# Tag Info

33

No, it is not a good idea to hash phone numbers. There are only a limited number of phone numbers, so it is pretty easy for an adversary to try and hash all of them. Then you can simply compare the hash of each with the stored hash. Generally you don't have to deal with all telephone numbers, only a subsection of phone numbers anyway (for a specific country ...

20

Strictly speaking, all hash functions are compressing since the output can be smaller than the input, but I imagine you're asking about compressing data that can later be losslessly decompressed. This is impossible due to the pigeonhole principle. The fact that the fixed output space of a hash algorithm is smaller than the input space means that there will ...

17

The $\operatorname{SHA-224}$ is defined in the exact same manner as $\operatorname{SHA-256}$ with different initial values and the digest is obtained truncating the hash value, FIPS PUB 180-4 Page 23. The different initial value provides domain separation. With domain separation $$\operatorname{SHA-224}(m) \neq \operatorname{SHA-256}(m)|_{224}$$ where $|_{... 15 Since the initial release of Bitcoin is 9 January 2009, the designer had these NIST hash functions (NIST-FIPS 180-4) as available options: SHA-1( 1995), SHA-256 (2001), SHA-512 (2001), and some more. The main difference between SHA-256 and SHA-512 is the target CPU. SHA-256 is designed for 32-bit CPUs and SHA-512 is designed for 64-bit CPUs. That makes a ... 12 It is always a bad idea to hash data that has a limited set of length or characters. A phone number in Germany for example has normally no more than 12 digits. The first digit is always a 0 and the vast majority of numbers is longer as 3 digits, as those are normally reserved for emergency services. This effectively leaves us with 10^11-10^3 possible ... 12 In the general sense, The problem is known as the small input space on the hash functions, and in short simple hashing won't be secure. If you hash data ( here a phone number) and an attacker tries to find an input value that matches the hash value is called the pre-image attack. In a secure Cryptographic hash functions pre-image attack requires$\mathcal{O}(...

11

There is actually a system for hash based signatures being standardized, see here. Hou have described a system where the public key is derived from the private key in such a way that the private key cannot be recovered from the public key. However, it is unclear what operations you can perform with the public key that can only be reversed by persons having ...

11

$2^{64-1}$ bits that make 2.30584301 exabytes *. If you are not restricted to SHA256, then use SHA512 that allows files to have size at most $2^{128}-1$, or use SHA3 that has no limit. The NIST must use a limit due to the artifact of the MD construction. SHA256 is standardized in 2001 along with SHA512. They have internal block size 512 and 1024 respectively,...

10

AES-128 (2000) has been around for 20 years and there is no attack faster than brute-force, except the multi-target that affects all block ciphers and hash algorithms. As you can see, an algorithm can withstand attacks for a long time with a good design. SHA-1 (1995) has 160-bit output, which results in 80-bit generic collision resistance. It was good for ...

10

If the question was about (current form) Reversible cryptographic hash functions Then No! One-wayness property of the cryptographic secure hash functions will prevent that. Hash functions don't use keys. So if you can reverse, everybody will reverse and there will be no secure hash function at all. Besides, mathematically impossible, too; hash functions use ...

9

This is a problem created by the library. Either the HMAC algorithm is skipped entirely, or - more likely - the HMAC authentication tag is generated and then forgotten. The reason is likely the spotty to non-existent handing of authenticated ciphers in the OpenSSL command line and (likely) higher-level functions. The string is recognized by the cipher ...

9

Bob compares the SHA256 checksum that he generated from fake ISO file to the checksum found on official linux distribution's home page. Because Bob's fake ISO checksum matches the official ISO checksum, Bob doesn't notice that he has downloaded fake ISO I highlighted the incorrect assumption: the checksums wouldn't match. What the length extension attack ...

8

I could base a public and private key system just using a hashing algorithm Hash based signatures, such as Sphincs+, are essentially this (except that the relation between private key and public key is a tad more complicated. However, to answer the question you appear to be answering: the problem with developing a public key cryptosystem is not just the ...

7

As an alternative, you can salt the phone numbers to avoid pre-calculation attacks. A known salt will help against an adversary who has already done a hash of all possible phone numbers but just adds one order of magnitude of work (the adversary just has to recalculate all the hashs with the salted phone numbers). If you can keep the salt private raises the ...

7

Comparing double sha256 to sha512 is like comparing apples to oranges. For one, the result of sha512 is 512 bits in length. The result of double sha256 (or triple sha256, or quadruple sha256, for that matter) is 256 bits in length. There has been a lot of conjecture over the years as to why the creator of bitcoin chose to use double sha256 in the protocol. ...

6

XChaCha20 requires a 192-bit nonce size. As a rule of Stream ciphers, the nonce should not be repeated under the same key. Note that; Daniel J. Bernstein is pointed in the XSalsa20 article for Salsa20 which has 64-bit nonces. There is a standard argument that a 64-bit nonce is long enough. Nonce security does not mean unpredictability; it means uniqueness. ...

6

Is there an efficient way to do this? 32 bytes of "good randomness" provided by hashing some secret value gives much less entropy than required to directly compute a sample of 100 items from a list of 100000. If this is required for any cryptographic purpose, you must use a good CSPRNG to convert your starting entropy into as much randomness as ...

6

If we assume that the output of a SHA-256 hash is roughly random, this is ultimately just a combinatorics problem. That is: Given a random string of 64 hexadecimal digits, what is the probability that the sequence "20" appears 3 or more times. We can think of this as having three set items, the three "20"s, and then 58 other ...

6

The size is just restricted by a length encoding at the end of the last block that is hashed using SHA-256 - one of the two main hash functions that make up the SHA-2 family. If you extend that size then you'd have your secure hash function with extended input. However, there is an easier way. The SHA-512 hash function - the other main hash function in the ...

5

Sponge Construction In sponge construction, the block size $b$ has two parts, $r$ is the rate and it is the written part and $c$ capacity is the untouched part by the input/output, $b= c+r$ The output is taken from $r$ in the squeezing stage. Therefore not all of the $b$ is the output. This prevents the length extension attack like in the truncations of ...

5

SHA-256 and SHA-512 were designed to be very fast. Their primary goal ist to verify the integrity of long messages or files. Long means not 10-12 bytes but some megabytes and greater. It is not a good idea, to use hash function that is fast by design for password hashing. Instead, for password hashing should be used functions that need essentially more ...

5

The typical reason one uses double hashing is to deal with length-extension attacks. That's because any Merkle-Dåmgard algorithm that outputs its entire state (e.g., SHA-1, SHA-256, and SHA-512) is vulnerable to a length extension attack, where users who know a hash can append additional data and also produce a valid hash. There are other algorithms, such ...

5

If we're using the cyphersuite TLS_AES_128_GCM_SHA256; and GCM is using Encrypt-then-MAC (which appears to be always?) and is configured to use a 128-bit tag; and AES_128 is encrypting plaintext in blocks of 128 bits; then is GCM adding a 128-bit tag onto each one of those blocks (thus, halving the amount of cyphertext that can ultimately fit into the data ...

4

Is the salt format moot because the entropy of 32 binary bytes is the same as 42.7 base64 characters, or is one format really a better choice? TLDR: moot, for the reason stated, as long as the whole thing is hashed and not otherwise used, and the hash used is secure, and the iterated hash construction on top of that is otherwise sound. The fear that ...

4

Does it mean that the input is any length of zeros and ones and that is should hash to a value which is 3000 digits of zeros and ones? Yes, that's the meaning of $\{0,1\}^*→\{0,1\}^{3000}$. It would be better to reformulate using the usual shortcut for "digits of zeros and ones": bits. Also, $\{0,1\}^*$ is the set of all bitstrings. Could I just apply ...

4

ECDSA is specified in SEC1. It's instantiation with curve P-256 is specified in FIPS 186-4 (or equivalently in SEC2 under the name secp256r1), and tells that it must use the SHA-256 hash defined by FIPS 180-4. I'll leave aside ASN.1 decoration (since the question uses none), conversions between integer to bytestring of fixed width (which all are ...

4

When it comes to asymmetric cryptosystems like RSA and ECC, security in bits is not the figure quoted in bits for key size (which itself has no universal relationship with the number of bits in the actual form taken by either the public or private key, which I won't further discuss in this answer). Oversimplifying, and sticking to attacks using classical ...

4

Would it be easier to find collisions, preimages, etc? First note that if you can find preimages for a compressing function (like a hash) you can also find collisions. This means if we can't find collisions, then we also can't find preimages and thelike. Now, the scenario you are describing is called a "freestart collision attack". Freestart collisions are ...

4

I'd expect a 196-bit nonce derived from a 256-bit nonce by computing the SHA-256 hash and then truncating it to 24 bytes to behave like an approximately uniformly randomly generated 196-bit nonce [...] Actually, we don't need to appeal to the truncated SHA-256 being random; we can get away with appealing to collision resistance. Even if we allow an ...

4

The other answers did not mention the Length-Extension attacks on the Merkle–Damgård construction. Length extension is given a hash value $h$; $$h = \operatorname{SHA-256}(\text{IV},\text{secret_key}\mathbin\|\text{known_data}\mathbin\|pad1)$$ the attackers can produce an extension as; \operatorname{SHA-256}(\text{IV},\text{secret_key}\mathbin\|\text{...

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