# Tag Info

63

An ASN.1-encoded SSH private key contains the following integers in order: The public modulus $n$ and exponent $e$; The private exponent $d$; The prime factors $p$ and $q$ of $n$; The "reduced" private exponents $d_p=d\bmod(p-1)$ and $d_q=d\bmod(q-1)$; The "CRT coefficient" $q_{\text{inv}}=q^{-1}\bmod p$. The observation that the value of $d$ in such a ...

51

These types of cryptographic primitive can be distinguished by the security goals they fulfill (in the simple protocol of "appending to a message"): Integrity: Can the recipient be confident that the message has not been accidentally modified? Authentication: Can the recipient be confident that the message originates from the sender? Non-repudiation: If ...

20

Mathematically it work just fine. "Encrypt" with the private key, "decrypt" with the public key. Typically, however, we say sign with the private key and verify with the public key. As stated in the comments, it isn't just a straight forward signing of the message $m$. Typically a hash function and padding is involved. Also, often one has a separate key ...

18

A few observations: RC4 suffers from related key attacks. This means your idea of concatenating a 224 bit key and a 32 bit IV is not a good idea. You should rather use $\operatorname{SHA-256}(Key||IV)$ Remember that a (Key, IV) pair must not be reused, ever. A 32 bit IV can work if it's a counter, but IMO such a scheme is unnecessarily fragile. I'd rather ...

16

Asymmetric encryption and signing are entirely distinct concepts. The security and construction requirements are completely different. (Encryption for instance, needs to be an injection, whereas signature verification needs to be a surjection towards the verified message space. As a further hint of how they are intrinsically different, note that it lasted ...

16

TL;DR No, the approach is not secure. Use a standard like CMAC instead. Or even better, check your AES accelerator module to see if it supports any AEAD modes of encryption like GCM, CCM, EAX. Long Version In order for a message authentication code (MAC) to be secure, an adversary with oracle access to the MAC (basically this means the adversary can send ...

14

This is an active area of research. There have been attempts at creating usable security tools and lots of user studies of existing tools (typically with critical results). A good anthology for work in this area (a few years old now) is Security and Usability edited by Lorrie Cranor and Simson Garfinkel. There is also a workshop every year called Symposium ...

13

What happens if the sender is at another point in the sequence? ... the key is pressed while out of range to the car. In a rolling code (code hopping) system, the keyfob transmitter maintains a synchronization counter C, incremented every time a button is pushed. The car receiver stores the most recent validated synchronization counter it has received ...

12

I'm considering switching to ECDSA, would this require less space with the same level of encryption? The answer to that question is yes, both ECDSA signatures and public keys are much smaller than RSA signatures and public keys of similar security levels. If you compare a 192-bit ECDSA curve compared to a 1k RSA key (which are roughly the same security ...

12

If k is a constant, such as 3, it becomes possible to select a pair (N,g) such that the discrete log of k to the base g is known, which would enable the two-for-one guessing attack again.

11

There's an obvious solution using DH: Alice has a private key $a$ and a public key $g^a$; Bob has a private key $b$ and a public key $g^b$. When Bob sends a message, he computes the shared secret value $(g^a)^b$, converts that into a MAC key (possibly using a nonce to prevent key reuse), computes the MAC of the message, and sends the message and the MAC ...

10

Cryptography is being a lot of places already and people might just know about it. For example, whenever you access an HTTPS page, that's cryptography protecting you. For desktop applications, many people use the Truecrypt application to protect their files. You also see a similar application in Windows BitLocker. As for why more people aren't using ...

10

The two keys $(e,m)$ and $(d,m)$ in the RSA key pair are fully equivalent, in that $e$ and $d$ can be arbitrarily chosen integers (in the interval $0..m-1$) as long as they fulfill the well-known congruence $ed \equiv 1 \mod \phi(m)$. You can swap them and nothing will change. A key becomes public the moment you (as the key generator) decide to disclose it, ...

9

No, RC4 is not completely broken. It is possible to use it properly. It's just not very likely that an average developer will do so. RC4 is not a good choice for new systems. It is tricky to use properly. There are some serious pitfalls which, if you're not an expert cryptographer, can bite you in the butt. In fact, if you take a quick look in the ...

9

No, you are not leaking any information except how to MAC those specific values with the specific key you are using. Using a short message is exactly as secure as using a long message. For the following, remember the definition HMAC (K,m) = H((K ⊕ opad) || H((K ⊕ ipad) || m)). There are two hashes here, an outer hash and an inner hash nested inside the ...

9

The usual answer is that a salt can be make public; if that was a problem, then the salt would not be called a "salt" but a "key". In some protocols, unauthenticated obtention of the salt is the norm, and is not considered to be a problem. E.g. with SRP, a password-authenticated key exchange, where any salting and hashing must necessarily occur client-side. ...

9

It is not secure, because an attacker can "mix and match" the output blocks from different authentication tags on different input messages, or repeat output blocks for repeated input blocks. For example, if the attacker knows the tag $F_k(m)$ for a one-block message $m$, then it can forge the correct tag $F_k(m) \mid F_k(m)$ for the two-block message $m ... 9 First the theoretical explanations: Integrity and authenticity are different goals to achieve, but both are achieved (for symmetric encryption) with a MAC. You should probably be using encrypt-than-MAC or an authenticated cipher unless you have very good reasons not to. No blanket statements can be made though. HMAC: HMAC is a often used construct. It ... 8 You could encrypt them using some key derived from the user's password (to your site). Of course, this assumes that you get your user's passwords in plain text (or in any form which is always the same) - thus you need to have an encrypted connection to your user. Do not allow any non-SSL login. You can use some key derivation function like PBKDF or bcrypt ... 8 The main weakness in cryptography is that people are involved. I'm also wondering why the allegedly secure websites (financial/healthcare) that I use, still use password access and still have "security questions" in case I forget my password, where the security questions aren't very secure. There are more secure schemes, however they cost money. ... 8 If your password database will never be compromised, you can store plaintext passwords and nobody will be bothered. The only thing about plaintext passwords is that they can be accidentally remembered by admins who see them - a simple base64 will fix that. Equivalently, MD5 or SHA1, with salt or without, is just as fine. If your password database is ... 8 Short version: SPKI links not only names, but authorizations to keys. Also, it uses a better syntax (S-expressions) than X.509 certificates. Long version: What problem does it solve? Both the traditional CA-based public-key infrastructure (PKI) and PGP's web of trust (and other similar systems) do mainly one thing: Linking names to public keys. The ... 8 In voting schemes, there is the three-ballot system, if you can consider this as part of crypto (I do). 8 CRAM-MD5 is a protocol to demonstrate knowledge of a password. In the context of email, it is sometime used by an email client to authenticate to a POP, IMAP, or/and SMTP server. Basically, the password is used as the key of HMAC-MD5 in a challenge-response protocol. Among positive things there are to say about CRAM-MD5: The password is not exchanged in ... 7 You didn't mention at what level you're hoping for, so I'll provide a few resources, and you can figure out which ones best meet your needs. UMAC: Fast and secure message authentication. John Black, Shai Halevi, Hugo Krawczyk, Ted Krovetz and Phillip Rogaway, CRYPTO 1999. (This research paper describes UMAC, a fast Carter-Wegman style hash. It also gives ... 7 The GQ identification scheme is essentially a zero-knowledge proof of a value$x$such that$x^\mu \equiv J \pmod N$where$N$is an RSA modulus and$(\mu,N)$are system parameters and$J$is known to the verifier and$x$only known to the prover. Now your question is not directly concerned with the aforementioned proof where a user shows the possession of ... 7 For P2P authentication, you can go for web of trust concept. Simply this means, if someone is trusted by people you can trust, you can also trust that unknown person. In OpenPGP, a certificate can be signed by other users who trust the association of that public key with the person or entity listed in the certificate. So trust relationships can be ... 7 A is acting as a square-root oracle in that protocol. We can use that oracle to factor$n$and break the scheme. Suppose you are an attacker that wants to impersonate A. You: Pick a random$m$; Send$m^2$to A; Compute$p = \gcd(m_1 - m, n)$, thus factoring$n$. This works with probability$1/2$for each attempt. 7 The other answer is correct in general. However, if your messages are all exactly one block long (or all one block after padding), ECB is a secure MAC. A PRP looks like a PRF up to half its bit length, i.e. up to$2^{64}\$ blocks for AES. A secure PRF is a secure MAC of the same size. Thus, AES ECB used on 128-bit messages is a secure MAC as long as you use ...

7

First, the fact that the data is "easy" to guess (in the sense that an attacker has a one-in-2^32 or a one-in-2^64 chance of guessing correctly) doesn't mean much if the attacker has no way of checking if his guess is correct. Or at least, it's not a problem with the cryptography. Second, even if he does have that ability, the problem of protecting your ...

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