# Tag Info

22

Applied Cryptography is book which is becoming, say, not-so-recent. NSA has quite a lot of budget, but not an infinite amount, and there are other organization, in particular big private corporation, which also have impressive means. Google or Apple, for instance, are companies with R&D activity in the area of cryptography, and who are able to ...

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(Disclosure: I'm the author of the functionality that you're asking about (good question!).) Ubuntu's Encrypted Home Directory feature uses eCryptfs as the filesystem encryption technology. eCryptfs is a layered filesystem built directly into the Linux kernel. It mounts one directory on top of another. The top directory is really just a "virtual" ...

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It's a good question. As pg1989 said, this is the basis behind stream ciphers, which are very fast in practice. I thought I'd quickly expand upon your statement that "the one-time pad is the perfect cipher and impossible to crack." This is true, in a sense, but it's worth pointing out that sometimes an attacker wants to do something simpler than "cracking" ...

13

Well, the standard answer is to preserve compatibility with DES; a hardware circuit that implemented 3DES (with EDE) could also be used to do DES as well (by, say, making all three subkeys the same). Now, there is one slight problem with this straightforward argument; 3DES (EEE, that is, with three encrypt operations) would have this property as well; if we ...

12

Say you encrypt a message with a key $k$. With symmetric encryption (ie. symmetric ciphers), $k$ must be secret. The sender and recipient must agree (somehow) on $k$. No-one else can be allowed to find out $k$. Anyone else who finds out $k$, can decrypt all the messages encrypted with $k$. For that reason, symmetric ciphers are often called "secret key" ...

11

There are three efficiency issues to discuss here: CPU, network bandwidth, and functionalities. The "moral" reason of public key encryption being slower than private key encryption is that it must realize a qualitatively harder feature: to be able to publish the encryption key without revealing the decryption key. This requires heavier mathematics, compared ...

9

You can use bcrypt to derive a (long enough) key from your initial secret value, and then use the key for symmetric encryption. "Guessing" the key is an issue only when the key is part of a space of possible keys which is small enough that such guessing is a possibility -- i.e. when the "secret value" is a password. A 128-bit uniformly random key is immune ...

9

Padding None can be used with stream cyphers and AES-CTR in order to keep the cyphertext the same length as the plaintext. Padding Zeros cannot always be reliably removed, and so should be avoided. Any of the others can be reliably removed and are fine for use. Padding None leaks information about the length of the plaintext. Apart from that there is no ...

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There are two main reasons why asymmetric cryptography is practically never used to directly encrypt significant amount of data: 1) Size of cryptogram: symmetric encryption does not increase the size of the cryptogram (asymptotically), but asymmetric encryption does. If we take the example of RSAES-OAEP in PKCS#1v2 with a 1024-bit key and 160-bit SHA-1 ...

9

A PGP encrypted message can be hundreds or even thousands of bytes. Encrypting and decrypting large amounts of data using asymmetric algorithms is extremely slow. Encrypting only 32 to 16 bytes (the symmetric key) is much faster. Additionally, if you encrypt the same message twice with an asymmetric algorithm, you will get the exact same ciphertext. Using ...

9

I think you misunderstood a detail of PGP encryption. Only the random symmetric key is encrypted under the recipient's (asymmetric) public key. This way to encrypt stuff is quite common and is called KEM/DEM paradigm: Key Encapsulation Method/Data Encapsulation Method oy Hybrid Encryption. Some refs: en.wikipedia.org/wiki/Hybrid_cryptosystem and ...

8

A block cipher is a family of permutations where the key selects a particular permutation from that family. With a tweakable bockcipher both key and tweak are used to select a permuation. So tweak and key are pretty similar. The main difference are the security and performance requirements for a tweak: Changing a key can be expensive, changing a tweak ...

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DES is a block cipher. It consists of a pair of algorithms, one for encryption and one for decryption. Each algorithm takes two inputs: the key, and the block to encrypt or decrypt; the output is the encrypted or decrypted block. For DES, the size of a block is 64 bits. So DES only tells you how to encrypt or decrypt data that consists of exactly 64 bits. ...

7

I really don't have an answer (other than saying that storing a hash of the password is good as any other way of solving your immediate problem; there are other ways, but they all allow an attacker to run a dictionary attack on the database). On the other hand, I do have these comments on what you're doing: If getting decrypted gibberish will really crash ...

7

No, they do not gain an advantage; part of the definition of IV is "information that is used to encrypt data which can be sent in the clear". In fact, it is actually rather common to send the IV along with the encrypted message. TLS version 1.1, IPsec, IKEv2 are all protocols that do that. In CBC mode (which you didn't specifically ask about, but it is ...

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As @David Schwartz said, using something like 3AES should work to scale up the security (but see my comment on the meet-in-the-middle attack). But seriously, why would you even need 512-bits of security in a symmetric algorithm? According to the analysis on wikipedia it would take 50,955,671,114,250,100,000,000,000,000,000,000,000,000,000,000,000,000 years ...

7

I suspect that KCV's are in general not used because they don't add enough to be worth the small overhead. There are a number of cryptographical attacks on encrypted methods that involve the attacker modifying a valid ciphertext, and then having the receiver decrypt the modified message (and watch how the receiver reacts). Because of these attacks, it is ...

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A reason to use CBC (or CFB) over CTR and OFB could be that they are a bit more misuse-resistant: If you use CBC with a repeated initialization vector, a (read-only) attacker only can get the fact that the plaintexts are equal up to some block, and not much more (and from the first different block the rest is different). With CTR and OFB, a repeated ...

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Symmetric encryption and asymmetric encryption algorithms are built upon vastly different mathematical constructs. In typical symmetric encryption algorithms, the key is quite literally just a random number in $\left[0 .. 2^n\right]$, where $n$ is the key length. The strength of the key is based upon its resistance to brute-force attacks, where an attacker ...

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A few reasons: As mentioned, modular arithmetic allows groups. See @mikeazo's answer. Cryptography requires hard problems. Some problems become hard with modular arithmetic. For example, logarithms are easy to compute over all integers (and reals), but can become hard to compute when you introduce a modular reduction. Similarly with finding roots. ...

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One major reason is that modular arithmetic allows us to easily create groups, rings and fields which are fundamental building blocks of most modern public-key cryptosystems. For example, Diffie-Hellman uses the multiplicative group of integers modulo a prime $p$. There are other groups which would work (i.e., certain elliptic curves).

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For asymmetric encryption such as embodied by RSA, the "key length" is conventionally taken to be the length of one of the public key elements. Such a key is highly structured (only a very few sequences of bits of that length are valid keys) and security relies on robustness of this structure with regards to some specific analysis algorithms. E.g., in RSA, ...

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I think this is generally not a good idea, for two reasons. First, do not generate a key as a hash of a passphrase. Where possible, you should try to avoid deriving cryptographic keys from passphrases, as passphrases rarely have enough entropy to make a secure crypto key. If you must derive the crypto key from a passphrase, then be sure to use PBKDF2 (or ...

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Yes, this is a fine approach. This sort of technique is known as "key separation". Since your master key is a cryptographically secure key, you do not need to use a large iteration count. Also, you could use any PRF, in place of PBKDF2. (The iteration count is normally used if you are applying PBKDF2 to a passphrase, instead of a cryptographically secure ...

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Hashes like SHA-x are symmetric and unkeyed. I have never heard of asymmetric hashes. Your question is based on a misunderstanding. You can implement computations in an asynchronous fashion, where you request some computation, then your thread is free to do something else, and at some later point when the computation is finished you do something with the ...

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It is probably not the case of your example, but in some sense "asymmetric hash functions" do exists: they are called trapdoor hash functions (or also chameleon hash functions). Very briefly, they are collision resistant only if you don't know their trapdoor secret key. Such functions take 2 arguments (instead of the usual one), and the second argument is ...

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The Diffie-Hellman key exchange is a public-key technology. It is (by itself) not an encryption algorithm (or signature algorithm), though. Here is the basic function: (All calculations here happen in a discrete group of sufficient size, where the Diffie-Hellman problem is considered hard, usually the multiplicative group modulo a big prime (for classical ...

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Neither of those really represent how confusion might get introduced into an encryption function. Confusion typically gets introduced during the computation of the ciphertext, not before or after. As an example, we can look at AES. AES is a substitution-permutation network. This means the encryption is done by doing several rounds of substitutions and then ...

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Handing keys in general is known as key management. Symmetric keys should be kept secret. Secret key is often used as a synonym for symmetric key. The establishment of symmetric keys can be performed in several ways: (Authenticated) Key Agreement (KA) Sending of an (authenticated) encrypted key, also known as key wrapping Derivation from a base key using ...

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