Transport Layer Security

According to Wikipedia:

Transport Layer Security (TLS), and its now-deprecated predecessor, Secure Sockets Layer (SSL), are cryptographic protocols designed to provide communications security over a computer network. Several versions of the protocols find widespread use in applications such as web browsing, email, instant messaging, and voice over IP (VoIP). Websites can use TLS to secure all communications between their servers and web browsers.

Implementations

There are multiple TLS implementations available. OpenSSL should already be installed on your system as it is an indirect dependency of the base meta package (base > coreutils > openssl). GnuTLS might already be installed on your system as it is required by many packages.

OpenSSL — A robust, commercial-grade, and full-featured toolkit for the TLS and SSL protocols; also a general-purpose cryptography library.

https://www.openssl.org/ || openssl

GnuTLS — A free software implementation of the TLS, SSL and DTLS protocols. Offers APIs for X.509, PKCS #12 and other structures.

https://www.gnutls.org/ || gnutls

Network Security Services (NSS) — Implementation of cryptographic libraries supporting TLS/SSL and S/MIME. Also supports TLS acceleration and smart cards.

https://developer.mozilla.org/en-US/docs/Mozilla/Projects/NSS || nss

mbed TLS — Portable SSL/TLS implementation, aka PolarSSL.

https://tls.mbed.org/ || mbedtls

LibreSSL — A version of the TLS/crypto stack forked from OpenSSL in 2014 by the OpenBSD project, with goals of modernizing the codebase and improving security.

https://www.libressl.org/ || libressl

Certificate authorities

With TLS one of a set of certificate authorities (CAs) checks and signs for the authenticity of a public key certificate from a server. A client connecting to the server via TLS may verify its certificate's authenticity by relying on a digital signature of CA. To check the digital signature a client must have a public key of CA, obtained via a separate path and stored as a self-signed certificate. On Arch Linux the default set of CA certificates is provided by the ca-certificates package.

Arch Linux provides a centralized system-wide interface for managing CA certificates. This interface is the library from the package, which provides PKCS #11 API for certificates, stored in (the token "Default Trust") and (the token "System Trust").

For using the interface from a command line, the p11-kit package provides the trust(1) utility.

For libraries, that have not been ported to PKCS #11 and use a custom logic for managing CA certificates, the package provides the script. It copies CA certificates obtained through the centralized interface to and .

An overview of mechanisms for loading a default set of CA certificates

Implementation	Mechanism	Arch Linux configuration
OpenSSL	Provides API functions that load the certificates from a hardcoded directory or file. .	A default file is , a default directory is .
GnuTLS	Provides an API function that loads the certificates from a hardcoded directory, file, or configured PKCS #11 modules. In the last case, a hardcoded URL allows to load either an arbitrary trusted certificate, or trusted CA certificates on modules, marked with , optionally with additional filtration criteria. , .	Loads all trusted CA certificates from configured PKCS #11 modules, marked with .
Network Security Services	Automatically loads the certificates from a dynamically configured list of PKCS #11 modules, managed with a dedicated API. Configuration can be stored in any directory, pointed by a user. The list always contains the built-in module that stores objects in the same user-provided directory. , .
mbed TLS	A user should load the certificates. .
LibreSSL	Provides an API function that loads the certificates from a hardcoded directory or file. libressl-SSL_CTX_load_verify_locations(3).	A default file is `/etc/libressl/cert.pem`, a default directory is .

Trust management

For trust management the trust(1) utility is provided. The utility operates on a list of PKCS #11 modules with the setting, sorted by the setting. See for details about configuration of modules.

List trust store items

$ trust list

Add a certificate to a trust store

# trust anchor certificate.crt

The certificate should be in the persistence, DER or PEM format (including the OpenSSL-specific trusted certificate format). This command stores the certificate in the first writable token found by querying the list of modules.

Remove a certificate from a trust store

$ trust anchor --remove 'pkcs11:id=%00%11%22%33%44%55%66%77%88%99%AA%BB%CC%DD%EE%FF%00%11%22%33;type=cert'

Override default trust

The token representing certificates in is always write-protected, so to distrust some default certificate authority run such a command:

$ trust extract --format=pem-bundle --filter='pkcs11:id=%00%11%22%33%44%55%66%77%88%99%AA%BB%CC%DD%EE%FF%00%11%22%33;type=cert' /etc/ca-certificates/trust-source/blocklist/untrusted_authority.pem

Obtaining a certificate

The first step is to generate a private key. Before generating the key, set a restrictive file mode creation mask with umask (for example ). This ensures that the keys written by openssl are read-protected.

Note: Unlike in other Linux distributions, the openssl package in Arch does not properly safeguard the /etc/ssl/private directory, so a umask is needed. See FS#43059.

Keys can use either elliptic curve or RSA algorithms.

Elliptic curves are newer algorithms and are becoming increasingly adopted for modern systems. A 256-bit elliptic curve key is expected to provide sufficient security through the year 2030 . Curve25519 is an elliptic curve algorithm which has good security and performance properties.

RSA is an older cryptosystem and has higher compatibility, especially with clients that do not support recent versions of TLS. However, RSA relies on factorization, which is an area of cryptography which may be becoming weaker due to the development of faster factorization algorithms . A 2048-bit RSA private key is expected to provide security through most of the 2020s . A 4096-bit RSA private key is expected to provide security for longer (barring major advancements in factorization), but has a very large performance impact. The performance difference can be benchmarked with openssl speed rsa2048 rsa4096 .

After the key is generated, a certificate can be obtained from a certificate authority with a Certificate Signing Request (CSR), or a certificate may be self-signed. While self-signed certificates can be generated easily, clients will reject them by default, meaning that every client needs to be configured to trust the self-signed certificate.

For the actual generation commands refer to the article of the used implementation:

Server-side recommendations

Because there are various attacks against TLS the best practices should be considered:

Checking TLS

Programs to check TLS:

Websites to check TLS:

https://dev.ssllabs.com/ssltest/ (only HTTPS)
https://www.checktls.com/ (only email)
https://www.immuniweb.com/ssl/ (any port)
https://tls.imirhil.fr/tls (any port)

Miscellaneous

ACME clients

The Automated Certificate Management Environment (ACME) protocol lets you request valid X.509 certificates from certificate authorities, like Let's Encrypt.

OCSP

The Online Certificate Status Protocol (OCSP) is supported by Firefox. Chromium has its own mechanism.

HSTS

The HTTP Strict Transport Security (HSTS) mechanism is supported by Firefox, Chromium and wget ().

DNS CAA

See Wikipedia:DNS Certification Authority Authorization.

gollark: Or raw processor microcode some days, for that performant performance.

gollark: I write C by hand with the type checker turned off.

gollark: As someone who never makes any mistakes, I don't need a programming language to hold my hand!

gollark: And the thing (a mobile bee deployment platform) needs that DMP thing to track its orientation.

gollark: The code is all running on an RPi, but the "DMP" features in the MPU6050 are poorly documented and the reverse engineered drivers are in C++.