# Tag Info

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"Cycles" are CPU instruction cycles. Cycles per byte roughly measures how many instructions, in a given instruction set, are needed to produce each byte of output. They're a reasonably-good relative measure of the performance of different algorithms. Generally, when you measure an algorithm's cycles per byte, you use carefully controlled conditions. You ...

7

In this context 'security margin' is a measure for how much better we need to get at analyzing a cipher to break it. Such advances in cryptoanalysis require new ideas of how one might attack a cipher. Thus estimating how strong a cipher is, is hard. Ultimately we can only tell something is broken, after we've broken it. We typically look at a few ...

6

This is a simple substitution cipher, specifically a mixed/deranged alphabet cipher. See wikipedia's description: Substitution of single letters separately—simple substitution—can be demonstrated by writing out the alphabet in some order to represent the substitution. This is termed a substitution alphabet. The cipher alphabet may be shifted or reversed ...

6

In my practice (Smart Cards, often using DES and increasingly AES) Key Expansion is often used to designate production of subkeys in a block cipher. This process is often a mere bit extraction, as part of the algorithm's Key Schedule. Key Diversification is, almost exclusively, the process of producing a device key from its serial number (or other ...

6

The Diffie-Hellman key exchange is a public-key technology. It is (by itself) not an encryption algorithm (or signature algorithm), though. Here is the basic function: (All calculations here happen in a discrete group of sufficient size, where the Diffie-Hellman problem is considered hard, usually the multiplicative group modulo a big prime (for classical ...

6

As noted in this answer and this answer to another question, permutation is just a mathematical term for a function $\sigma:X{\rightarrow}X$ that maps a finite set $X$ onto itself, in such way that for each $y \in X$ there exists exactly one $x \in X$ such that $\sigma(x) = y$. This is also equivalent to how the term substitution is used in cryptography, so ...

6

According to J.-P. Aumasson (who's one of the authors of another SHA-3 finalist, BLAKE, and who participated in the cryptanalysis of Keccak), the name "Keccak" is a variant spelling of "Kecak", a type of Balinese dance. So far, that's the most authoritative reference I've been able to come up with. It should be noted that naming crypto primitives after ...

5

A mode of operation is an explicit method by which we use a block cipher (eg AES) to do more than just encrypt one block of data. For example, it may allow us to encrypt multiple blocks of data (eg ECB,CBC etc), provide us with some authenticated encryption (eg GCM) or a method for encrypting disc storage (eg XTS). Rijndael,DES etc are block ciphers. That ...

4

I don't have my copy of Katz & Lindell in front of me, but using the term "simulator" in the context of, say, an IND-CPA definition, is not exactly in line with standard usage in current literature. (But still an ok choice of words which is excusable for a textbook.) Here's the breakdown of "game-based" vs. "simulation-based" definitions: Both styles ...

4

Fair exchange protocols aren't new by any means, but there is a lack of layman-friendly material out there, unfortunately. I think the high prevalence of theoretical cryptography in fair exchange protocols may be partially responsible for that. At any rate, here is the basic idea behind a fair exchange protocol. Suppose you have two parties, Alice and Bob, ...

4

A "cipher" is the algorithm which encrypts and decrypts data, while the "cipher-mode" defines how the cipher encrypts and decrypts it. In other words: ciphers are the cryptographic algorithms that you use to encrypt/decrypt data, while cipher-modes define the "mode of operation" for applying the cipher. Both are complementary and can be chosen separately. ...

4

Many block ciphers are defined by specifying a round and then running that specification multiple times. For example, in AES, a round consists of the operations SubBytes, ShiftRows, MixColumns, AddRoundKey. That is one round and, to get AES, you run that multiple times (plus some setup and some post-processing). Thus a round is defined by each cipher and ...

3

In general, a cipher is simply any algorithm for encrypting data. It's a really broad term, and might cover anything from ancient substitution ciphers like the Caesar cipher to modern-day public-key ciphers like RSA. In modern cryptography, there are two commonly encountered types of symmetric (i.e. not public-key) ciphers: block ciphers and stream ...

3

Hard-core bits are related to one-way functions. For some intuition on what hard-core bits are, consider a one-way function $f$. SInce it's a one-way function, it's hard to invert: that is, if I select a random $x$ in the function's domain and give you $f(x)$, you cannot find a $x'$ such that $f(x) = f(x')$ with non-negligible probability in probabilistic ...

3

It's sometimes called a keyword cipher. As dr jimbob notes, it's a particular type of monoalphabetic substitution cipher. Ps. See also this recent question about breaking such ciphers.

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As @CodesInChaos explains: It might refer to blind signatures. It also might refer to a method to harden (typically) RSA implementations against timing/side-channel attacks, by blinding the data before operating on it. Example: suppose you are writing code to decrypt data, i.e., to compute $y=x^d \bmod n$, given the input $x$. The naive way to do is just ...

2

Protocol refers to a highly structured mode of operation. A protocol acts in a very specific way which is laid out, it is not possible to deviate from a protocol without failure. Scheme refers to a more generalised set of solutions to a problem, not defining any particular way that it should be done. For example, A cryptographic communication scheme ...

2

In a signature scheme with appendix (such as RSASSA-PSS), the signature $s=Sign(M,PrivateKey)$ of the message $M$ is usually appended to the unmodified message $M$, forming $(M,s)$. This is effectively sent, and verified; the signature is an appendix to the message. Signature scheme with appendix opposes to signature scheme with message recovery (such as ...

2

A cryptographic algorithm where the operations for encryption and decryption are identical is called a reciprocal cipher. As dfaranha mentioned, this is mathematically an involution. Most mechanical cipher machines use a reciprocal cipher, so it wouldn't need a seperate "encode mode" and "decode mode". Some of the more famous reciprocal ciphers are: The ...

2

Linear functions when expressed as polynomials only have terms of degree 1 or 0. Non-linear functions have at least one term of degree 2 or higher. For example, here is a linear boolean function: $y = ax + bz + c$, where $y$ is the output bit, $x$ and $y$ are input variables, and $a$, $b$, and $c$ are constants. Notice that none of the variables are ...

1

Efficiency of PIR largely depends whether you have a single-server or a multi-server PIR (replicate the database $n>1$ times). Multi-server PIR seems more attractive (Ian Goldberg does a lot of research into this direction, e.g., this paper is the basis for most multi-server PIR approaches). Considering the database to be organized in a $r\times s$ ...

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Unified addition means that the formula you use for adding two points doesn't depend on whether they are equal. Simplifying point addition in the Weierstrass form somewhat $s=(y_A-y_b)/(x_A-x_b)$ when $A\neq B$ - this is "Adding". Otherwise $s=(3x_A^2-p)/{2y_A}$ when doubling a point. The implementation of these steps would normally involve different code ...

1

I believe the closest term for what you describe is "involutional cipher". The encryption operation would not be strictly identical to the decryption operation, but the only difference would be the order in which the key schedule is applied: http://www.iacr.org/archive/fse2003/28870048/28870048.pdf

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Scheme would be more like the plan, design, or program of action to be followed. It differs from protocol in this context because it might not effect or manipulate any data or other tangible asset. Protocol relates more to the conventions and treatment surrounding the formatting of the data used in electronic communications systems

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