# Tag Info

14

If this requires a single answer among 1/2/3/4 (rather than none), I would select 3, by the following reasoning: Digital Signature provides confidentiality while message authentication code can not We can summarily exclude this, since since Digital Signature simply do not provide confidentiality. Digital Signatures works faster than ...

7

Alice could just generate a random number (to be their shared key), sign it, encrypt it with Bob's public key, and send it to Bob. I, as an eavesdropper, can capture this exchange. In fact, I can capture many of these as I want with other people communicating with Bob. Then, fast forward to some point in the future, if I can compromise Bob's private ...

7

The correct term for bytes to be signed is “message”. Generally, it does not really matter if a message to be signed is human readable or not. Sometimes, you may also find it mentioned as “digital message”… which practically is the same and merely extends the term to explicitly hint at the fact the message is digitally stored and/or processed. References ...

6

As pointed by CodesInChaos, you'll need to know the padding used; depending on application that could be RSASSA-PKCS1-V1_5, or RSASSA-PSS, or some of the three schemes of ISO/IEC 9796-2, etc.. Hashing, and padding check, are a significant part of the code. In any case, yes, it is possible to implement RSA-2048 signature verification on a Cortex-M0 ...

6

Can I compute the only hash of the modulus for integrity? Well, if we allow the attacker to modify the value of $e$ you use (because it's in untrusted storage and you don't verify it), how can he exploit that? Well, the most obvious approach for him would be to modify $e$ to be the value 1; that would make generating forgeries really quite simple. ...

6

Note that the signature is $(s,e)$ where $s=k-xe$. If you can learn $k$ since it is predictable, then you can learn the secret signing key by computing $x = (s-k)/e$. Note that even without a concrete attack, the proof of security completely breaks down if the value $k$ is not chosen randomly. Having said this, it is possible to change the scheme to be ...

5

No, plain RSA signatures are existentially forgeable under a key only attack. This is because of the following attack strategy: Given a verification key $(e,N)$, set the message to $m=s^e \bmod{N}$ for an arbitrary $s$ in the message space and set the corresponding signature to $s$. Output the message signature pair $(m,s)$ as a forgery. It is easy to ...

5

The standard way to do this is with a hash list. That is, you would hash each of the messages $m_i$ to produce a hash $h_i = H(m_i)$, and then combine all the hashes and hash them to obtain a master hash $h = H(h_0 \| h_1 \| h_2 \| \dots \| h_n)$. Finally, you can e.g. digitally sign the master hash to prove that the hash, and by extension all the ...

5

Although I can't see any immediate weaknesses, I also don't see how it adds significant value over DSA (while being significantly slower). It claims to be based on two hard problems, discrete log and factoring. However, it doesn't give any particular proof that if you could forge signatures, you can solve both problems. It also doesn't look particularly ...

5

Towards the security of the signature scheme, no precaution against timing attack is necessary when verifying an asymmetric signature. That's because there is no secret involved, thus no information leak to fear. However it can happen that the message, or the signature itself, is intended to be secret; a leak by timing dependency (during computation of the ...

5

My answer is not original, as I am simply summing up information from different questions already solved in this site. Nevertheless, I thought it could be interesting to collect everything in one answer. The first thing you have to know is to differentiate between a digital signature and a message authentication code (MAC). In this case, HMAC is a MAC and ...

5

I'd say that most of the time the signature is accompanied by the certificate of the signer. This certificate contains the public key. Most container formats such as CMS (used in S/MIME, also known as PKCS#7) or XML digsig contain specific fields that may contain certificates - and usually do. When the certificate is received the Public Key Infrastructure ...

4

The short answer is that there's no link between your physical signature and any cryptographic signature. Indeed, from the high-level description of how DocuSign works and their security manifesto there's no reason to believe that any cryptography goes into the signature process itself. Note that “signature” is an overloaded word. In this post, I will refer ...

4

The main benefit of adding randomness in RSA signature padding is that it simplifies and strengthens security arguments. At least that's claimed by PKCS#1v2, paragraph above 8.1.1 (emphasis mine) RSASSA-PSS is different from other RSA-based signature schemes in that it is probabilistic rather than deterministic, incorporating a randomly generated salt ...

4

DSA relies on $k$ being independent from $d$. You define $k$ as: $k=z^\prime d\mod n$ Substituting $k$ in the signing equation you get: $s = k^{-1} (z+rd) \mod n$ $s z^\prime d = z + rd \mod n$ $d=z (sz^\prime -r)^{-1} \mod n$ The attacker knows everything on the right side and can recover the private key.

4

The question "why is preimage resistance needed for hash functions" is not really relevant. This is because collision resistance implies preimage resistance. Thus, it is just a fact that if you have collision resistance then you must have preimage resistance. So, instead, I will relate to what preimage resistance is good for at all. In more technical ...

4

If he gets the signature for the message 00000..00000, then the checksum will be $t_1 2^w$. For any other message, the checksum will be smaller, and hence the there will be at least one digit $i$ within the checksum for which the $c_i$ digit with value $v$ for the signed message will be larger than the corresponding digit for the new message. The attacker ...

4

First lets be precise on some definitions : Integrity = only the authorized users can modify the information. Confidentiality = only the authorized users can access the information. Here the information is in plain view. Authentication = Proof of the identity of the content/sender (sort of proof of identity), be sure to not mistake it with identification. ...

4

One of the main differences is that Message Authentication Codes don't prove authorship of the message. Imagine the situation, when Bob sent a signed contract to Alice. In case of digital signature Alice can go to court claiming that Bob has signed the contract. A judge can verify the signature and make sure that the contract was really signed by Bob as only ...

3

Proofs of Storage (PoS) are challenge-response protocols that allow a client to verify that a server is truthfully storing a file. See this paper from Ateniese, Kamara and Katz for an example of PoS. The basic idea is explained in this quote from that paper: Viewing the file $\vec f$ as an $n$-dimensional vector, the client begins by tagging each ...

3

By fixing the first byte of both the private and the public key you actually reduce the key space by about 16 bits, because only about one in $2^{16}$ key-pairs has that property – as you notice. (If there is no way to exploit the restriction on both keys at the same time, you may lose less security but I would assume the worst.) Doing this is fine, unless ...

3

I'm trying to get my head around how the crypto solves this problem. It doesn't. You need to trust the platform you use to do the signing. For instance, my bank has replaced the "signature" generation device that I previously used with one that displays the actual transaction, so I don't have to trust the information on the computer screen that much.

3

If we signed a secret message $m$ by publishing its signature $σ$ computed as $m^d\bmod N$, at least two very bad things would happen: The message would not be so secret anymore That's because anyone knows the public key $(N,e)$, and thus from $σ$ can compute $σ^e\bmod N$, which is $m\bmod N$. This reveals a lot of information about $m$, which goes ...

3

An attack is described in Section 4.1.6 of the SEC1 document. Regarding xagawa's answer: The attack you describe is different from that described in Section 4.5 of the Blake-Wilson--Menezes paper. Specifically, their attack: (a) does not require knowledge of the secret ephemeral key $k$, and (b) changes the reference point $P$, which is not allowed in ...

3

First to explain you, why you get 512-bit outputs from a 256-bit curve: The output is basically a point (x-coordinate is enough) and a message-dependant value, with the x-coordinate being expressed as integer. You can verify the signature by checking for a specific relationship between the point and the message-dependant value and the public key point. In ...

3

In this notation, $f^k(x)$ means "apply $f$ $k$ times in succession". For example, $f^3(x)$ is defined to be $f(f(f(x)))$. Because of this definition $f^a(f^b(x)) = f^{a+b}(x)$ holds trivially (even though we known nothing else about $f$), as the the left side means "do $f$ $b$ times, and then do it $a$ times", while the right means "do $f$ $a+b$ times". ...

3

At first I want to cite Lindell and Katz book: A "plain Rabin" encryption scheme, constructed in a manner analogous to plain RSA encryption, is vulnerable to a chosen-ciphertext attack that enables an adversary to learn the entire private key. Although plain RSA is not CCA-secure either, known chosen-ciphertext attacks on plain RSA are less damaging ...

3

Among several aspects of the question, I'll cover only protection against replay of commands. A common technique (among several) is to have commands tied to a nonce, that somewhat is accepted only once by the slave device receiving the command. The nonce is included in the input of a MAC or public-key signature algorithm that protects the integrity of the ...

3

If Eve can find an $n'$ that is prime (and $n'-1$ is relatively prime to $e$),then she can easily sign any message with that $n', e$ pair. So, the question is: what is the probability that there exists a prime $n'$ such that there is only one bit difference between $n$ and $n'$, and $n' \not\equiv 1 \pmod {e}$ ? The answer is that it is quite good if \$e ...

3

Peter Schwabe, one of the authors of Ed25519, directed me to a recent paper titled "EdDSA for more curves". The section "Security notes on prehashing", page 5, says that the Ed25519 algorithm without prehashing the message is resistant to collisions in the hash function, while using the algorithm with prehashing is not. Of course the hash function is not ...

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