{"licence":{"name":"CC BY-SA 4.0","spdx":"CC-BY-SA-4.0","url":"https://creativecommons.org/licenses/by-sa/4.0/","attribution":"Atlas, a bilingual technical dictionary (https://cmaintz.github.io/tech-atlas/)"},"id":"cs/encryption","url":{"en":"https://cmaintz.github.io/tech-atlas/en/terms/cs/encryption/","da":"https://cmaintz.github.io/tech-atlas/da/terms/cs/encryption/"},"term":{"en":"Encryption","da":"Kryptering"},"aka":{"en":["enciphering"],"da":["chiffrering"]},"domain":["cs","security"],"cluster":"networking","layer":"data","status":"current","summary":{"en":"Scrambling data with a secret key so only someone holding the matching key can read it.","da":"At kode data med en hemmelig nøgle, så kun den, der har den tilsvarende nøgle, kan læse dem."},"body":{"formal":{"en":"A mathematical process that uses a key to transform readable data into a form that cannot be read, such that only a holder of the matching key can turn it back. The key may be shared by both sides (symmetric) or split into a public and a private part.","da":"En matematisk proces, der ved hjælp af en nøgle omdanner læsbare data til en form, der ikke kan læses, så kun den, der har den tilsvarende nøgle, kan vende processen om. Nøglen kan være fælles for begge parter (symmetrisk) eller delt i en offentlig og en privat del."},"plain":{"en":"Like writing a letter in a secret code only you and a friend know - anyone who steals the letter sees only gibberish.","da":"Som at skrive et brev i en hemmelig kode, kun du og en ven kender - enhver, der stjæler brevet, ser kun volapyk."},"inPractice":{"en":"A nurse in a municipality's home care leaves her work laptop on the bus; its disk is encrypted, so whoever finds it cannot read the citizens' care notes without her password.","da":"En sygeplejerske i kommunens hjemmepleje glemmer sin arbejdsbærbar i bussen; disken er krypteret, så finderen ikke kan læse borgernes plejenotater uden hendes adgangskode."},"whyItMatters":{"en":"Data is copied, lost and overheard all the time; with encryption a stolen disk or tapped connection reveals nothing readable, which can turn a serious data breach into a minor incident - as long as the key stays safe.","da":"Data bliver kopieret, tabt og aflyttet hele tiden; med kryptering afslører en stjålet disk eller en aflyttet forbindelse intet læsbart, og et alvorligt databrud kan blive til en mindre hændelse - så længe nøglen holdes sikker."}},"deepDive":{"en":"Modern encryption follows Kerckhoffs's principle: the algorithm is public and security rests on the key alone. The workhorse symmetric cipher is AES (FIPS 197, 2001), a block cipher with a 128-bit block and 128-, 192- or 256-bit keys; ChaCha20 is the common stream-cipher alternative on hardware without AES instructions. A block cipher on its own encrypts only one block, so a mode of operation matters as much as the cipher: ECB leaks patterns because identical blocks give identical ciphertext, and CBC without a separate MAC is malleable and has repeatedly fallen to padding-oracle attacks. Current practice is authenticated encryption with associated data (AEAD), such as AES-GCM (NIST SP 800-38D) or ChaCha20-Poly1305 (RFC 8439), which provides confidentiality and integrity together. GCM's weak spot is nonce reuse: encrypting two messages with the same key and nonce exposes the XOR of the plaintexts and lets an attacker forge authentication tags.\n\nAsymmetric schemes (RSA, elliptic-curve cryptography) are slow and are used to establish or wrap symmetric keys rather than to encrypt bulk data. RSA encryption needs proper padding (OAEP); the older PKCS#1 v1.5 padding enabled Bleichenbacher's 1998 oracle attack, which resurfaced as ROBOT in 2017. TLS 1.3 therefore uses ephemeral (EC)DH key agreement for forward secrecy, and cloud KMS services use envelope encryption: data is encrypted with a data-encryption key (DEK), which is itself encrypted by a key-encryption key (KEK) held in the KMS or an HSM. NIST SP 800-57 Part 1 maps key sizes to security strength: RSA-2048 gives roughly 112 bits, RSA-3072 and the P-256 curve roughly 128 bits.\n\nA large enough quantum computer running Shor's algorithm would break RSA and elliptic-curve schemes, while Grover's algorithm only halves the effective strength of symmetric keys, which is why AES-256 is favoured for long-lived data. NIST published its first post-quantum standards in August 2024: FIPS 203 (ML-KEM), FIPS 204 (ML-DSA) and FIPS 205 (SLH-DSA). Because traffic recorded today can be decrypted later (\"harvest now, decrypt later\"), major browsers and TLS libraries have already deployed hybrid key exchange that combines X25519 with ML-KEM.\n\nEncryption is frequently overestimated. Full-disk encryption such as BitLocker protects a powered-off or locked device; once the system is unlocked, the operating system decrypts transparently for every process, including ransomware. Database transparent data encryption protects files and backups, not data fetched through SQL injection. Unauthenticated encryption does not guarantee integrity, and encryption is not hashing. In practice most failures are about keys and randomness - hard-coded keys, weak random number generators, keys stored beside the data - rather than broken algorithms. Legally, GDPR Art. 32(1)(a) names encryption as an example of an appropriate measure, Art. 34(3)(a) can remove the duty to notify data subjects when breached data was unintelligible to unauthorised persons, and NIS2 Art. 21(2)(h) requires policies on cryptography and encryption.","da":"Moderne kryptering følger Kerckhoffs' princip: Algoritmen er offentlig, og sikkerheden hviler alene på nøglen. Den gennemgående symmetriske algoritme er AES (FIPS 197, 2001), en blokchiffer med 128-bit blokke og nøgler på 128, 192 eller 256 bit; ChaCha20 er det udbredte alternativ som strømchiffer på hardware uden AES-instruktioner. En blokchiffer krypterer i sig selv kun én blok, så driftsformen (mode of operation) betyder lige så meget som selve algoritmen: ECB lækker mønstre, fordi ens blokke giver ens chiffertekst, og CBC uden separat MAC kan manipuleres og er gentagne gange faldet for padding-oracle-angreb. Nutidens praksis er autentificeret kryptering (AEAD), fx AES-GCM (NIST SP 800-38D) eller ChaCha20-Poly1305 (RFC 8439), som giver fortrolighed og integritet på én gang. GCM's svage punkt er genbrug af nonce: Krypteres to beskeder med samme nøgle og nonce, afsløres XOR af klarteksterne, og en angriber kan forfalske autentifikationstags.\n\nAsymmetriske metoder (RSA, elliptisk kurve-kryptografi) er langsomme og bruges til at etablere eller indpakke symmetriske nøgler frem for at kryptere store datamængder. RSA-kryptering kræver korrekt padding (OAEP); den ældre PKCS#1 v1.5-padding muliggjorde Bleichenbachers oracle-angreb fra 1998, som dukkede op igen som ROBOT i 2017. TLS 1.3 bruger derfor flygtig (EC)DH-nøgleudveksling for at opnå forward secrecy, og KMS-tjenester i skyen bruger envelope encryption: Data krypteres med en datakrypteringsnøgle (DEK), som selv er krypteret med en nøglekrypteringsnøgle (KEK), der ligger i KMS'en eller et HSM. NIST SP 800-57 del 1 knytter nøglestørrelser til sikkerhedsstyrke: RSA-2048 giver omtrent 112 bit, RSA-3072 og kurven P-256 omtrent 128 bit.\n\nEn tilstrækkelig stor kvantecomputer med Shors algoritme ville bryde RSA og elliptiske kurver, mens Grovers algoritme kun halverer den effektive styrke af symmetriske nøgler - derfor foretrækkes AES-256 til data med lang levetid. NIST udgav de første post-kvante-standarder i august 2024: FIPS 203 (ML-KEM), FIPS 204 (ML-DSA) og FIPS 205 (SLH-DSA). Fordi trafik, der optages i dag, kan dekrypteres senere (\"harvest now, decrypt later\"), har de store browsere og TLS-biblioteker allerede indført hybrid nøgleudveksling, der kombinerer X25519 med ML-KEM.\n\nKryptering bliver ofte overvurderet. Fuld diskkryptering som BitLocker beskytter en slukket eller låst enhed; når systemet er låst op, dekrypterer styresystemet gennemsigtigt for alle processer - også ransomware. Transparent data encryption i databaser beskytter filer og backup, ikke data, der hentes ud via SQL injection. Kryptering uden autentifikation sikrer ikke integritet, og kryptering er ikke det samme som hashing. I praksis skyldes de fleste fejl nøgler og tilfældighed - hårdkodede nøgler, svage tilfældighedsgeneratorer, nøgler gemt ved siden af data - snarere end brudte algoritmer. Juridisk nævner databeskyttelsesforordningens art. 32, stk. 1, litra a, kryptering som eksempel på en passende foranstaltning; art. 34, stk. 3, litra a, kan fjerne pligten til at underrette de registrerede, når de kompromitterede data var uforståelige for uvedkommende; og NIS2 art. 21, stk. 2, litra h, kræver politikker for brug af kryptografi og kryptering."},"edges":[{"type":"kind-of","to":"security/control","confidence":"high","strength":"normal"},{"type":"implements","to":"security/confidentiality","why":{"en":"Encrypted data stays unreadable to anyone without the secret, which is exactly what confidentiality asks for.","da":"Krypterede data kan ikke læses af nogen uden hemmeligheden, hvilket netop er, hvad fortrolighed kræver."},"confidence":"high","strength":"primary"},{"type":"contrasts-with","to":"cs/hashing","why":{"en":"Encryption can be reversed by whoever holds the key; hashing is one-way and can never give the original data back.","da":"Kryptering kan vendes om af den, der har nøglen; hashing er envejs og kan aldrig give de oprindelige data tilbage."},"confidence":"high","strength":"primary"},{"type":"mitigates","to":"security/data-breach","why":{"en":"Stolen data that is encrypted is useless to the thief as long as the secret stays safe.","da":"Stjålne data, der er krypterede, er værdiløse for tyven, så længe hemmeligheden holdes sikker."},"confidence":"high","strength":"primary"},{"type":"used-with","to":"cs/tls","confidence":"high","strength":"normal"},{"type":"used-with","to":"cs/vpn","confidence":"high","strength":"normal"}],"depth":0,"sources":[{"title":"Kurose & Ross, Computer Networking: A Top-Down Approach","tier":"textbook"}],"draft":true}