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This is one of the earliest questions that was asked in modern cryptography. There is a proof that you cannot achieve completely fair contract signing. However, there are some reasonable alternatives. There is one direction called "gradual release" which I personally do not like. A model that I think has a lot of promise is called the "optimistic model". In ...

5

Rabin-Williams signature verification with 3072 bit keys is much faster than EdDSA signature verification of comparable security (when done in software). How much depends on care of coding, hardware, EdDSA parameters. Two data points: in the eBATS benchmarks for a skylake CPU, ronald3072 signature verification (RSA with $e=3$ as an OpenSSL wrapper, by ...

5

The usual methods of RSA signature with message recovery (that is, embedding a part of the message in the signature, known as the revoverable message) are those in ISO/IEC 9796-2:2010 (partial preview). There are three: Scheme 1; it is an ad-hoc scheme that essentially concatenates the recoverable message, the hash of the whole message, and a little fixed ...

4

OK, so this takes a bit of guessing, but I'm assuming the following: A is the identity of A, which can be used to select the right public key of A; Kxa() is a signing operation (with message recovery) that signs the random Ra and a tag Ts which is used as proof; Kya() is an encryption operation with the public key A, so that the session key Ks is kept ...

3

Suppose you want to obtain the signature $s = m^d \bmod n$ on a chosen message $m$. Here is that attack. You ask the signer to sign a random message $m_1$ and obtain the corresponding signature $s_1 = m_1^d \bmod n$; You compute message $m_2 := m\cdot m_1^{-1} \bmod n$ and ask the signer to sign message $m_2$; you obtain the signature $s_2 = m_2^d \bmod n$...

3

So is PKCS7 a signature format or a certificate format or both? Neither. PKCS7 is now Cryptographic Message Syntax(CMS). From the RFC 5652: This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. CMS enables interoperability between different products which can operate on the same document without ...

2

When a contract in digital from is to be signed online by Alice and Bob, an issue concerning the fairness of the signing process crops up as follows: If Alice first signs the document and sends it to Bob, it means she has committed to something (e.g. ready to purchase an article from Bob at a certain price), Bob can however, if he desires, to some extent ...

2

The Procedure Step 1: Factor the original signature $s$ into $s=\prod_{i=1}^n s_i$ and then exponentiate each signature with $e$ as in: $m=\prod_{i=1}^n s_i^e=\prod_{i=1}^n m_i$. Different methods to obtain multiple $s_i,m_i$ pairs work just as well, such as asking the signing oracle. Step 2: Build a new message with a valid signature as the product of any ...

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