51 votes
Accepted

Why haven't (most of) the Zodiac Killer's letters been decrypted?

The Zodiac killer ciphers are an interesting case. As there were four ciphers sent to the local papers, I will address each in turn. They do share some common traits however. They are each their own ...
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50 votes

SSH - If Eve has the passphrase and public key, can she derive the private key?

The private key is unrelated to the passphrase. So is the public key. The public key is also generally stored unencrypted, even when the private key is protected by a passphrase. (Exceptions may exist ...
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  • 7,660
26 votes

Why haven't (most of) the Zodiac Killer's letters been decrypted?

You're assuming that they're actually encrypted. A lot of crazy people have written things that nobody understands. Just because the author thinks they're in code doesn't necessarily mean that the ...
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  • 82k
23 votes

SSH - If Eve has the passphrase and public key, can she derive the private key?

The passphrase guards against accessing the private key The passphrase is meant to guard the private key in the event of physical access. If the hacker can sign on to your server and can access your ...
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  • 9,061
12 votes
Accepted

Is there a method to get plain-text from cipher-text in HMAC SHA-1, given that the key is known?

Nope Generally speaking: No. Hashing is not encryption. Hashing is not reversible. At all. It always generates a fixed length output. So with an output fixed to say 32 characters, and an input of 33 ...
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12 votes
Accepted

Does it weaken the encryption of SSH to use compression?

Compression before encryption is a problem if the attacker can control parts of the transferred data and then use the detectable compression ratio (i.e. amount of transferred data vs. original data) ...
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10 votes
Accepted

Is it ever possible to crack an encryption key character by character?

If an encryption system allows for character-per-character cracking, then it is awfully weak, and should not be used. Mathematically, block ciphers are defined as pseudorandom permutations. A block ...
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  • 168k
10 votes
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Will prepending random data (of random length) result in more secure communication?

No, this won't provide any improvement for encryption with any decent cipher. If a cipher is so bad that a known-plaintext attack is capable of fatally breaking it, then the cipher is worthless. A ...
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  • 63.7k
9 votes

Is AES encryption vulnerable?

Technically "cracked" only means that a method was found that reduces guessing from random guesses to slightly less than random guesses. The flaw that was discovered is one such minor crack. A ...
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  • 41.8k
8 votes
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Crypt-analysis methods for breaking or finding weakness in an encryption algorithm?

For cryptanalysis, the usual three-point method applies: Write down the problem. Think real hard. Write down the answer. And that's about all that can be said generically. The methodology of a ...
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7 votes

With a very large number of GUIDs, can one predict the previous and subsequent GUIDs that will be generated?

It would depend on the implementation of the GUID/UUID library. Determining the internal state of a pseudo-random number generator varies widely based on which one you're talking about. Many GUID/...
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  • 1,905
7 votes
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Should we be using SHA3? (2017)

I'm not convinced that we should. SHA-3 has some nice features for sure, but for the reasons I list below, I would probably suggest using SHA-2 or BLAKE2 for the time being. Even NIST themselves say:...
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  • 2,926
7 votes

Are stronger passwords safe in case of a breach?

A stronger password is almost always better than a weak password. In the event of a breach, the details of the hashing and salting of passwords become very important. If the server used plain text, ...
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  • 1,632
7 votes
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Can a Keypass file theoretically be cracked offline?

What about a password manager's database? You know nothing about the content of the file. How do you know you did manage to crack it? This assumption is wrong. You don't know the entries, but in this ...
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  • 12.5k
6 votes

Is it ever possible to crack an encryption key character by character?

Yes. The current best-known example isn't strictly character-by-character, but Wi-Fi Protected Setup splits the key in two halves and verifies them independently, which permits an attacker to brute-...
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  • 34.4k
6 votes
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How is a hash's algorithm detected based on the hash?

You cannot, in full generality, infer the hash algorithm from the output. An output is just a sequence of bits, and hash functions that produce n-bit outputs can at least theoretically produce any ...
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6 votes
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Find DES encryption key having input and encrypted output

This is not DES encryption. This is password hashing with the old DES-based "crypt" scheme. The terminology is, of course, very confusing. DES is an encryption function, but here we are talking about ...
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  • 168k
6 votes

How the NSA can break trillions of encrypted Web and VPN connections

Edit #2 (11/09/15) - I read through a few articles and a paper. See down at bottom for more comments. I read the article, although I have no direct data on the following, so guess work on my part. ...
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5 votes

How to cryptoanalyze a one time pad that uses a plain text document as the pad?

Well, first of, the number of files in the project Gutenberg, though vast by human standards, is really small for a computer, so it is workable to simply try them all. Apart from that remark, non-...
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  • 168k
5 votes

Soundness of GRC.com Haystack padding concept

Of course entropy theoretically makes a password stronger but from the perspective of a password cracker the first password likely would take much longer to crack. This is because of the length of the ...
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  • 2,800
5 votes
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What keeps the private key safe if an attacker has both clear and encrypted copies of a message and the public key?

What you describe is known as a "known-plaintext" attack - an excellent answer to this question can be found on the crypto stack exchange: https://crypto.stackexchange.com/questions/6624/is-a-known-...
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  • 186
5 votes

Are stronger passwords safe in case of a breach?

When there's a breach … when talking to to someone with a genuinely random password – eg one generated by a good password manager – is it fair to say you are probably safe? Theoretically, yes (under ...
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  • 1,296
5 votes

SSH - If Eve has the passphrase and public key, can she derive the private key?

It might be possible to derive your private key from the public key (it is only thought that no-one knows how to do it efficiently), but the passphrase won’t help at all. You should understand that ...
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4 votes

Cryptanalysis of encrypted data at rest

The whole point of symmetric encryption is that if an attacker doesn't have the key, they can't decrypt the data. AES has no publicly-known computationally feasible attacks (significantly better than ...
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  • 38.6k
4 votes

How to determine what type of encoding/encryption has been used?

This is very weak security on all fronts! The plaintext is P4$$w0rdP4$$w0rd and it's encrypted using XOR encryption, with the key CdZ4MLMPgYtAE9gQ80gMtg==. This produces the ciphertext posted by the ...
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  • 19.6k
4 votes
Accepted

Can a botnet be used to effectively break encryption keys?

No, most cryptographic methods that are considered secure are based around assuming every computer in the world is working on the problem non-stop and it still takes to the heat death of the universe. ...
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  • 41.8k
4 votes
Accepted

How can an attacker identify the encryption method with only ciphertext?

If the attacker knows the encryption method, then assuming it's a decent algorithm it won't help them break the cipher in any meaningful way. Cryptography is designed and analyzed under the assumption ...
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  • 7,183
4 votes

How can an attacker identify the encryption method with only ciphertext?

What you describe (figuring out which cipher was used, given only the ciphertext) is a type of distinguishing attack. Modern ciphers are generally highly resistant to such attacks: the ideal cipher ...
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  • 34.4k

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