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Term applied to cryptographic systems that are highly resistant to cryptanalysis

Strong cryptography or cryptographically strong are general terms used to designate the cryptographic algorithms that, when used correctly, provide a very high (usually insurmountable) level of protection against any eavesdropper, including the government agencies. There is no precise definition of the boundary line between the strong cryptography and (breakable) weak cryptography, as this border constantly shifts due to improvements in hardware and cryptanalysis techniques. These improvements eventually place the capabilities once available only to the NSA within the reach of a skilled individual, so in practice there are only two levels of cryptographic security, "cryptography that will stop your kid sister from reading your files, and cryptography that will stop major governments from reading your files" (Bruce Schneier). The strong cryptography algorithms have high security strength, for practical purposes usually defined as a number of bits in the key. For example, the United States government, when dealing with export control of encryption, considered any implementation of the symmetric encryption algorithm with the key length above 56 bits or its public key equivalent to be strong and thus potentially a subject to the export licensing. To be strong, an algorithm needs to have a sufficiently long key and be free of known mathematical weaknesses, as exploitation of these effectively reduces the key size. At the beginning of the 21st century, the typical security strength of the strong symmetrical encryption algorithms is 128 bits (slightly lower values still can be strong, but usually there is little technical gain in using smaller key sizes). Demonstrating the resistance of any cryptographic scheme to attack is a complex matter, requiring extensive testing and reviews, preferably in a public forum. Good algorithms and protocols are required (similarly, good materials are required to construct a strong building), but good system design and implementation is needed as well: "it is possible to build a cryptographically weak system using strong algorithms and protocols" (just like the use of good materials in construction does not guarantee a solid structure). Many real life systems turn out to be weak when the strong cryptography is not used properly, for example, random nonces are reused A successful attack might not even involve algorithm at all, for example, if the key is generated from a password, guessing a weak password is easy and does not depend on the strength of the cryptographic primitives. A user can become the weakest link in the overall picture, for example, by sharing passwords and hardware tokens with the colleagues.

The level of expense required for strong cryptography originally restricted its use to the government and military agencies, until the middle of the 20th century the process of encryption required a lot of human labor and errors (preventing the decryption) were very common, so only a small share of written information could have been encrypted. US government, in particular, was able to keep a monopoly on the development and use of cryptography in the US into the 1960s. In the 1970, the increased availability of powerful computers and unclassified research breakthroughs (Data Encryption Standard, the Diffie Hellman and RSA algorithms) made strong cryptography available for civilian use. Mid 1990s saw the worldwide proliferation of knowledge and tools for strong cryptography. By the 21st century the technical limitations were gone, although the majority of the communication were still unencrypted. At the same the cost of building and running systems with strong cryptography became roughly the same as the one for the weak cryptography. The use of computers changed the process of cryptanalysis, famously with Bletchley Park's Colossus. But just as the development of digital computers and electronics helped in cryptanalysis, it also made possible much more complex ciphers. It is typically the case that use of a quality cipher is very efficient, while breaking it requires an effort many orders of magnitude larger making cryptanalysis so inefficient and impractical as to be effectively impossible.

Widespread use of encryption increases the costs of surveillance, so the government policies aim to regulate the use of the strong cryptography. In the 2000s, the effect of encryption on the surveillance capabilities was limited by the ever increasing share of communications going through the global social media platforms, that did not use the strong encryption and provided governments with the requested data. Murphy talks about a legislative balance that needs to be struck between the power of the government that are broad enough to be able to follow the quickly evolving technology, yet sufficiently narrow for the public and overseeing agencies to understand the future use of the legislation.

The position of the EU, in comparison to the US, had always been tilting more towards privacy. In particular, EU had rejected the key escrow idea as early as 1997. European Union Agency for Cybersecurity (ENISA) holds the opinion that the backdoors are not efficient for the legitimate surveillance, yet pose great danger to the general digital security.

The Five Eyes (post Brexit) represent a group of states with similar views one the issues of security and privacy. The group might have enough heft to drive the global agenda on the lawful interception. The efforts of this group are not entirely coordinated: for example, the 2019 demand for Facebook not to implement end to end encryption was not supported by either Canada or New Zealand, and did not result in a regulation.

President and government of Russia in 90s has issued a few decrees formally banning uncertified cryptosystems from use by government agencies. Presidential decree of 1995 also attempted to ban individuals from producing and selling cryptography systems without having appropriate license, but it wasn't enforced in any way as it was suspected to be contradictory the Russian Constitution of 1993 and wasn't a law per se. The decree of No.313 issued in 2012 further amended previous ones allowing to produce and distribute products with embedded cryptosystems and requiring no license as such, even though it declares some restrictions. France had quite strict regulations in this field, but has relaxed them in recent years.

PGP is generally considered an example of strong cryptography, with versions running under most popular operating systems and on various hardware platforms. The open source standard for PGP operations is OpenPGP, and GnuPG is an implementation of that standard from the FSF. However, the IDEA signature key in classical PGP is only 64 bits long, therefore no longer immune to collision attacks. OpenPGP therefore uses the SHA 2 hash function and AES cryptography. The AES algorithm is considered strong after being selected in a lengthy selection process that was open and involved numerous tests. Elliptic curve cryptography is another system which is based on a graphical geometrical function. The latest version of TLS protocol (version 1.3), used to secure Internet transactions, is generally considered strong. Several vulnerabilities exist in previous versions, including demonstrated attacks such as POODLE. Worse, some cipher suites are deliberately weakened to use a 40 bit effective key to allow export under pre 1996 U. S. regulations.