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I'm an engineer with experience in applied cryptography, in particular in Smart Card systems.


15h
comment Is signing a message just encrypting it with private key?
No, proper RSA signing of a text is not as simple applying the private-key function to the (hash of a) text ("encrypting a text with private key" as questionably worded in the question). $\;$ That is not safe for many definitions of safe, including most ones used in cryptography. $\;$ For an introduction to that, see the second part of this answer, and the final reference.
2d
revised Is a small size block cipher usable?
Make it clear that I made a wild conjecture
2d
revised Achieving 32-bit verification code with 16-bit CRC?
Polish
2d
answered Achieving 32-bit verification code with 16-bit CRC?
2d
revised Finding strong primes
Fix syntax of a link; and a typo
Jan
24
revised Finding strong primes
Polish end of first part
Jan
24
revised Finding strong primes
Polish
Jan
24
revised Finding strong primes
Polish
Jan
24
comment Finding strong primes
Are you sure of the PKCS reference? The definition of strong prime that you give is not in PKCS#1 (at least, the current version), which would seem to be the logical place. $\;$ Also, it is quite feasible to generate a prime $p$ such that $(p-1)/2$ and $(p+1)/2$ are primes (the trick is to use a sieve of width a few times $(\log p)^3$ to keep only candidates for $p$ such that any of $p$, $(p-1)/2$ and $(p+1)/2$ are divisible by small primes; then apply a probabilistic primality test).
Jan
24
answered Finding strong primes
Jan
22
reviewed Approve How does cryptanalysis of the Playfair cipher work?
Jan
22
comment Why the data length is shifted 3-bit in MD4 Java implementation?
This is a basic Computer Science question, unrelated to cryptography. The left shift by 3 converts the length in octets previously accumulated by the Java code into the length in bits prescribed by the quoted text of RFC 1320, in order to form the padding.
Jan
21
comment Practical Attack on RSA
Yes, much like for yogurt.
Jan
21
comment Practical Attack on RSA
My point is that "such key length will be vulnerable at about that time" (as in "estimate for how many years a given keylength may last you", what this is about) is very different from "use that key length and you should be okay" (a safe recommendation, what keylength is about).
Jan
21
revised How big an RSA key is considered secure today?
Update "main practical threat" section. Remove a redundant reference.
Jan
21
revised How big an RSA key is considered secure today?
remove a redundant reference
Jan
21
revised How big an RSA key is considered secure today?
Move promotion of keylength.com for choosing a keylength in the first paragraph. Link to attacks on the key generator of a smartcard.
Jan
21
comment Practical Attack on RSA
keylength does NOT "try to estimate for how many years a given keylength may last you". Rather, it is about estimating what key length is appropriate so that one can be next to certain that at some prescribed date, brute-forcing a public key will fail (or, equivalently, about selecting a key length that conforms to prescribed standards with that aim). The former is about a best guess, the second is all about being conservative. $\;$ A decision maker has use of the former when evaluating a non-critical legacy system, and of the later for a new system.
Jan
21
comment Practical Attack on RSA
@Nayef: 768 bits was the academic factoring record by the end of 2009, and that might translate to perhaps 900 to 1000 bits nowadays (with comparable effort for some meaning of that). Where you put the bar of a "practical attack" is a matter of policy, and what risks and facts there are to consider (like, feasibility of changing an already deployed system).
Jan
20
comment Practical Attack on RSA
Check this for practical brute force attacks versus key size. $\;$ Practical attacks often are independent of key size; like, abusing a certification authority, or hacking into a computer that can use a secure device that holds the key.