# Tag Info

15

Curve25519 was designed to take advantage of the Montgomery ladder, which combined with Montgomery curves forgoes the $Y$ coordinates, is side-channel resistant, and enables public keys to be any 255-bit string. The ladder looks something like this (pseudocode): Q[0] = P; Q[1] = 2*P; for(int i = log2(exponent) - 2; i >= 0; --i) { Q[ bit(exponent, i)] ...

13

A simple way to imagine the effect of the hash function is a truncation. A "good" hash function ought to behave like a random oracle. If your source has entropy $s$ bits, then this means that the source somehow assumes $2^s$ possible values. When processed with a random oracle with an $n$-bit output, you force the $2^s$ input values into $2^n$ possible ...

10

Randomness is not a property of strings of bits (or characters of any sort). Rather it is a property of the process that generates those strings. However, it is convenient to conflate the string with the thing that produced the string, and thus to speak about strings being “random” or “not random”. The string 00000, for example, is random if it was the ...

8

No, because even SHA-512 was considered overkill from a security perspective. It has 256-bit collision resistance, which is unbreakable. (The link is about keys but a similar argument applies.) If you think large quantum computers will be efficient, a 512-bit hash makes some sense, but even then a 1024-bit one wouldn't. A quantum computer requires ...

8

Pure Threefish has received less attention than Skein. Shortly speaking, it has a large security margin, and can be safely used for encryption. In more details, Threefish has been tweaked twice. The first two versions were vulnerable to rotational cryptanalysis in weak models (related-key attacks or distinguishers) up to 57 rounds. All these attacks are ...

8

The idea of "safe curve" is somewhat overrated. What you really want is a safe implementation which won't leak secret information when employed in some practical context. Leakage may occur in a variety of ways; some examples include timing attacks and implementation behaviour when encountering anomalous input. This is not an exhaustive list, and, depending ...

7

In RSA as usually practiced (encryption or signature per PKCS#1, signature per X9.31, ISO/IEC 9796-2, FIPS 186), it is NOT necessary, or even common, to require $n=p⋅q$ with $p=2⋅p′+1$ and $q=2⋅q′+1$ with $p'$ and $q'$ huge primes, as stated in the question. IF that's done, it ensures that: any small odd $e>2$ (including the common $e=3$ and $e=65537$) ...

7

It's called a keyword cipher. See this question for some ways to break it.

7

Uniformity is a tricky one. SHA-256 (as well as SHA-3 for that matter) follows a heuristic approach. That is, the design is not based on a hardness assumption (for example, the factoring or discrete-log assumption) but on criteria that have only been verified empirically. As such, also the study of uniformity is an empirical study. The development of ...

7

Yes, they are (deterministically) equivalent. The original RSA paper (Section IX.C), working off Miller's results (Theorem 3), showed how knowing the secret exponent $d$ was probabilistically equivalent to factoring $n$. Later, using more advanced techniques, Coron and May showed how to deterministically reduce finding $d$ to factoring $n$.

6

Did you take a look at DjB's paper? One of his design criterias in order to improve performance is "Use a fixed position for the leading 1 in the secret key". The set of secret keys is defined to be $\{\underline{n} : n \in 2^{254} + 8\{0, 1, 2, 3,\ldots, 2^{251}-1\}\}$.

6

Safe primes (that are two times a prime plus one) and strong primes were at some point in time considered sensible. One reason was that safe primes ensures that Pollard's $p-1$ factoring algorithm stops working. However, safe primes are not enough. There are other related factoring algorithms, such as the $p+1$ method, and strong primes also stop them. The ...

6

There is no uniform permutation; there is a permutation uniformly chosen from the set of all possible permutations over $Z_2^{128}$. It is evident that AES is not a uniformly chosen permutation, since its permutation is fixed for any key. One can consider a family $\{AES_K\}$ of AES permutations under all possible keys $K$. Even if the key is chosen ...

6

Using a MAC on the plaintext may potentially leak information about the plaintext (MAC algorithms do not necessarily ensure confidentiality of the data they are applied to, although some MAC algorithms like HMAC seem pretty safe). If you want to avoid this (theoretical) problem, then you should encrypt the MAC on the plaintext (i.e. MAC-then-encrypt, not ...

6

To answer point 2: No. When using a good encryption scheme, one aims to prove that the ciphertext is only negligibly different from random data. As such, without breaking the encryption scheme completely no information (for example the entropy) of the plaintext is leaked.

6

As far as I can tell, NIST has only one official document about entropy collection. SP-800-90B. The purpose of NIST Special Publication (SP) 800-90B is to specify the design and testing requirements for entropy sources that can be validated as approved entropy sources by NIST‘s CAVP and CMVP. It essentially defines a bunch of statistical tests to ...

5

Start with NIST Computer Security Division: FIPS (Federal Information Processing Standards) specifies multiple use scenarios in their Publications and Special Publications, many of which include the type of scenarios on your short list, as well as related scenarios. These are primarily for federal use, but if your use case matches that of the government ...

5

To be concise, true randomness boils down to the selected data being causally unrelated. That is, if each piece of data is the result of no common cause, then there is no relation by which the rest of the data can be predicted or inferred. So being unpredictable is a consequence of being truly random, but it is the lack of causal relationship that is the ...

5

Randomness is the information loss of any causal relationship between events. The universe needn't be a clockwork universe for the assumption of pervasive causality - if events are "sticky" and accrue localised causality in the same way that a molecular cloud accretes into stars and planets. The underlying cause of the speed of light might also be the prime ...

5

The most well known example of a cipher practically broken with linear attacks is by no doubt DES, a cipher with 56-bit key and 64-bit block. Equipped with a cluster of PCs in the year 1994, Mitsuru Matsui has experimentally found a secret key after 10 days of the analysis (the data generation took additional 40 days on the same machine set). By that time ...

5

Actually, there are also other reasons why one wants to use safe primes in the RSA setting (when working with hidden order groups in cryptographic protocols). When choosing the RSA modulus $n=pq$ to be the product of safe primes $p=2p'+1$ and $q=2q'+1$, then we also have the following: The subgroup of $Z_n^*$ of qadratic residues is cyclic and has order ...

5

Full disclosure — I'm a Skein/Threefish co-author. Also, when I mention Skein/Threefish without any other qualification, I mean Skein/Threefish-512. The security proofs we did for Skein prove that if there's a weakness in Skein, it implies an underlying weakness in its components (Threefish or UBI). As Dmitry says above, Threefish is very strong, and there ...

5

We need clear goals. The question asks for "plausible deniability" or "deniable encryption", and these terms needs a precise definition in a public-key context (implied by RSA). I assume that in addition to the IND-CPA and IND-CCA1 properties of a cipher, including hybrid (as implied by AES), it is desired that: One without the private key can't ...

5

A video camera can obtain entropy, but only at a fairly low rate and only if allowed to see "unusual" scenes… like someone making funny faces, unusual movements, etc. Of course, this only works in a room with no video bugs. Theoretical explanations… Depending on your knowledge-range, the following sources may be able to explain ways webcams can be used ...

5

I think you don't quite understand how RSA signatures work (and why they are the size they are). When generating an RSA signature, we follow a two-step process: We take that hash of the message we're signing, and convert (and pad) it into an integer $M$ which is between 0 and $N$ (where $N$ is a large integer that specified by the RSA key) We use the RSA ...

5

The problem doesn't lie with curves in Weierstrass form necessarily, but with naive implementations of elliptic curve arithmetic on such curves. Basically, if you implement an ECC scheme (ECDH, ECDSA or whatever) on a smart card using a curve in Weierstrass form in the most straightforward way possible (by writing a simple double-and-add loop for ...

5

I assume $R(x)$ is the original generator, returning $r$ uniformly distributed with $0\le r<x$ for $x<2^{n-1}$, as does Java's int nextInt(int) for $n=32$; and we want to extend that to $R'(y)$, returning $r$ uniformly distributed with $0\le r<y$ for $y<2^n$. $R$ and $R'$ should treat an argument less than $2$ in the same way (perhaps accept it ...

5

Under the assumption that $(K,\text{Msg})\to H_K(\text{Msg})$ is a secure MAC (be it HMAC or any other MAC), and $\text{Nonce}$ does not repeat and is of fixed size, both $H_K(\text{Msg}||\text{Nonce})$ and $H_K(\text{Nonce}||\text{Msg})$ are demonstrably secure, in the sense that an adversary not knowing $K$ can't distinguish either from random, even for ...

4

Here's a nice paper I came across a while ago: Wooding, Mark (2008), "New proofs for old modes", Cryptology ePrint Archive, report 2008/121: "Abstract: We study the standard block cipher modes of operation: CBC, CFB, and OFB and analyse their security. We don't look at ECB other than briefly to note its insecurity, and we have no new results on counter ...

4

Google (and other companies) have decided to enable one-time passwords for their 2-factor authentication as a step to improve password security. Here is the webpage that explains what Google is doing in more detail (including source code): https://code.google.com/p/google-authenticator/ In a nutshell, they implement two IETF RFCs, namely RFC 6238 and RFC ...

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