# Tag Info

8

What is usually meant by "group encryption" is not what you are after. Group encryption algorithms strive to achieve the following: a given message is encrypted, and may be decrypted only if sufficiently many group members collaborate. This is not what you seek; what you want is a system such that a given message can be encrypted once and every member of the ...

7

ElGamal appears to be used instead of Diffie-Hellman (or IES) in OpenPGP mostly because when that format was put together, there were some unresolved intellectual property issues surrounding both RSA and Diffie-Hellman, while ElGamal was unproblematic. This trend for ElGamal seems to stick around, mostly by force of habit, e.g. when switching to ...

7

ECDSA is a digial signature algorithm ECIES is an Intergrated Encryption scheme ECDH is a key secure key exchange algorithm. First you should understand what are the purpose of these algorithms. Digital signature algorithms are used to authenticate a digital content.A valid digital signature gives a recipient reason to believe that the message was created ...

6

So your protocol goes like this: Alice generates a key pair $(a_{priv}, a_{pub})$ and sends $a_{pub}$ to Bob. Bob generates a key pair $(b_{priv}, b_{pub})$ and sends $b_{pub}$ to Alice. Alice generates a message $m$ and sends $Enc(Sign(m, a_{priv}), b_{pub})$ (or $Sign(Enc(m, b_{pub}), a_{priv})$, I'm not sure which of both is usually used by PGP) to Bob. ...

5

One observation is that if we modify the problem so that $M, A, B$ are random invertible matrices, then it is easy to prove the security of the system. In fact, we can prove that the system is informationally secure; that is, for any observed $C_1, C_2$ pair, for any possible value of $K$, there is a unique set of values of $A, B, M$ that yield that $K$ ...

5

First, I am assuming, per https://security.stackexchange.com/questions/29172/what-changed-between-tls-and-dtls, that the client handshake protocol in DTLS is not different from that in TLS over TCP. This seems a safe bet since the client/server encrypted handshake protocol in OpenVPN's UDP implementation is the same as in standard TLS over TCP. I am not ...

5

Let’s take your questions in order. Note that I’m a physicist working in quantum cryptography, so my opinion on this might be biased 1. What about authentication ? The classical channel between Alice and Bob has to be authenticated in order for the protocol to work. Formally, this is a pre-requisite for quantum key distribution (QKD), and is not part of ...

5

What's to guarantee authentication or message integrity (particularly when Alice and Bob are exchanging which filters were correct and so on)? A pre-authenticated classical channel is an essential requirement in addition to the quantum channel on which the quantum key exchange (QKE) is performed. This implies that Alice and Bob must share an initial ...

4

That was a bad edit to Wikipedia. The phrase "Carry-forward verification" is not a standard, well-known term in the cryptographic literature. It should not have been included in Wikipedia without a reference to something more specific. But oh well, no one is perfect, sometimes these things happen. Your request for an elaborate survey of MITM defence is ...

3

The major thing missing from Diffie-Hellman is that it provides no protection from someone running a man-in-the-middle attack. Your changes don't actually do anything to prevent that. That is, suppose Eve was between Alice and Bob; when Alice sends the first message to Bob, Eve intercepts the message, and performs the exchange with Alice. At the same ...

3

A possible deficiency is that if the use made of any $K_j$ allows it to leak, all later security is lost. That makes $K_j$ plain unsuitable in some uses, e.g. directly as keystream for short messages. The $K_j$ must be wide enough that it is extremely unlikely that a cycle is ever reached in deriving them. For plausible parameters that translates to ...

3

I will address your question below, however I have a serious concern that I want to bring up first. I glanced at the $p$ used in ngx_ssl_dhparam, and it is not immediately obvious that it was chosen correctly. Unless you know that whoever generated that value knew what they were doing, you should select a different value. The security of DH depends on, ...

3

PKCS#3 is an older standard which only defines the DH primitive itself. It contains the following information: parameter generation, the Diffie Hellman key agreement algorithm, integer/octet string conversions (as in PKCS#1, RSA) and the specification of an ASN.1 structure for the parameters. The ASN.1 structure is very limited, containing only the necessary ...

3

Are there any advantages to “1.”, especially when users must communicate the password/key through a separate channel in both cases? As the comments (1, 2) already indicated: the first option “1.” will be easier to communicate. When you talk about a “high-entropy key”, I assume you are generating that high-entropy with a cryptographically secure random ...

3

ECDH or DH for that matter doesn't provide any authentication of a user. ECDSA as a public key scheme does provide authentication, but lacks validation. You need to certify that the exchanged public keys are indeed from Alice or Bob. So Alice and Bob must let an authority certify their own public keys such that Alice trusts the authority of Bob and Bob ...

3

If we assume that $E$ is just semantically secure, without providing authenticity and integrity of the encrypted message then this scheme is has a huge drawback. It would be possible for an attacker to pose himself as either A or B, or to alter any message send from A to B. So without authenticated encryption, this scheme may protect against eavesdropping, ...

3

I assume that Alice is capable of accepting a connection while negotiating another, and let $A_2$ and $A_1$ denote her two roles. $\;\; A_1 \to M \:$ : $\:$ Alice, $nonce_1$ $\;\; M\to A_2 \:$ : $\:$ Bob, $nonce_1$ $\;\; A_2 \to M \:$ : $\:$ $nonce_2$, $E_{k_{AB}}\hspace{-0.04 in}(nonce_1||k_2)$ $\;\; M\to A_1 \:$ : $\:$ $nonce_2$, ...

3

In TLS, the key exchange step results in a key called the master secret which is then derived into as much key material as needed with a custom key derivation function, called in TLS terminology the PRF. It is not slow -- contrary to PBKDF2, the "PRF" of TLS is not for handling password and thus has no need to be slow.

3

There are several kind of quantum key distribution (QKD) protocols as of today. Are you looking for a particular one? The best known QKD protocol goes by the name BB84 after its inventors Bennett and Brassard and the year in which they presented their work. Searching on the Internet, I found this link http://fredhenle.net/bb84/demo.php with a simulation ...

2

First, understand that keys need to last only as long as you need to recover what they are protecting. If you are storing a secret in a box until next year, you have to keep the key until next year. But if we're talking on the phone, we only have to keep the key for the duration of the phone call. If Alice and Bob are going to speak securely, they don't ...

2

In an ordinary ID-based scheme, you won't get strong PFS. The center always knows a secret that can be used to recover your private key and thus can violate PFS. One approach is a hybrid scheme, such as the following. You could do a (non-ID-based) Diffie-Hellman or ECDH key exchange, with messages signed and authenticated using an ID-based signature ...

2

I know how Diffie-Hellman Key Exchange works. Is this the main way of encrypting with PGP, ssh, ssl (https), DKIM, ...? As the name says Diffie-Hellman key exchange is a key exchange protocol, i.e., a protocol where two parties agree on a common secret without having exchanged any secret prior to that, in an interactive way, i.e., both parties are ...

2

Actually recently I found out about a complete QKD simulation toolkit that has become available, accessible online via this link, QKD simulator. It is a parameter-based simulator, so different scenarios (qubit numbers, Eve's influence, etc.) can be set up and simulated.

2

Is there any chance of finding symmetric key from encrypted public key? The answer is no, assuming the symmetric key has sufficient entropy and a secure encryption algorithm and mode is used correctly. Modern ciphers like AES with proper secret keys are not vulnerable to known plaintext attacks. Why? This has been answered in an earlier question: Why ...

2

From my understanding, this protocol makes use of a trusted third party in order from A and B to exchange a symmetric key, $K_{AB}$. For the protocol to work, it is assumed that both A and B must share a master key, $K_{AS}$ and $K_{BS}$ respectively with the trusted S and A wants to communicate with B but they has no shared secret. Since there is no ...

2

Actually, I never heard of the notation of an implicit signature before. However, when looking at MQV, there is a quantity which is denoted as an implicit signature, which is nicely described here: The MQV elliptic curve key agreement method is used to establish a shared secret between parties who already possess trusted copies of each other’s static ...

2

In addition to the earlier remarks about the missing background of your question please also consider that TLS and IKEv2 are actually not just a single authentication and key exchange protocol but rather a framework that supports many different AKA protocols. Let us use TLS as an example. In TLS you have the concept of ciphersuites and they allow you to ...

2

If the attacker M is impersonating both A and S, then he obviously doesn't have to bother sending the third message in the protocol to himself. Thus, the protocol reduces to: M(A) → B: A, Na B → M(S): B, {A, Na, Tb}Kbs, Nb M(A) → B: {A, Kab, Tb}Kbs, {Nb}Kab where M(A) and M(S) denote M impersonating A and S respectively. By itself, this is not a ...

2

Your proposal is theoretically sound. If an attacker gets $K_5$ the only way to get previous keys would be to "rollback" the hash. If the hash function is secure, this should not be feasible. You could probably even prove it using the random oracle model, for example.

2

What you are describing is known as the Photon Number Splitting Attack (PNS), described for the first time (I think) by Brassard, Lütkenhaus, Mor ans Sanders in this 1999 paper. Several countermeasures have been invented since (single photon sources, robust protocols, decoy states), but detailing them would stray away from of your question. If one sends 2 ...

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