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10

Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? Yes. That is a semi-serious answer; both are very good choices, assuming, of course, that a Message Authentication Code is the appropriate solution (that is, both sides share a secret key), and you don't need extreme speed. How much HMAC-SHA256 is slower than HMAC-SHA1? Those ...


10

SHA-224, part of FIPS 180 since FIPS 180-3 FIPS 180-2 change notice 1 of 2004, was introduced to match the second of the security strengths {80, 112, 128, 192, 256} defined in the document that became NIST Special Publication 800-57 – Recommendation for Key Management – Part 1: General (Revision 3). That security strength itself was kept ...


8

No, because even SHA-512 was considered overkill from a security perspective. It has 256-bit collision resistance, which is unbreakable. (The link is about keys but a similar argument applies.) If you think large quantum computers will be efficient, a 512-bit hash makes some sense, but even then a 1024-bit one wouldn't. A quantum computer requires ...


7

In early years of hash function design it was unclear how to choose constants (not only initial vectors), and it was widely assumed that the more random they look, the more secure the function is. There is still not much research in this direction. However, there have been several attacks (rotational cryptanalysis, slide attacks, internal difference attacks) ...


6

The attack which you link to, on ECDSA, is related to the following: the signer computes several values $kG$, for random $k$ values chosen uniformly modulo $n$ ($n$ is the size of the subgroup generated by $G$). One such value is generated for each signature. It is important that the selection is uniform: even small biases can be exploited in order to make a ...


5

What you are describing is essentially the same things as a hash list. A hash list is a sequence of hashes over which another hash is calculated. Your scheme does the same thing after sorting. The sorting won't matter for the security of the scheme; it won't increase the chance of collisions. Hash lists are also used for a well known structure called a ...


4

The Secure Hash Standard and corresponding FIPS-180/202 do not specify any hash to meet a security requirement above 256-bits (using a 512-bit hash). This is unlikely to change. SHA-2 was built with state and word sizes to meet the security requirements on commodity computers (x86 and Alpha), which use 32 and 64-bit maximum CPU word sizes for general ...


4

The main thing that makes HMAC secure in typical use even with MD5 is that it is used with a secret key. That means only preimage attacks are really relevant, since finding a collision is always an online attack if you don't know the key. With known attacks the preimage resistance of both MD5 and SHA-1 is > 100 bits. Additionally, HMAC may be secure even ...


4

It meets the security requirement for 112-bit collision and preimage resistance, while being 32 bits shorter than SHA-256. This may not seem like a lot, but when you have thousands or even millions of hashes or signatures to worry about in a system, those extra 4 bytes add up. Think of a webmail service, where a hash of each email is used for deduplication ...


3

Elliptic Curves over binary fields In naive implementation of Elliptic Curves, either $GF(p)$ or $GF(2^{n})$ will be vulnerable to some timing attacks. The paper you provided is on OpenSSL's implementation of EC with $GF(2^{n})$. This implementation uses Montgomery's ladder scalar multiplication, which is in fact very good for making sure that most of the ...


3

Honestly, in practice, there are very few if any reasons to use SHA-224. As fgrieu notes, SHA-224 is simply SHA-256 with a different IV and with 32 of the output bits thrown away. For most purposes, if you want a hash with more than 128 but less than 256 bits, simply using SHA-256 and truncating the output yourself to the desired bit length is simpler and ...


3

SHA-2, like SHA-1, is an ARX hash function: that is, it uses Addition, Rotation, and eXclusive-or for bit diffusion. The purpose of each one is explained very simply and clearly by Khovratovich & Nikolić in their paper "Rotational Cryptanalysis of ARX", so I will simply quote them here: Addition provides diffusion and nonlinearity, while XOR does ...


2

Does input truncation using SHA-256 expose any potential weaknesses? No, hashing the passphrase with SHA-256 will be no stronger or weaker than feeding it in directly. If you go with Scrypt (which I would recommend you do), there are no restrictions on the size of the passphrase... and Scrypt consumes it internally with one round of PBKDF2-HMAC-SHA256 ...


2

Does allowing users to test VALUEs increase the likelihood that SECRET will be broken or illegal hashed values generated, relative to the CONTROL scenario? By definition a cryptographic Message Authentication Code such as HMAC is secure only if resists existential forgery under chosen-plaintext attacks. i.e. if allowing users to test VALUEs increases ...


2

SHA is related to AES in that they are both US government standards. They are not similar algorithmically. SHA and AES are cryptographic primitives, TLS is a protocol. As the name describes SHA is a family of hash algorithms. AES is a block cipher. TLS uses many encryption algorithms, including AES in various modes, and several hash algorithms, including ...


1

All the arithmetic in SHA-256 should be 32-bit, it can be implemented at a low level using only 32-bit registers. If you use a representation that can go over 32 bits, then you need to truncate back to 32 bits on every calculation in your chosen language where it will not do so itself - i.e. on every addition.


1

Is this a sound way of doing things? Depends. Where do the keys come from? If they come from a user's memory (e.g., a password) then no. The reason for this is that a simple hash if fairly fast to compute. Typically we recommend people use something like PBKDF2 or scrypt as they run through thousands of iterations. The effect of this is that computing ...


1

There is vast literature on timing attacks on AES, but to the best of my knowledge no such attack on SHA-2 or any construction that uses SHA-2 (e.g., HMAC-SHA256).



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