Cryptography Glossary

A5/1 A stream cipher used in GSM mobile phone communication. 36 Active Attack A cryptanalysis attack that relies on actively disrupting communication 44 or forcefully getting access to data. Adaptive chosen-ciphertext Attack Similar to chosen-ciphertext attack but Eve can choose subsequent ci- 44 phertexts based on information learnt from previous encryptions. Adaptive chosen-plaintext Attack Similar to chosen-plaintext attack but Eve can choose subsequent plain- 44 texts based on information learnt from previous encryptions. AddRoundKey The round key addition operation used during AES encryption and de- 28 cryption. AES The advanced encryption standard that is the successor of DES. 26 Alice The sender of an encrypted message. 11 Asymmetric Cipher Cipher that uses different (not trivially related) keys for encryption and 11, 54 decryption. Asymmetric Cryptography The study of algorithms and protocols for asymmetric ciphers. 54

Birthday Paradox A probability theoretical fact stating that in a random group of ¢¡ people 47

the likelihood that two of them will have the same birthday is more than £¥¤§¦

Block A ﬁxed-length group of bits. 11 Block Cipher A symmetric key cipher, which operates on ﬁxed-length groups of bits, 11 named blocks. Bob The intended receiver of an encrypted message. Bob is assumed to have 11 the key to decrypt it.

C2 Short for Cryptomeria. 24 CA Short for Certiﬁcation Authority. 71

Caesar Cipher Shift cipher with ﬁxed shift of ¡ . 6

© § ¨ © !¨ Carmichael Number A number ¨ for which for all but that is 64 nevertheless composite. CBC Short for Cipher Block Chaining 20 CBC-MAC A method to produce a MAC from a block cipher. 51 Certificate Also called Digital Certificate. 71 Certification Authority A trusted third party that issues and validates digital certificates 71 CFB Short for Cipher Feedback mode 21 Chosen-ciphertext Attack Eve can obtain the plaintexts corresponding to arbitrary ciphertexts of 44 her choice. Chosen-plaintext Attack Eve can obtain the ciphertexts corresponding to arbitrary plaintexts of 44 her choice. Cipher Substitution at the level of letters/characters/symbols. 4 Cipher Block Chaining mode A mode of operation for block ciphers that propagates context informa- 20 tion. Cipher Feedback mode A mode of operation that turns a block cipher effectively into a stream 21 cipher by using context information. Cipher-alphabet The alphabet used for encryption. 6 Ciphertext An enciphered message. 4 Ciphertext-only Attack Eve has access only to a collection of ciphertexts or codetexts. 44 Code Substitution at the level of words 4 Codetext An encoded message. 4

Collision Free Function A hash function is called collision free, if for any " it is computationally 46 %&'"$)(!%&'"*#+ infeasible to find an "$# such as . Commitment Scheme A method of sending secret information such that it cannot be altered at 73 a later stage; neither by the sender nor the receiver. Connection Polynomial Polynomial used to describe and analyse a LFSR based stream cipher. 34 Content Scrambling System A proprietary standard to encrypt multimedia DVDs for copy protection. 35 Counter mode A mode of operation that turns a block cipher effectively into a stream 23 cipher by using a counter. Cross-Certification Several CAs are linked by exchanging certificates directly with each 72 other. Cryptanalysis The science of breaking ciphertexts. 4 Cryptanalyst A practitioner of cryptanalysis. 4 Cryptographer A practitioner of cryptography. 4 Cryptographic Hash Function A function that transforms an arbitrarily long message into fixed lengths 46 string to authenticate it has not been tampered with. Cryptography The science of scrambling messages. 3, 4 Cryptologist A practitioner of cryptology. 4 Cryptology The branch of mathematics encompassing both cryptography and crypt- 4 analysis Cryptomeria A Feistel Cipher used as content protection mechanism for multimedia 24 DVDs. CSS Short for Content Scrambling System. 35 CTR Short for Counter mode 23, 31

Davies-Meyer A method to build hash functions from block ciphers. 50 Decipher The process of decrypting a message with respect to some cipher. 4 Decode The process of decrypting a message with respect to some code. 4 Decrypt The process of unscrambling a message. 4 Deletion Attack Data is modified by deleting parts of the ciphertext. 44 Denial of Service Attack This is an attempt to make a computer resource unavailable to its in- 45 tended users. Differential Cryptanalysis is the study of how differences in an input can affect the resulting differ- 44 ence at the output. Diffie-Hellman Key Exchange The very first asymmetric cryptography algorithm invented, that uses 56 discrete exponentiation to agree on a key. Diffie-Hellman Problem A computationally infeasible problem. 56 Digital Signature A cryptographic scheme to ensure message identity based on public key 66 cryptography. Digital Signature Algorithm A digital signature derived from RSA, whose security is based on the 67, 68 factorisation of numbers. Discrete Logarithm Problem A computationally infeasible problem. 56 DoS Short for Denial of Service attack. 45 DSA Short for Digital Signature Algorithm. 68

ECB Short for Electronic Codebook mode 19 Electronic Codebook mode A na¨ıve mode of operation for block ciphers that encrypts every block 19 independently. ElGamal A public key cipher based on discrete exponentiation. 57 Encipher The process of encrypting a message with respect to some cipher. 4 Encode The process of encrypting a message with respect to some code. 4 Encrypt The process of scrambling a message. 4 Encryption Schemes Techniques to introduce a degree of randomness into public key ciphers 63 often by preprocessing the messages. Eve The eavesdropper who tries to intercept and to cryptanalyse messages 11 passed between Bob and Alice.

Feedback Function A function that determines a new bit for a shift register by tapping the 33 content of some of the registers. Feistel Cipher A basic block cipher developed by Horst Feistel at IBM. It forms the 12 bases of many modern block ciphers. Feistel function The function applying the round key in the Feistel cipher, thereby effec- 12 tively parameterising the round function. Fermat’s Test A probabilistic test algorithm to establish the likelihood that a given 63 number is a prime, based on Fermat’s little theorem. Finalisation Function A ﬁnal function applied to strengthen an output hash in the Merkle- 47 Damgard˚ Construction. Fingerprint The result of a hash function. Also known as hash sum. 46 Frequency Analysis A statistical method to decipher messages based on the frequency of 7 occurrences of letters in particular languages.

Hash The result of a hash function. 46 Hierarchical CAs Several CAs are certiﬁed by a single root CA. 72 HMAC A method to produce a MAC from a hash function. 51

Initial Value An arbitrary value used to kick off encryption in block cipher modes. 20

Initial values are normally denoted as ¢¡ and do not need to be kept secret. Initialisation Vector See Initial Value. 20 Insertion Attack Data is modiﬁed by inserting parts into the ciphertext. 44 Integer Factorisation A computationally infeasible problem for large integers. 56 InverseMixColumns A column manipulation operations used during AES decryption. Inverse 28 of MixColumns. InverseShiftRows A byte permutation used during AES decryption. Inverse of Shift Rows. 28 InverseSubBytes An S-Box like substitution algorithm used during AES decryption. The 27 inverse of SubBytes. IP Spooﬁng A computer poses as another computer by illegitimately assuming its IP 45 address.

Kerberos A symmetric key exchange protocol derived from Needham-Schroeder, 42 but based on timestamps. It is used in Windows 2000. Key Auxiliary information that is used to vary the encryption procedure 6 Key Exchange Protocols Precisely deﬁned communication methods to exchange keys for crypto- 39 graphic algorithms. Key Generator A function that generates keys that can be used in a public key cipher. 55, 66 Key management Problems related to choosing, distributing, and securely storing keys for 11 symmetric ciphers. Key Schedule A function to generate round keys from one input key. 14, 16 Keystream A stream of bits used of encryption by xor-ing it with the plaintext in a 31 stream cipher. Known-plaintext Attack Eve has a set of ciphertexts to which she knows the corresponding plain- 44 text.

Lamport Signatures A generic signature scheme that is based on one-way functions. It is 67, 69 considered secure even against quantum cryptanalysis, since it uses signatures exactly once. LFSR Short for Linear Feedback Shift Register. 33 Linear Cryptanalysis is the approximation of the action of a cipher by some function. 44 Linear Feedback Shift Register A shift register with a linear feedback function that is a building block 33 for many modern stream ciphers. Lucifer Cipher First simple Feistel cipher. 12 MAC Short for Message Authentication Code. 51, 66 Mallory A malicious attacker, who can modify messages, substitute her own mes- 39 sages, replay old messages, and so on. Man-in-the-Middle Attack Mallory spoofs Alice into believing she’s Bob, and spoofs Bob into be- 45 lieving she’s Alice, thus gaining access to all messages in both directions without the trouble of any cryptanalytic effort. Masquerading Attack An attack in which one person or program successfully masquerades as 45 another by falsifying data and thereby gaining an illegitimate advantage. Matyas-Meyer-Oseas A method to build hash functions from block ciphers. 50 MD Short for Message-Digest. 48 MD4 A hash function of the MD family producing 128-bit hashes. 48 MD5 A hash function of the MD family producing 128-bit hashes. 49 Mental Coin Flipping A commitment scheme for making a binary decision. 73 Mental Poker A commitment scheme to play poker without physically exchanging 74 cards. Merkle-Damgaard Construction A method to extend a ﬁxed size compression function to an arbitrary size 47 compression function. Merkle-Damgaard strengthening A padding method that adds a bit representation of the message length 47 at the end of a message before applying a hash function. Message Authentication Code A symmetric key algorithm to ensure the authenticity of messages. 46, 51, 66 Message Digest A family of hash functions that generally have MD as preﬁx. 48 Miller-Rabin Test A probabilistic test algorithm to establish the likelihood that a given 64 number is a prime based on number theory. It is an improvement over Fermat’s test, since it can successfully detect Carmichael numbers as composite. MITM Short for Man-in-the-Middle attack. 45 MixColumns A column manipulation operations used during AES encryption. 28 Miyaguchi-Preneel A method to build hash functions from block ciphers. 50 Modes of Operation Ways of applying block ciphers to encode large messages. 19 Monoalphabetic Substitution Cipher Using a single cipher-alphabet for encryption. 6

Needham-Schroeder A symmetric key exchange protocol based on nonces. 41 Non-linear Keyspaces A technique to have a cipher secure only for some keys in the keyspace 39 and deliberately weaken it for other keys, possibly unknown to the user. Nonce A number that is only used once. It is usually a random or pseudo- 23 random number. Nulls Empty or meaningless words or letters in a cipher or code. 5

OFB Short for Output Feedback mode 22, 31 One Time Pad A secure but impractical cipher that is unbreakable. 9, 31 One-way Function A function that is easy to compute but hard to invert. 46, 55 Otway-Rees A symmetric key exchange protocol based on nonces. 42 Output Feedback mode A mode of operation that turns a block cipher effectively into a stream 22 cipher by using context information.

P-Box Short for Permutation Box. 15 Padding Adding nulls at the end of messages to obtain the required block size. 19 It is important that the nulls do not obscure the message, such that its original length can be recovered. The simplest is padding is achieved by adding null bits. Passive Attack A cryptanalysis attack that relies essentially on eavesdropping. 44 Peggy The prover in a zero knowledge proof protocol. 75 Permutation Box An operation that takes a set of input blocks, combines them and applies 15 a permutation. Permutation Cipher A more elaborate transposition cipher. 5 PGP Short for Pretty Good Privacy. 72 Phishing A legitimate web page such as a bank’s site is reproduced in look and 45 feel on another server under Mallory’s control. PKI Short for Public Key Infrastructure. 71

Plaintext A message before it is encrypted. 4 ¡ Plaintext-alphabet The regular alphabet which is in our case letters but could also contain 6 numbers etc. PMAC A method to produce a MAC from a block cipher using parallelisation. 52 Polyalphabetic Substitution Cipher Using several cipher-alphabets for encryption. 7 Pretty Good Privacy A program suite that provides cryptographic tools for cryptographic pri- 72 vacy and authentication. PRG Short for Pseudo-random Generator. 31 Private Key The key that is kept secret in a public key cipher and which is generally 54, 55 used for decryption. Product cipher A cipher that applies the same basic encryption scheme iteratively for 12 several rounds. Prover The secret holding party in a zero knowledge protocol. 75 Pseudo-random Generator An algorithm that produces a bit string that looks like a random se- 31 quence. Pseudo-random String A bit string that looks like a random sequence but was generated by a 31 pseudo-random generator. Public Key The key that is publicly known in a public key cipher and which is gen- 54, 55 erally used for encryption. Public Key Cipher An asymmetric cipher using two keys per participant: a private and a 54 public key. Public Key Cryptography The study of algorithms and protocols for public key ciphers. Nowadays 54 often used synonymously with asymmetric cryptography. Public-key Infrastructure An infrastructure for trusted public key encryption, using digital certiﬁ- 71 cates, signatures, and a certiﬁcation authority.

Rail Fence Cipher A simple transposition cipher. 5 RC4 A stream cipher used in many protocols such as SSL/TLS and 802.11b 32 WEP. Related-key Attack Similar to a chosen-plaintext attack, except that Eve can obtain cipher- 44 texts encrypted under two different keys. The keys are unknown, but the relationship between them is known; for example, two keys that differ in one bit. Replay Attack Mallory repeats or delays a valid data transmission maliciously or fraud- 45 ulently. Revoking Certificates Publishing certified lists of certificates that have become invalid. 72 Rijndael The original name of AES. 26 Round function The parameterised encryption function applied during one round of a 12 Feistel cipher. RSA The first, and probably most widely used public key cipher based on 61 discrete exponentiation modulo a composite number. RSA Signature A digital signature derived from RSA, whose security is based on fac- 67, 69

torising numbers.

£¢¥¤ $¨ RSA Trapdoor Function RSA’s function that can only be easily inverted if the fac- 61

torisation of ¨ is known.

S-Box Short for Substitution Box. 14 Schnorr Signature A digital signature derived from ElGamal, whose security is based on 67 discrete logarithms. Secure Hash Algorithms A family of hash functions that generally have SHA as prefix. They are 48 seen as successors to the MD algorithms. Seed The initial value for a pseudo-random generator. 31 SHA Short for Secure Hash Algorithm. 48 SHA-1 A hash function of the SHA family producing 160-bit hashes. 50 SHA-2 A new generation of SHA algorithms, individually named after their 50 hash length, e.g., SHA-256, SHA-384, SHA-512. Shift Cipher Substitution cipher that shifts every letter in the message by a set number 6 of places in the alphabet. Shift Register A group of single bit cells that shift by one cell at every clock cycle. 33 ShiftRows A byte permutation used during AES encryption. 28 Signature Function A function that is used to produce a unique identity in a digital signature. 66 Spoofing Attack See Masquerading Attack. 45 Steganography The science of hiding messages. 3 Stream Cipher A symmetric cipher that encrypts plaintext continuously. 11, 31 Sub-key A round key derived from one key valid for the entire algorithm. 12 SubBytes An S-Box like substitution operation used during AES encryption. 27 Substitution Attack Data is modified by substituting parts of the ciphertext. 44 Substitution Box A lookup table that substitutes a 4-bit block for a 6-bit block. 14 Substitution Cipher Encrypting texts by replacing characters in the plaintext with characters 4, 6 from a cipher-alphabet. Symmetric Cipher Ciphers that use the same or trivially related keys for encryption and 11 decryption.

Symmetric Group The set of all permutations on ¨ numbers. i

Tapping Taking certain, but not all bits from a shift register. 34 Tapping Sequence The sequence of bits tapped from a shift register by its feedback function. 34 Transposition Cipher Encrypting texts by transposing characters with one another. 4, 5 Trapdoor One-way Function A function that is hard to invert, unless some secret information, the 55 trapdoor, is known. Trapdoor Permutation Another expression for Trapdoor One-way Function. 55 Trent A trusted third party that is often central to symmetric key exchange. 39

Verification Function A function that is used to verify the authenticity of a digital signature. 66 Verifier The party in a zero knowledge protocol that needs to be convinced that 75 the prover possesses the secret. VeriSign A commercial certification authority. 72 Victor The verifier in a zero knowledge proof protocol. 75 Vigenere Cipher A polyalphabetic substitution cipher. 7

Web of Trust A decentralised public key infrastructure, in which certiﬁcation is 72 achieved by authentication via several already known and trusted oth- ers. Whirlpool A hash function build from a modiﬁed version of the AES block cipher. 51 Wide-Mouth Frog A very basic symmetric key exchange protocol based on timestamps. 40

Zero Knowledge Proof A two party protocol where one party convinces another party that it is 75 in possession of a secret without revealing the secret itself.