Technology materials

ZIP

When Zip was understood, the sauce of Info-ZIP was very useful.
readme.txt of Advanced ZIPPassword Recovery of ElcomLtd. gave me a hint to the speed-up.

About the structure of the password of Zip.

The password of Zip is an extreme head 80 characters.

How to look for a password.

A total hit is certain to begin to find one from a lot of combinations. At present, PikaZip checks 280,000 passwords in 1 second though the password of test.zip inside pikazip.zip is examined when "6x86MX-PR200" is used for the CPU. It takes time too much in this.So, a method except for it, that is, dictionary attack is used. Because a person inputs a password, the combination (for example, parsonalcomputer) of the word (for example, parsonal) or the word will be inputted. Those lists (dictionary) are prepared for in advance. Time becomes overwhelmingly short. But, it isn't surely always found because it is not a total hit. The thing of the dictionary which the number of the words is abundant is good. A dictionary formation engine is important again, too. The combination of the word is formed from the word, and a dictionary formation engine makes a word change. (for example, parsonal to Parsonal, power to p0wer). This contributes to increasing the number of the words with covering the size of the dictionary.

A password is computed with the data on random 12Byte put when a file is compressed first when it is inputted. It is compared with the data on 1Byte of the calculation result and 24-32bit of CRC. The password may be a correct answer if two data are the same at this time. But, if it doesn't actually try to extract with the password, it doesn't know whether it is a correct answer. It is because 1/250 of all the passwords has the same value with this check.

test.zip which is actually packed into pikazip.zip together can be experienced as for this by extracting. Try to input a suitable password this archive when it extracts. The message that a password is wrong may have been indicated. (It becomes input of a password again with Lhasa.). Well, try to input it next with 53. Though extract is actually done, it becomes a CRC error. (with Lhasa, damage file and indication). You must try to extract the considerable number of times, the archive with 1/250. It takes time if it becomes number M though there is no great thing in little archive, either. Therefore, you must hold the burden of the extract.

When therefore more than one file is in archive, they are supposed to be compressed with the same password. (On/Off is made of the optional same password in this.). A file several minutes, 1/250 are done by this. Extract number of times decreases in this. (ElcomLtd. reference). The loads of the extract decrease if the thing that No.1 file size inside archive is small in advance is made to extract again.

Lhasa: Copyright Takemura

 

RAR

RAR is very late because extract is only being repeated.

The thing that sauce was seen and which it noticed.

The size of the multi-volume can be specified individually. For example it can be divided into MO one sheet + 2 sheets of floppy disk. It can extract as usual with WINRAR.