premail(1) on Linux
The text b73c48796ad4, as Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze), Debian 5.0 (Lenny), Debian 4.0 (Etch), Debian 3.1 (Sarge), Debian 3.0 (Woody), Debian 2.1 (Slink) shipped it.
An E-Mail Privacy Package. Easy E-Mail Encryption, Decryption, Signing and Anonymization.