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Sep
26
comment Roots of polynomial in Shamir secret sharing
1) Since shares can have a 0 y value they can be roots, in which case the attacker trivially learns some roots. 2) What you describe in your comment has almost nothing to do with SSS, except that both use polynomials in finite fields. 3) Just to be sure, with roots you mean the points at which y=0 and not the coefficients of the polynomial, right?
Sep
26
comment Roots of polynomial in Shamir secret sharing
Why do you want to know that? Unless it's just idle curiosity, I'm pretty sure that you're asking the wrong question.
Sep
24
comment Generation of N bit prime numbers -> what is the actual range?
Personally I'd approximate the $\sqrt{2}$ by 1.5, which results in the simple algorithm of setting the two most significant bits to 1.
Sep
23
comment Is it possible to find out whether a number is greater than another number without knowing the numbers?
Similar to Yao's Millionaires' Problem, except here it's one party knowing both values instead of two parties wanting to compare their values.
Sep
23
comment Deprecation of RSA-SHA-1 in DKIM keys?
The feasible attack against SHA-1 is a collision attack, not even a chosen prefix attack, let alone a second pre-image attack.
Sep
22
comment Is it possible to hand-negotiate an SSL/TLS session?
It's also important to note that even if you managed to pass through binary data, you'd need to be able to perform cryptographic operations in your head, no small feat.
Sep
22
comment Using modulus in one time pad?
1) What would you use instead? It's correct and it's convenient. What more do you want? 2) Nowadays we typically use xor (addition modulo 2) which is fast in software and encryption/decryption are the same function. 3) why do you claim M is a large number? It's often 2 or 256 and very rarely larger than 64 bits or so.
Sep
22
comment Random vs. Fixed Paddings
Are you only talking about paddings used with symmetric block ciphers, as in your examples? Or also about paddings used for RSA/Rabin encryption? These kinds of paddings are quite different and should not be discussed in a single question.
Sep
22
comment Random vs. Fixed Paddings
I don't understand what you want to say. 1) If you're talking about padding oracles (adaptive chosen ciphertext), the proper defense is a MAC. 2) If you're talking about semantic security and known/chosen plaintext attacks, it's the job of IVs/nonces to achieve that. 3) If you're talking about asymmetric encryption (RSA, Rabin) you need randomized padding, but RSA padding is conceptually very different from the block cipher paddings the OP mentioned. RSA padding includes properties which in the symmetric case are handled by MAC and IV instead of the padding.
Sep
22
comment Random vs. Fixed Paddings
For symmetric encryption the IV introduces all the randomness you need.
Sep
22
comment Bouncy Castle elliptic curve from explicit parameters (E-521)
BouncyCastle only supports weierstrass for curves. So you'd need to convert the curve and the points from edwards to weierstrass before use. Of course this negates most of the advantage of E521 over curves like P521, the fast and constant time arithmetic in edwards or montgomery form.
Sep
20
comment Why is 224 bit ecdsa faster than 192 bit ecdsa?
Based on the size, P224 should be 50-60% more expensive since the size of the exponent increases as well. So the scaling has an exponent between 2.6 and 3 depending on the multiplication algorithm.
Sep
19
comment Why is 224 bit ecdsa faster than 192 bit ecdsa?
Implementers put a lot of effort into speeding up the variants we use in practice. For example the paper Emilia Käsper - Fast Elliptic Curve Cryptography in OpenSSL presents an optimized P224 implementation.
Sep
19
comment Is it possible (how difficult) to find MORE than one valid RSA signature?
@poncho Which means that I can check that the signer didn't choose $\mathrm{GCD}(e,\phi(n))>1$ by asking for a certain number of signatures. This can be made non interactive with the usual hashing techniques. Still it's yet another pitfall for blind signature schemes, made particularly annoying since they are often used with multiple public exponents.
Sep
19
comment Is it possible (how difficult) to find MORE than one valid RSA signature?
For a maliciously generated key with $\mathrm{GCD}(e, \phi(n))>1$ there should be several different values that become the same after raising to the $e$th power.
Sep
18
comment Finding algorithm: desire RSA's uniqueness and ECDSA's space efficiency
I think BLS is (or can be made) unique. Its signatures have half the size of ECDSA signatures.
Sep
16
comment Scalar multiplication of elliptic curve point by a fraction
You need the inverse modulo the order of the group, not modulo the inverse in the underlying field.
Sep
16
comment Why would an RSA library tell me that the public key must be at least 512 bits in size?
229110545576645850236522690668306544921 = 13118050575083334077 * 17465289088900344973 so this looks like a plausible modulus, albeit very weak. 13082845549543033994073971762152947067 = 37 * 1128586338367 * 313303828194079496938273 It's quite unusual to use a large e, typically we use small primes with low hamming weight like 3 or 65537.
Sep
16
comment Why would an RSA library tell me that the public key must be at least 512 bits in size?
I also want to note that keys used for confidentiality effectively never time out, since the attacker can store the ciphertext forever.
Sep
16
comment Why would an RSA library tell me that the public key must be at least 512 bits in size?
You should replace the "any kind of asymmetric encryption ... at least 2048 bits" part. This size recommendation if appropriate for RSA or finite field Diffie-Hellman based encryption. There are other algorithms which need much smaller (e.g. 224 bits with ECC) or much larger (~1 Mbit for McEliece) keys for a similar security level.