Сквозное шифрование: защита переписки от посторонних глаз.

Encryption model where only the sender and recipient can read the ciphertext

End to end encryption (E2EE) is a private communication system in which only communicating users can participate. As such, no one, including the communication system provider, telecom providers, Internet providers or malicious actors, can access the cryptographic keys needed to converse. End to end encryption is intended to prevent data being read or secretly modified, other than by the true sender and recipient(s). The messages are encrypted by the sender but the third party does not have a means to decrypt them, and stores them encrypted. The recipients retrieve the encrypted data and decrypt it themselves. Because no third parties can decipher the data being communicated or stored, for example, companies that provide end to end encryption are unable to hand over texts of their customers' messages to the authorities. In 2022, the UK's Information Commissioner's Office, the government body responsible for enforcing online data standards, stated that opposition to E2EE was misinformed and the debate too unbalanced, with too little focus on benefits, since E2EE "helped keep children safe online" and law enforcement access to stored data on servers was "not the only way" to find abusers.

In many messaging systems, including email and many chat networks, messages pass through intermediaries and are stored by a third party, from which they are retrieved by the recipient. Even if the messages are encrypted, they are only encrypted 'in transit', and are thus accessible by the service provider, regardless of whether server side disk encryption is used. Server side disk encryption simply prevents unauthorized users from viewing this information. It does not prevent the company itself from viewing the information, as they have the key and can simply decrypt this data. This allows the third party to provide search and other features, or to scan for illegal and unacceptable content, but also means they can be read and misused by anyone who has access to the stored messages on the third party system, whether this is by design or via a backdoor. This can be seen as a concern in many cases where privacy is very important, such as businesses whose reputation depends on their ability to protect third party data, negotiations and communications that are important enough to have a risk of targeted 'hacking' or surveillance, and where sensitive subjects such as health, and information about minors are involved. It is important to note that E2EE alone does not guarantee privacy or security. For example, data may be held unencrypted on the user's own device, or be accessible via their own app, if their login is compromised.

The term "end to end encryption" originally only meant that the communication is never decrypted during its transport from the sender to the receiver. For example, around 2003, E2EE has been proposed as an additional layer of encryption for GSM or TETRA, in addition to the existing radio encryption protecting the communication between the mobile device and the network infrastructure. This has been standardized by SFPG for TETRA. Note that in TETRA E2EE, the keys are generated by a Key Management Centre (KMC) or a Key Management Facility (KMF), not by the communicating users. Later, around 2014, the meaning of "end to end encryption" started to evolve when WhatsApp encrypted a portion of its network, requiring that not only the communication stays encrypted during transport, This new meaning is now the widely accepted one.

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 sensitive content. 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 or plugin to implement an end to end encryption scheme over an otherwise non E2EE protocol. Some non E2EE systems, such as 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 enabling end to end encryption by default. Telegram did not enable end to end encryption by default on VoIP calls while users were using desktop software version, but that problem was fixed quickly. However, as of 2020, Telegram still features no end to end encryption by default, no end to end encryption for group chats, and no end to end encryption for its desktop clients. Some encrypted backup and file sharing services provide client side encryption. The encryption they offer is here not referred to as end to end encryption, because the services are not meant for sharing messages between users. However, the term "end to end encryption" is sometimes incorrectly used to describe client side encryption.

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 their 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 they share with the actual recipient, or their 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 (MITM).

The end to end encryption paradigm does not directly address risks at the communications endpoints themselves. Each user's computer can still be hacked to steal their cryptographic key (to create a MITM attack) or simply read the recipients’ decrypted messages both in real time and from log files. Even the most perfectly encrypted communication pipe is only as secure as the mailbox on the other end. However, since plaintext input and output are still visible to the host system, malware can monitor conversations in real time. A more robust approach is to isolate all sensitive data to a fully air gapped computer. PGP has been recommended by experts for this purpose. However, as Bruce Schneier points out, Stuxnet developed by US and Israel successfully jumped air gap and reached Natanz nuclear plant's network in Iran. To deal with key exfiltration with malware, one approach is to split the Trusted Computing Base behind two unidirectionally connected computers that prevent either insertion of malware, or exfiltration of sensitive data with inserted malware.

A backdoor is usually a secret method of bypassing normal authentication or encryption in a computer system, a product, an embedded device, etc. Companies may also willingly or unwillingly introduce backdoors to their software that help subvert key negotiation or bypass encryption altogether. In 2013, information leaked by Edward Snowden showed that Skype had a backdoor which allowed Microsoft to hand over their users' messages to the NSA despite the fact that those messages were officially end to end encrypted. Following terrorist attacks in San Bernardino in 2015 and Pensacola in 2019, the FBI requested backdoors to Apple's iPhone software. The company, however, refused to create a backdoor for the government, citing concern that such a tool could pose risk for its consumers’ privacy.

While E2EE can offer privacy benefits that make it desirable in consumer grade services, many businesses have to balance these benefits with their regulatory requirements. For example, many organizations are subject to mandates that require them to be able to decrypt any communication between their employees or between their employees and third parties. This might be needed for archival purposes, for inspection by Data Loss Prevention (DLP) systems, for litigation related eDiscovery or for detection of malware and other threats in the data streams. For this reason, some enterprise focused communications and information protection systems might implement encryption in a way that ensures all transmissions are encrypted with the encryption being terminated at their internal systems (on premises or cloud based) so they can have access to the information for inspection and processing.