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10

This depends on the public-key system (algorithm). For RSA, technically the private and public key (i.e. the exponents, the keys share the same modulus) are symmetric, you can swap them, and it still works. But you usually don't want to do this: The public exponent is usually a small number (like $3$ or $2^{16} + 1$) in order to speed up ...


4

When using a Discrete Logarithm based scheme, such as SRP, the rule of thumb is to always use private exponents with a bit length twice the desired security strength. Hence, a 128 bit exponent $a$ will at most give you 64 bits of security. If you want 128 bit security, you need (at least) a 256 bit exponent. This is because the algebraic structure of the ...


3

If RSA keys in PGP format are fine, you can download them from any keyserver. Many keyservers, at least MIT, have a robots.txt that disallows automated downloads, so be sure to check that before attempting to bulk download anything. If you need a large collection, you could download a keyserver dump and filter out what you need.


3

The problem you described is addressed and solved in different way by the Broadcast Encryption: roughly the problem is how to encrypt a message intended to more than one recipient. You described a naive approach to the problem, your solution is, indeed, probably the simplest one. The Broadcast Encryption concept was formalized and defined by Fiat and Naor ...


3

This sounds like "fair exchange," the subject of many good research papers. In general you need a third party to give any security guarantees, but "optimistic fair exchange" involves the third party only when one of the parties tries to cheat (i.e., when both play honestly there is no involvement from the third party). Incidentally, Diffie-Hellman is most ...


2

You will always need to have some way of identifying the other party, be it a pre-distributed key or out-of-band procedures as you are describing. There is no purely digital method that can do this without some kind of shared knowledge.


2

I know how Diffie-Hellman Key Exchange works. Is this the main way of encrypting with PGP, ssh, ssl (https), DKIM, ...? As the name says Diffie-Hellman key exchange is a key exchange protocol, i.e., a protocol where two parties agree on a common secret without having exchanged any secret prior to that, in an interactive way, i.e., both parties are ...


2

For the signatures, hash-based signatures provide a nice solution to your problem: You can use so called hash combiners to instantiate the signature. These are functions that construct a hash function given two or more hash functions and preserve certain properties as long as at least one of the hash function has this property. For example the concatenation ...



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