1. Akademska in raziskovalna mreža Slovenije 1 Cryptography Overview Gorazd Božič Academic and Research Network of Slovenia gorazd.bozic@arnes.si gorazd.bozic@arnes.si

2. Akademska in raziskovalna mreža Slovenije 2 What it is plaintextciphertextplaintext encryptiondecryption

3. Akademska in raziskovalna mreža Slovenije 3 Goals of cryptography authentication –receiver is able to determine the origin of the message integrity –receiver can verify that message has not been modified in transit non-repudiation –sender is not able to deny sending the message

4. Akademska in raziskovalna mreža Slovenije 4 Encryption algorithms mathematical functions used for encryption and decryption restricted algorithms –inadequate for today’s standards –hard to keep them secret –no quality control (peer rewiev) public algorithms –rely on strength of the key –exposed to wide public review

5. Akademska in raziskovalna mreža Slovenije 5 Encryption keys plaintext (M) E K (M)ciphertext (C)D K (C) plaintext (M) key

6. Akademska in raziskovalna mreža Slovenije 6 Symmetric algorithms keys for encryption and decryption are the same (or one can be calculated from the other) sender and receiver need to agree on the key –key must somehow be transmitted –this presents a security risk mode of operation –stream ciphers (operate on characeters or bits) –block ciphers (operate on larger groups of bits)

7. Akademska in raziskovalna mreža Slovenije 7 Asymmetric (public-key) algorithms presented in 1976 decryption key can not be calculated from the encryption key (not in a reasonable time) enable digital signatures much slower than symmetrical encryption key can be public

8. Akademska in raziskovalna mreža Slovenije 8Cryptanalysis recovering plaintext without the key strength resides in the key Bruce Schneier: “... one would assume that the CIA does not make a habbit of telling Mossad about its algorithms, but Mossad probably finds out anyway.” numerous methods –brute-force attack –ciphertext-only attack –known-plaintext attack –chosen-plaintext attack –...

9. Akademska in raziskovalna mreža Slovenije 9 Security of algorithms usually information needs to stay secret for a limited time estimates for complexity –2 128 operations on million of computers that perform million operations per second take 10 19 years (billion times the age of universe) breakthroughs in mathematics can instantly change everything

10. Akademska in raziskovalna mreža Slovenije 10Code cryptosystem dealing with linguistical units much used in history example: –“The apple is in the box.” meaning “Our troops have landed.” used only in special environments

11. Akademska in raziskovalna mreža Slovenije 11Steganography hiding secret messages in other messages –invisible ink –tiny marks –matrices exposing selected positions in text –graphic images

12. Akademska in raziskovalna mreža Slovenije 12 Substitution and transposition ciphers substitution –each character is replaced by another HELLO -> URYYB –most famous: Caesar Cipher transposition –exchanges character positions in message HELLO -> LHOEL

13. Akademska in raziskovalna mreža Slovenije 13 Simple XOR simple to break in two steps with a computer still companies produce software with this solution one special case where key is the same length as the message

14. Akademska in raziskovalna mreža Slovenije 14 One-time pads invented in 1917 large non-repeating set of random key characters sender and receiver share the same set of keys after encryption, used character keys are destroyed theoretically unbreakable storage and distribution problems

15. Akademska in raziskovalna mreža Slovenije 15 “Snake oil” universal medicine to cure all disease encryption snake oil warning signs –“Trust us, we know what we’re doing.” –technobabble –secret algorithms –revolutionary breakthroughs –“military grade” –revirewed by “security experts” –unbreakable –recoverable keys with no key-escrow

16. Akademska in raziskovalna mreža Slovenije 16 Hash functions way to “fingerprint” messages (files) function converts fixed-length hash from a variable-length message inverse function does not exist extremely hard to find two messages with the same hash used for digital signatures in public-key encryption

17. Akademska in raziskovalna mreža Slovenije 17 Session encryptions (example) request for communication sends public key sends generated session key encrypted with the public key secure channel with symmetric encryption with session key

18. Akademska in raziskovalna mreža Slovenije 18 E-mail encryptions two main contenders –PGP widely used commercial and open-source products (PGP, GnuPG) need for establishing “web of trust” –S/MIME is gaining more usage, but slowly relies on digital certificates issued by Certificate Authorities slow establishment of CAs

19. Akademska in raziskovalna mreža Slovenije 19 Certificate authorities (CA) neutral third parties issuing digital certificates ideally, a hierarchy of CAs different levels of identity check –personal IDs, physical presence –local companies and court registries –some sort of proof sent over fax is enough

20. Akademska in raziskovalna mreža Slovenije 20 Encryption and law digital communication is replacing paper parties can agree on a particular scheme –flexible for business transactions –no prescribed technology or procedures low requirements for CA more strict requirements for qualified CA