Hackepedia - User contributions [en]

Hashcash

2005-10-10T05:01:07Z

Roy:

[[Hashcash]] is a "proof of work" system originally designed as an anti-spam measure. [[Hashcash]] tokens are "minted" by finding a [[hash collision]] for a unique input string. The result is included in the header of an email message as "proof" that the sender has done a certain amount of work to "pay for" the delivery of the message. A 20-bit collision may take only a fraction of a second to compute, depending on the computer system. 25 bits on the same system would take 11 seconds, and 30 bits would take over 6 minutes. The recipient determines how much work is required to pay for delivery.

The idea behind [[hashcash]] is that a legitimate email sender can easily afford a few seconds to mint a stamp for an outgoing email, but spammers who want to send millions of emails would be bogged down.

The [[hashcash]] project may be found [http://www.hashcash.org here].

Hash collision

2005-10-10T04:49:04Z

Roy:

The desirable property of a [[One way hash]] algorithm is that for any given input, the output is unique. Given that the hash output is almost always smaller than the input, this property can't be achieved completely. When two sets of input data result in the same hash output, it is called a [[hash collision]].

Hash collisions aren't always a problem. Some techniques, like [[hashcash]], rely on finding collisions as a "proof of work". The problems arise when similar data sets collide. This is the basis of the [[MD5]] collision paper presented at Crypto 2004. Further research has produced a method of creating quite similar documents that have the same [[MD5]] hash value. When the two documents are, for example, slightly different versions of a contract, and the [[MD5]] hash value is used to attest that the original document has not been altered, the [[hash collision]] problem becomes obvious.

One way hash

2005-10-10T04:35:04Z

Roy:

A [[One way hash]] is a mathematical way to create a (usually smaller) unique representation of a given set of data. It's sometimes referred to as a "fingerprint". It's called "One Way" because while it's easy to create the hash value from the input data, it is, for all intents and purposes, impossible to reconstruct the original data from the hash value. [[MD5]] is perhaps the most well-known [[One way hash]]. [[SHA]], the Secure Hashing Algorithm, is widely used today to replace [[MD5]].

Md5

2005-10-10T04:33:34Z

Roy:

[[MD5]] is a cryptographic one-way hashing algorithm first described in [[RFC]] 1321 back in 1992.

[[MD5]] is no longer recommended for use, following a presentation at the 2004 Crypto Conference that revealed [[hash collision]] problems with the algorithm.

One way hash

2005-10-10T04:28:18Z

Roy:

A [[One Way Hash]] is a mathematical way to create a (usually smaller) unique representation of a given set of data. It's sometimes referred to as a "fingerprint". It's called "One Way" because while it's easy to create the hash value from the input data, it is, for all intents and purposes, impossible to reconstruct the original data from the hash value. [[MD5]] is perhaps the most well-known [[One Way Hash]]. [[SHA]], the Secure Hashing Algorithm, is widely used today to replace [[MD5]].

Rot13

2005-10-10T04:13:44Z

Roy:

Rot13 rotates all 26 letters of the alphabet by 13 positions rolling over to position 1 after position 26. A sample display of how rot13 "encrypts" is shown below:

$ echo A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | rot13
N O P Q R S T U V W X Y Z A B C D E F G H I J K L M

So you see A becomes N and N becomes A. This encryption can be found in many places online, such as Usenet and geocaching.com.

"But wait!", you say. "My system, has no 'rot13' program!" Fear not. It is but one line of Perl:

#!/usr/bin/perl -p
y/A-Za-z/N-ZA-Mn-za-m/;

(OK, two lines, if you count the shellexec header). rot13 is also found in the BSD games package.

Rot13

2005-10-10T04:13:29Z

Roy:

Rot13

2005-10-10T04:02:17Z

Roy: