# Tag Info

15

Examining his claims about "Thundercloud": You can use it with "any existing software, operating system, or device" (a massive amount of effort---by whom?) Has its "own cryptographic language that is completely independent of any existing security technology" (this is a negative thing: abandoning the entire knowledge base of cryptography is incredibly ...

10

This depends on the public-key system (algorithm). For RSA, technically the private and public key (i.e. the exponents, the keys share the same modulus) are symmetric, you can swap them, and it still works. But you usually don't want to do this: The public exponent is usually a small number (like $3$ or $2^{16} + 1$) in order to speed up ...

8

It looks like, given your adversary model, things should be secure. HMAC as a randomness extractor has been shown to be good, especially when we can assume the hash function is collision resistant. That paper also has some results which tell how you could guard against the collision resistance being broken (basically use a hash function with larger output ...

8

Definitions In RSA, an encryption key is a pair of integers $(N,e)$ with $N$ the product of $m\ge2$ distinct odds secret primes $r_i$ (with $0<i\le m$), and $e$ is such that $\gcd(e,\lambda(N))=1$ where $\lambda(N)=\operatorname{lcm}(r_1-1,\dots,r_m-1)$ is the Charmichael function. It follows that $e$ is odd. Typically, other conditions are added, like ...

6

The only reason you are seeing this is because you are dealing with such small primes. With primes like we would use in practice (1024 bits), the probability of this happening is very, very small. And, it can only happen when $e>\sqrt{\lambda(n)}$. Since we typically use $e=65537$ in practice, it is guaranteed to not happen. Anyways, there is no mistake ...

6

While it may be confusing, that Wikipedia article is actually correct! Let me try to explain it a bit better… Definition of key whitening Key whitening is an extremely simple technique to make block ciphers like DES much more resistant against brute-force attacks. Like you’ve already discovered yourself, this is the basic scheme: Or, defining it a bit ...

5

Key length is the length of the key. It's a term whose meaning has evolved over time; these days, it typically means length in bits. With digital symmetric ciphers, it's fairly simple, because those tend to have a key that's just a string of some number of bits, and any string of that length is a valid key. With RSA, it's more complicated - the key has a ...

5

Sorry for the late answer, I got busy... So, you know that $\mathsf{Gen}$ is a probabilistic algorithm. What's a probabilistic algorithm? It's an algorithm which, during its execution, can make some random choices, which can be modeled as coin tosses. In programming terms, the algorithm can use a special coin-tossing function, which returns $0$ or $1$ each ...

4

Am I going to regret posting this? There seems to be enough non-classified information available about GPS to answer this question. I see 3 reasons why P(Y) encryption is different and less likely to be hacked than game console encryption: Hardware containing the GPS decryption key is more difficult to obtain than hardware containing the game console ...

4

Trevor Perrin wrote a library doing exactly that. Explanation can be found on in the curves mailing list archives. To convert a Curve25519 public key $x_C$ into an Ed25519 public key $y_E$, with a Ed25519 sign bit of $0$: $$y_E = \frac{x_C - 1}{x_C + 1} \mod 2^{255}-19$$ The Ed25519 private key may need to be adjusted to match the sign bit of $0$: if ...

4

A key derivation function lets you derive keys from others. In this case I would use HKDF, which means using HMAC in a predefined way. Your key material is the keys $X$ and $Y$, so you can concatenate those to get the PRK for HKDF-Expand. An output key would then be $\operatorname{HMAC}(X||Y, \text{info} || \text{0x01})$, if the size of the HMAC is long ...

4

There are two things here: Encryption uses mode of operation, and not "AES alone". Some of them are randomized by an initialization vector - that means the encryption of the same text under the same algorithm is still randomized and not deterministic. The encryption methods take care of that. You only need the correct key to decrypt. Passwords are not ...

3

Your description of how RFC 5959 works isn't quite right. It is not quite correct to state that RFC 5959 encrypts using AES in ECB mode. A correct statement is: if the plaintext is exactly 128 bits, then use ECB mode, otherwise use a non-trivial mode of operation found in RFC 3394. In the former case, ECB mode is fine, since it's just a single block of ...

3

Yes. $\:$ "simply XORing" is obviously malleable, which may allow related-key attacks. "When storing a short key, e.g. a 256-bit ECC private key," the "good reason to use AES" is that "the XOR with a single PBKDF2 (or other KDF) output block" is not necessarily sufficient, since an adversary might also have changed the stored public key.

3

One way key whitening improves security is by increasing resistance to bruteforce attacks (and doing this essentially for free). Consider, for example, DES. Key is 56 bits, so given a single pair $(M, E=DES(K,M))$ attacker will find $K$ in $2^{55}$ operations on average. By employing key whitening it is possible to increase required effort substantially: we ...

3

Simply put: No. First recall that this is a mis-use of the term "One Time Pad" So lets call it a vigenere cipher instead. You can determine this is insecure with a simple algebraic combination: $\text{attack} = cipher_1 + cipher_2 + cipher_3 + cipher_4 \\ \text{Simplify: } \\ \text{attack} = character_1 + key_1 + IV_1 + character_2 + key_2 + IV_1 + ... 3 A keyspace is the set of all possible keys; it's a set. The cardinality of the keyspace is an integer, and is the number of elements in the keyspace. There is no possibility of confusion, because one is a set and the other is an integer. 3 In the context of encryption schemes, the key is whatever piece of information the legitimate recipient of an encrypted message possesses, which allows him to decrypt the ciphertext efficiently. Hence, the key must be kept hidden from an attacker, since otherwise the attacker could decrypt efficiently just as the legitimate recipent does. 2 HMAC: The hmac version is considered slightly more secure than sha-256, assuming it's also based on SHA-256, because the HMAC formulation folds in the key material with 2 rounds of hashing, making it harder to use a chosen plaintext attack on the digest. SHA-256: SHA-256 should be relatively secure against chosen plaintext attacks, but it's better to be ... 2 Put another way, you can say that the key is whatever information the recipient possesses which allows him to decrypt the message, and which must be kept secret from everybody else. Thus, "algorithm" and "key" are not mutually exclusive: if knowledge of the algorithm allows one to decrypt a message, then the algorithm is the key. 2 Picking up what has been said in the comments: to simplify: symmetric ciphers are like mathematical operations with 2 operands and 1 result. There is The plaintext message$m$and$k$as the key and they result in the ciphertext$c$. In your example, the algorithm can be cut down to a addition and modulo:$c = (m + k) \mod k_{max}$And of course there is ... 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 Short answer, before someone marks this as a duplicate or answers it with an essay or something: You're exactly right. This was one of the biggest consequences of the infamous Heartbleed exploit in OpenSSL, which exposed the memory of processes using OpenSSL for TLS to anyone with an Internet connection. It's also significant for cold boot attacks, where ... 2 There are many possibilities here, depending on the particulars of where exactly you will use it. Your use case may require a random looking derived key, where the 1024 bytes of entropy have been distributed evenly over all the bytes of the final key. In that case there's no avoiding a key derivation function. You will have to use either a block cipher, a ... 2 CryptGenRandom is supposed to produce cryptographically strong random numbers, so you shouldn't need to process it before using as a key. However, if you want to treat it as of suspect quality, I would go with SP 800-90B recommendations: assume it has entropy at least half the bits, so request double what you need. Then run it through HMAC with a suitable ... 2 The once part inside of the nonce in CTR mode means effectively "once for this particular key". If you use a fresh key for each message (e.g. by encrypting it using public-key crypto or similar), you can use the same nonce for all the messages (or a size-zero nonce). The important part is that the combination of nonce and ctr-value (i.e. what is input into ... 2 It seems that you are trying to implement your own KBKDF (Key Based Key Derivation Function) using HMAC. Maybe it is better to use a pre-defined one. It would be more sensible maybe to use an HSM that is FIPS certified for NIST SP 800-108. These use one of the KBKDFs defined in NIST SP 800-108. You can still use the idea of the random by putting it in the ... 2 There are two inputs to PBKDF2, and key derivation functions in general: the password and a salt. Assuming the attacker knows the salt, all the password hash can do is slow down a brute force or dictionary search (i.e. 1000x the complexity of HMAC with PBKDF2 default iterations), as well as force the attacker to search for each user's password individually. ... 2 Not always, it depends on the particular encryption scheme. Strictly speaking, the proofs only say that breaking indistinguishability is equivalent to breaking the hardness assumption they are based on. There are some cryptosystems, like Rabin's, where the security of the key is equivalent to the security of the ciphertexts, i.e. factoring <=> key ... 2 I understand that if a block cipher has$k$-bit keys and$n$-bit input/output blocks, then if$k>n$, we can expect one message-ciphertext pair to narrow us down (I think?) to$2^{k−n}\$ possible keys, right? That is approximately correct (if the block cipher with the wrong key acts like a random permutation; this is generally a safe assumption); if ...

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