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4

It can be safe, but using the same key in both directions adds several things you need to be careful about: One thing you need to make sure is not a problem is if an attacker takes a message from Alice to Bob, and sends it back to Alice as if it were from Bob. Since Bob to Alice communications use the same key, Alice might decrypt the message, and act as ...


3

You can call C from Java. You can call C libraries from just about anything. Java isn't suitable for some cryptographic applications because you can't guard against garbage collector attacks, not because it's slow. It's not particularly slow. C does have a byte type, it's called "char". Object Pascal (Delphi) isn't a systems programming language, and so ...


2

I don't believe he is answering the right question. You essentially asked "why are public keys so much larger than symmetric keys", and after his first sentence (which started to address the question, but was a bit vague), he tried to answer the distinct question "why are public key operations so much slower" (not that he got the details of that correct; ...


2

Java isn't as fast as C for cryptographic operations, it is a factor of 2 to 10 times slower, depending on the algorithm - according to my 15 year experience. With the current processors that's often a smaller issue than you may think, you can still get very respectable speeds with Java (much higher than with non-native scripting code for instance). That ...


1

Asymmetric keys have to be much larger than symmetric keys because 1) there are less asymmetric keys for a given number of bits (key space), and 2) there are patterns within the asymmetric keys themselves. To compare, consider that the ECRYPT II recommendations on key length suggest a 128-bit symmetric key is as strong as a 3,248-bit asymmetric key, and ...


1

I don't think it is right. The reason why RSA in particular uses such a high bit count, is that RSA's security is based on factorization of integers and integers with up to 100 digits (roughly 300 bits) can be "easily" factorized with the Quadratic Sieve. In general, there are asymmetric ciphers like those based on elliptic curve cryptography that also use ...


1

No, it won't "leak" information, as long as you're using a modern symmetric algorithm that's resistant to known-plaintext attacks. However, depending on the encryption mode used (and whether there's integrity checking or not), there can be other security implications, such as the data in the known spot being substituted.


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Rijndael-128, Rijndael-192 and Rijndael-256 are actually 3 different variants of block cipher that are very similar. Simon, Speck, Threefish and RC5 also define different block size variants in similar way. Rijndael is not unique in this regard. Block cipher that really has variable block length is XXTEA as its block length is not limited.


1

It depends greatly on what form and mode of encryption you use. For any stream cipher or a block cipher used in a psuedo-stream mode such as CTR or GCM then reusing a key/IV pair even once is absolutely fatal. Never, ever use a static key with them, and if you do, never, ever reuse an IV. They are strictly for one time use keys negotiated via a key exchange ...


1

In short, yes, key re-use will eventually lead to a growing vulnerability to a persistent and dedicated attacker over a very large data set. Depending on what encryption method you are using, the details get a very complicated very quickly. I believe in a standard AES CBC implementation with a random IV a key change is recommended after 264 bytes of data. ...



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