End-to-end encryption (E2EE) is a system of communication where only the communicating users can read the messages. In principle, it prevents potential eavesdroppers – including telecom providers, Internet providers, and even the provider of the communication service – from being able to access the cryptographic keys needed to decrypt the conversation. The systems are designed to defeat any attempts at surveillance and/or tampering because no third parties can decipher the data being communicated or stored. For example, companies that use end-to-end encryption are unable to hand over texts of their customers' messages to the authorities.
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Key exchange
In an E2EE system, encryption keys must only be known to the communicating parties. To achieve this goal, E2EE systems can encrypt data using a pre-arranged string of symbols, called a pre-shared secret (PGP), or a one-time secret derived from such a pre-shared secret (DUKPT). They can also negotiate a secret key on the spot using Diffie-Hellman key exchange (OTR).
Modern usage
Examples of end-to-end encryption include PGP, GnuPG, Protonmail, Mailfence, S/MIME, Inky, or pEp for email; OTR, iMessage, Signal, Threema, or WhatsApp for instant messaging; ZRTP or FaceTime for telephony; and TETRA for radio.
As of 2016, typical server-based communications systems do not include end-to-end encryption. These systems can only guarantee the protection of communications between clients and servers, meaning that users have to trust the third-parties who are running the servers with the original texts. End-to-end encryption is regarded as safer because it reduces the number of parties who might be able to interfere or break the encryption. In the case of instant messaging, users may use a third party client (e.g. Pidgin) to implement an end-to-end encryption scheme (e.g. OTR) over an otherwise non-E2EE protocol.
Some non-E2EE systems, for example Lavabit and Hushmail, have described themselves as offering "end-to-end" encryption when they did not. Other systems, such as Telegram and Google Allo, have been criticized for not having end-to-end encryption, which they offer, enabled by default.
Some backup services such as SpiderOak and Tresorit provide client-side encryption. The encryption they offer is not referred to as end-to-end encryption, because the services are not meant for communication between users.
Man-in-the-Middle attacks
End-to-end encryption ensures that data is transferred securely between endpoints. But, rather than try to break the encryption, an eavesdropper may impersonate a message recipient (during key exchange or by substituting his public key for the recipient's), so that messages are encrypted with a key known to the attacker. After decrypting the message, the snoop can then encrypt it with a key that he/she shares with the actual recipient, or his/her public key in case of asymmetric systems, and send the message on again to avoid detection. This is known as a man-in-the-middle attack.
Authentication
Most end-to-end encryption protocols include some form of endpoint authentication specifically to prevent MITM attacks. For example, one could rely on certification authorities or a web of trust. An alternative technique is to generate cryptographic hashes (fingerprints) based on the communicating users’ public keys or shared secret keys. The parties compare their fingerprints using an outside (out-of-band) communication channel that guarantees integrity and authenticity of communication (but not necessarily secrecy), before starting their conversation. If the fingerprints match, there is in theory, no man in the middle.
When displayed for human inspection, fingerprints are usually encoded into hexadecimal strings. These strings are then formatted into groups of characters for readability. For example, a 128-bit MD5 fingerprint would be displayed as follows:
43:51:43:a1:b5:fc:8b:b7:0a:3a:a9:b1:0f:66:73:a8Some protocols display natural language representations of the hexadecimal blocks. As the approach consists of a one-to-one mapping between fingerprint blocks and words, there is no loss in entropy. The protocol may choose to display words in the user's native (system) language. This can, however, make cross-language comparisons prone to errors. In order to improve localization, some protocols have chosen to display fingerprints as base 10 strings instead of hexadecimal or natural language strings. Modern messaging applications can also display fingerprints as QR codes that users can scan off each other's devices.
Endpoint security
The end-to-end encryption paradigm does not directly address risks at the communications endpoints themselves. Each users’ computer can still be hacked to steal his or her cryptographic key (to create a MITM attack) or simply read the recipients’ decrypted messages. Even the most perfectly encrypted communication pipe is only as secure as the mailbox on the other end.
Backdoors
Companies may also willingly or unwillingly introduce back doors to their software that help subvert key negotiation or bypass encryption altogether. In 2013, information leaked by Edward Snowden showed that Skype had a back door which allowed Microsoft to hand over their users' messages to the NSA despite the fact that those messages were officially end-to-end encrypted.