15 questions linked to/from The effect of truncated hash on entropy
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### Is there a way to use bcrypt with passwords longer than 72 bytes securely?

From my understanding BCrypt truncates the password to 72 bytes. If a password is longer than 72 bytes, what is a way to store that password using bcrypt securely without compromising it? Or is this ...
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### Theoretical notion of white-box

Is there any theoretically safe notion of white-box? I have been reading about white-box [1, 2, 3] recently. It seems white-box deals with code obfuscation and its not known whether a perfect white-...
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### Expected entropy in $P(x)\oplus x$ for random $x$, where $P$ is a random permutation

Let $P$ be a random permutation of $n>1$ bits. Let $F$ be the function on the same domain $\{0,1\}^n$, defined by $F(x)=P(x)\oplus x$. When $P$ is a block cipher with key a message block, that's ...
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### Entropy when iterating cryptographic hash functions

Consider a cryptographic hash function that maps $n$-bit strings to $n$-bit strings:  \DeclareMathOperator{\H}{H} \DeclareMathOperator{\SHA}{SHA-256} \H(x) : \left\{0,1\right\}^{n} \mapsto \left\{0,...
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### Why not use ordinary hash functions for password hashing?

Assume we have a password and add a unique salt to prevent various precomputation attacks. Why don't we do this: SHA256(SHA256(SHA256 ...(Password || Salt))) ...
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### If P256 ECDH shared secret contains ~128 bits of security, does using first half of the secret (= 128 bits) contain only 64 bits of security?

There's this rather popular open source project (I'd rather not name it before possible responsible disclosure) that computes a P256 ECDH shared secret (256 bits) and uses only first half of it, rams ...
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### What if using a block cipher as compression function in Merkle–Damgård?

The Merkle–Damgård construction builds a hash by iterating a compression function $F$, with $S_{j+1}=F(B_j,S_j)$ where $B_j$ is one of $n$ padded message blocks, $S_0$ is the IV, and $S_n$ is the hash....
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### Entropy preservation through cryptographic hash function

Background First, while studying MinEntropy a bit, I came across an NIST paper, "DRAFT SP 800-90B (second draft)," which suggests "twice" the entropy of the underlying block of a cryptographic hash ...
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### Minimalist memory-hard function?

What would be a minimalist memory-hard function, reasonably conjecturable to require $\approx2^k$ bits of memory per running evaluation, $k\approx32$; require $\approx2^n$ R/W accesses to $2^w$-bit ...
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### Can we estimate the loss of entropy when applying a N-bit hash function to and N-bit random input? [duplicate]

Someone pointed out recently to me that a cryptographic hash function " is not designed as a bijective mapping from N bit input to N bit output". So if I feed an N-bit cryptographic hash function ...
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### Hash functions, bijectiveness, and entropy

For those who don't know, a bijective function is one for which each input yields one and only one output. A block cipher, for example, is guaranteed to be bijective or you could not decrypt. When a ...
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### Is it OK to just hash a symmetric key to derive an authentication key?

I'm using xsalsa20poly1305 for encryption in my program from libsodium. The recommended size for chunks is 4 kB. In that case, poly1305 will have authentication only for the the individual 4k chunks. ...
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### repeatedly hashing result and using reverse sequence as RNG secure?

I want to have a source of randomness for which: no one but me can predict the next number in the sequence. I know the entire sequence in advance, but I have no way of manipulating it once the first ...