). Check out the compatibility list.

Force

--force for being able to renew the certificate before the next renewal time (60 days by default).

Deployment

The previous command creates the certificate and key in a directory next to where the acme.sh script is.

DIR=~/.acme.sh/example.com_ecc

rsync $DIR/fullchain.cer   your_live_server:
rsync $DIR/example.com.key your_live_server: