Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication. PGP is used for signing, encrypting, and decrypting texts, e-mails, files, directories, and whole disk partitions and to increase the security of e-mail communications. Phil Zimmermann developed PGP in 1991.
Public-key cryptography, or asymmetric cryptography, is the field of cryptographic systems that use pairs of related keys. Each key pair consists of a public key and a corresponding private key. Key pairs are generated with cryptographic algorithms based on mathematical problems termed one-way functions. Security of public-key cryptography depends on keeping the private key secret; the public key can be openly distributed without compromising security.
GNU Privacy Guard is a free-software replacement for Symantec's cryptographic software suite PGP. The software is compliant with RFC 4880, the IETF standards-track specification of OpenPGP. Modern versions of PGP are interoperable with GnuPG and other OpenPGP-compliant systems. GnuPG is however expected, as of April 2024, to break compliance with the upcoming revision of OpenPGP and thus with other implementations that will continue to comply.
An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.
In cryptography, a web of trust is a concept used in PGP, GnuPG, and other OpenPGP-compatible systems to establish the authenticity of the binding between a public key and its owner. Its decentralized trust model is an alternative to the centralized trust model of a public key infrastructure (PKI), which relies exclusively on a certificate authority. As with computer networks, there are many independent webs of trust, and any user can be a part of, and a link between, multiple webs.
Hushmail is an encrypted proprietary web-based email service offering PGP-encrypted e-mail and vanity domain service. Hushmail uses OpenPGP standards. If public encryption keys are available to both recipient and sender, Hushmail can convey authenticated, encrypted messages in both directions. For recipients for whom no public key is available, Hushmail will allow a message to be encrypted by a password and stored for pickup by the recipient, or the message can be sent in cleartext. In July 2016, the company launched an iOS app that offers end-to-end encryption and full integration with the webmail settings. The company is located in Vancouver, British Columbia, Canada.
Enigmail is a data encryption and decryption extension for Mozilla Thunderbird and the Postbox that provides OpenPGP public key e-mail encryption and signing. Enigmail works under Microsoft Windows, Unix-like, and Mac OS X operating systems. Enigmail can operate with other mail clients compatible with PGP/MIME and inline PGP such as: Microsoft Outlook with Gpg4win package installed, Gnome Evolution, KMail, Claws Mail, Gnus, Mutt. Its cryptographic functionality is handled by GNU Privacy Guard.
Key exchange is a method in cryptography by which cryptographic keys are exchanged between two parties, allowing use of a cryptographic algorithm.
S/MIME is a standard for public-key encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFC 8551. It was originally developed by RSA Data Security, and the original specification used the IETF MIME specification with the de facto industry standard PKCS #7 secure message format. Change control to S/MIME has since been vested in the IETF, and the specification is now layered on Cryptographic Message Syntax (CMS), an IETF specification that is identical in most respects with PKCS #7. S/MIME functionality is built into the majority of modern email software and interoperates between them. Since it is built on CMS, MIME can also hold an advanced digital signature.
Internet security is a branch of computer security. It encompasses the Internet, browser security, web site security, and network security as it applies to other applications or operating systems as a whole. Its objective is to establish rules and measures to use against attacks over the Internet. The Internet is an inherently insecure channel for information exchange, with high risk of intrusion or fraud, such as phishing, online viruses, trojans, ransomware and worms.
In cryptography and steganography, plausibly deniable encryption describes encryption techniques where the existence of an encrypted file or message is deniable in the sense that an adversary cannot prove that the plaintext data exists.
The following tables compare general and technical features of notable email client programs.
Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring and possibly modifying a message.
DomainKeys Identified Mail (DKIM) is an email authentication method designed to detect forged sender addresses in email, a technique often used in phishing and email spam.
Email encryption is encryption of email messages to protect the content from being read by entities other than the intended recipients. Email encryption may also include authentication.
Invisible mail, also referred to as iMail, i-mail or Bote mail, is a method of exchanging digital messages from an author to one or more recipients in a secure and untraceable way. It is an open protocol and its java implementation (I2P-Bote) is free and open-source software, licensed under the GPLv3.
Proton Mail is a Swiss end-to-end encrypted email service founded in 2013 headquartered in Plan-les-Ouates, Switzerland. It uses client-side encryption to protect email content and user data before they are sent to Proton Mail servers, unlike other common email providers such as Gmail and Outlook.com. The service can be accessed through a webmail client, the Tor network, Windows, macOS and Linux (beta) desktop apps and iOS and Android apps.
Mailvelope is free software for end-to-end encryption of email traffic inside of a web browser that integrates itself into existing webmail applications. It can be used to encrypt and sign electronic messages, including attached files, without the use of a separate, native email client using the OpenPGP standard.
Mailfence is secure encrypted email service that offers OpenPGP based end-to-end encryption and digital signatures. It was launched in November 2013 by Belgium-based company ContactOffice Group that has been operating an online collaboration suite since 1999.
OpenKeychain is a free and open-source mobile app for the Android operating system that provides strong, user-based encryption which is compatible with the OpenPGP standard. This allows users to encrypt, decrypt, sign, and verify signatures for text, emails, and files. The app allows the user to store the public keys of other users with whom they interact, and to encrypt files such that only a specified user can decrypt them. In the same manner, if a file is received from another user and its public keys are saved, the receiver can verify the authenticity of that file and decrypt it if necessary. As of August 2021, it is no longer actively developed.
