# Tag Info

## Hot answers tagged terminology

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

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

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 ...

5

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 ...

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

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

"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 ...

3

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 ...

3

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 ...

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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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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

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 ...

1

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 ...

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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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