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10

Yes, of course there is a benefit to signing unencrypted emails. The article you cite is solely about the combination of signature and encryption; it doesn't directly say anything about signing unencrypted emails. There is an important concern raised by the article which does apply to unencrypted emails, but that's because that concern applies equally ...


8

The article you linked to predates the S/MIME 3.2 spec. If your client is sending S/MIME 3.2 messages, it should support header protection. Refer to RFC 5751 Section 3.1: In order to protect outer, non-content-related message header fields (for instance, the "Subject", "To", "From", and "Cc" fields), the sending client MAY wrap a full MIME message ...


7

Short answer No, RSA encryption with a private key is not the same as RSA signature generation. RSA encryption can only be performed with an RSA public key according to the RSA standard. The terms Raw RSA or textbook RSA are often used to indicate RSA without a padding scheme. Raw RSA simply consists of modular exponentiation. Raw RSA is vulnerable to many ...


6

It mainly depends on how the algorithm was selected. If it was selected by a public competition like for AES, then it is likely to be secure. If it was forced in by the NSA such as Dual-EC random number generator, then you may have some doubts. Other questions you may want to ask yourself are: Is this an "original" algorithm or was the problem that it ...


6

OpenPGP as defined by RFC 4880 knows two different encodings. Binary encoding Obviously, there is no reasonable limitation to an (ASCII) character subset in binary encoding. Radix 64 Radix 64 is also often entitled ASCII armored. In the end, it is a base64 encoding with a checksum. The content may consist of [a-zA-Y0-0+/=]. ASCII-armored OpenPGP ...


6

Informally, a signature scheme with message recovery is one where some or all of the message is embedded in the signature, allowing to conserve bandwidth when transmitting a signed message, compared to a signature scheme with appendix. Total message recovery A signature scheme with total message recovery [some sources make total implicit, e.g. the HAC ...


5

SafeCurves lists some ways to compare the security of elliptic curves. Their security criteria are split to "ECDLP security" and "ECC security". Failing the former basically means "there is no way to use this curve securely in general" while the latter "it is difficult to implement this curve securely". None of the (few) BouncyCastle-supported curves that ...


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 ...


4

It depends on what you mean by RSA. If you mean the plain textbook RSA where $P = C^d \bmod n$ (decryption with private key $d$) and $S = M^d \bmod n$ (signature generation), then yes, they are the same. However, textbook RSA is inherently unsafe, and for real-life RSA such as RSA-OAEP+ (encryption) or RSA-PSS (signatures) signing is not the same as ...


4

Guillou and Quisquater (link) present a zero-knowledge proof of an RSA signature. Basically, the scheme is as follows: Public knowledge: RSA modulus $n$, public RSA exponent $v$, preimage $X$. Secret knowledge for prover: $A$, such that $A^v = X \mod n$. $$ \begin{matrix} \mathcal{P} & & \mathcal{V} \\ r \xleftarrow{\$} \mathbb{Z}_n^* ...


4

You can use multi-signatures. One example is the BN06 scheme described in the paper: Bellare, Neven - Multi-signatures in the plain public-Key model and a general forking lemma


4

The benefit to signing a non-encrypted email is that any recipient can verify that it was indeed you who wrote that non-encrypted email, unless your key was compromised (or the signing protocol has an exploit).


4

$q$ does not divide $s^e-h(m)$, but $p$ does, so since the gcd must divide both $s^e-h(m)$ and $n$ it's $p$. To be even more explicit, we know that $p$ divides both $s^e-h(m)$ and $n$. The only larger divisor of $n$ that is also divisible by $p$ is $n$ itself, but if $n$ would divide $s^e-h(m)$, then $q$ would also divide $s^e-h(m)$, which we already assumed ...


4

Yes, you can, but you would need access to raw or textbook RSA encryption and you would have to implement the PKCS#1 v1.5 or PSS padding primitives yourself. Beware that PKCS#1 v1.5 compatible padding is different for encryption signature generation. If you only have PKCS#1 v1.5 encryption or OAEP encryption available then the encryption routine will ...


4

Yes! (restrictions apply). ISO/IEC 9796-2 (scheme 1, SHA-1 hash, option 1 also know as implicit hash identifier, alternative signature production function) is a fully standard signature scheme, based on RSA, widely used in the Smart Card industry for public key certificates and message authentication, that adds only 22 bytes of signature overhead (if the ...


4

Well, if the hash function is weak, then the attacker might be able to take a valid signature for a signed message, and find a second message for which the signature for this first would also validate for the second. For example, if Alice signs the message "I like chocolate", what Bob might do is find a second message "Alice owes Bob $13,106,107.57", and ...


3

Here and in many other signature schemes, $f$ is modeled as a "random oracle." This means that on each distinct input, $f$ outputs a uniformly random value in $\mathbb{Z}_q$ that is independent of all other outputs. (When queried on the same input multiple times, it always returns the same answer.) The trick here is that the simulator has the power to ...


3

RSASSA-PKCS1-v1_5 does not use random padding, the scheme is deterministic. RSASSA-PSS is different from other RSA-based signature schemes in that it is probabilistic rather than deterministic, incorporating a randomly generated salt value. The signature is verified using a hash over the message hash and the salt. So the verifier does not ...


3

No, these sorts of attacks are not of any use against RSA -- they are much harder to perform than other existing attacks (and in particular, attacks that factor an RSA modulus). Here is how this precomputation attack works; you assume that someone generating the keys will always MAC (or sign) a specific message: $$S_i = MAC_{K_i}( FixedMessage )$$ And so ...


3

In general, no. There are: $$ {2^{64} \choose 2^n} = \frac{2^{64}!}{2^{n}!(2^{64}-2^n)!} $$ possible ways of selecting $2^n$ distinct 64-bit vectors. This is a huge number; using Stirling's approximation of factorials, when $2^{n}$ is substantially smaller than $2^{64}$ (i.e. when $n$ is smaller than $55$ or so), this number of combinations is approximately ...


3

I guess the answer is no, as long as you are using ECIES then this protocol does not work - you cannot trust the public key of Bob, which is required for ECIES. You could however use ephemeral-static Diffie-Hellman, using ECDH as cryptographic algorithm. Alice would supply the static part as her public key is trusted, Bob may use any key pair. That means ...


3

Basically because of Fermat's little theorem: if $a$ is not divisible by $p$ then $a^{p-1} = 1$ $mod$ $p$. A part of the expression for $\delta$ appears as a power of $a$ in the ElGamal signature verification equation, which "happens" to work because it is reduced modulo $p-1$ so Fermat's little theorem applies.


3

The paper itself has more details on this: ECDSA, like many other signature systems, asks users to generate not merely a random long-term secret key, but also a new random secret session key $r$ for each message to be signed. ... If the same value r is ever used for 2 diff erent messages the secret key can be computed as well, as ElGamal... It ...


3

If you want $N$ serial numbers, your serial numbers will have to use $n$ bits for uniqueness, where $n = \log_2 N$. So if you have 100 bits to use for the serial, you could use 20 to get about a million serials and have 80 bits to use for a cryptographic MAC or signature. Now there are two approaches, the symmetric and the asymmetric. In the symmetric ...


3

If you look at exact security, the height matters. The reason is that it defines the number of OTS key pairs and hence the possible number of one time signatures per MSS key pair. To forge a MSS signature, it is enough to generate a forgery for 1 out of $2^h$ OTS signatures. Hence you get a reduction in the bit security of $h$ bits.


3

This sounds like "fair exchange," the subject of many good research papers. In general you need a third party to give any security guarantees, but "optimistic fair exchange" involves the third party only when one of the parties tries to cheat (i.e., when both play honestly there is no involvement from the third party). Incidentally, Diffie-Hellman is most ...


3

You can prove that a document was signed after a certain date by including data that was not known to anyone before that date, such as stock market data. You cannot prove that a document was signed before a certain date by purely cryptographic means. Information doesn't go stale, so when you show a signature, it could have been signed at any time. You can ...


3

I think you have some misunderstanding here. Finding collisions when knowing the trapdoor is a required feature, but leaking the trapoor when knowing collisions is an undesirable "feature" (which some constructions suffer from). A chameleon hash function (aka trapdoor commitment) allows you given the trapdoor to find pairs $(m,r)$ and $(m',r')$ with $m\neq ...


3

No, not really. Elliptic curve signatures are the smallest you'll find in common use. An $n$-bit elliptic curve produces $2n$-bit ECDSA signatures. The smallest standard curves that offer some security are 160-bit, and those are not really recommended (e.g. NIST recommends 224+ bits). That would give you 40 byte signatures. Lower than 64, but not 32. So 40 ...


3

The only way you could do this if if you could affect the padding schemes appropriately. Mathematically, textbook RSA encryption with the private key is the same as textbook RSA signature generation. Nobody should use textbook RSA, however. In practice, padding schemes are used and they differ between the two operations. So unless you can turn off padding ...



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