Archive for 2012-08-05

Caesar Cipher

In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a shift of 3, A would be replaced by D, B would become E, and so on. The method is named after Julius Caesar, who used it in his private correspondence.

How it works?

Plain:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
Cipher:   DEFGHIJKLMNOPQRSTUVWXYZABC

The encryption can also be represented using modular arithmetic by first transforming the letters into numbers, according to the scheme, A = 0, B = 1,..., Z = 25. Encryption of a letter $x$ by a shift n can be described mathematically as,

$E_n(x) = (x + n) \mod {26}.$

Decryption is performed similarly,

$D_n(x) = (x - n) \mod {26}.$

following shows a message :

Ciphertext: WKH TXLFN EURZQ IRA MXPSV RYHU WKH ODCB GRJ
Plaintext:  the quick brown fox jumps over the lazy dog

History!

The Caesar cipher is named after Julius Caesar, who, according to Suetonius, used it with a shift of three to protect messages of military significance. While Caesar's was the first recorded use of this scheme, other substitution ciphers are known to have been used earlier.

1. Check Windows Update and Office Update regularly (http://office.microsoft.com/productupdates) have your Office CD ready. Click on the Automatic Updates tab in the System control panel and choose the appropriate options.

2. Install a personal firewall. Both SyGate (_www.sygate.com) and ZoneAlarm (_www.zonelabs.com) offer free versions.

3. Install a free spyware blocker. (http://security.kolla.de). SpyBot is also paranoid and ruthless in hunting out tracking cookies.

4. Use strong passwords and change them periodically. Passwords should have at least seven characters; use letters and numbers and have at least one symbol. A decent example would be f8izKro@l. This will make it much harder for anyone to gain access to your accounts. and even harder to remember at first but if you make some combinations, it will work.

5. If you're using Outlook or Outlook Express, use the current version or one with the Outlook Security Update installed. The update and current versions patch numerous vulnerabilities.

6. Buy antivirus software and keep it up to date. If you're not willing to pay, try Grisoft AVG Free Edition (Grisoft Inc., www.grisoft.com). And doublecheck your AV with the free, online-only scanners available at www.pandasoftware.com/activescan and http://housecall.trendmicro.com.

7. If you have a wireless network, turn on the security features: Use MAC filtering, turn off SSID broadcast, and even use WEP with the biggest key you can get.

8. Be skeptical of things on the Internet. Don't assume that e-mail "From:" a particular person is actually from that person until you have further reason to believe it's that person. Don't assume that an attachment is what it says it is. Don't give out your password to anyone, even if that person claims to be from "support."

Today I'll be talking about crypters - to bypass antivirus

Crypters

Crypting is one of the popular methods used for antivirus evading due to it’s simplicity and also because it does not require any prior knowledge about any programming language.

How Crypters Work?

A crypter is a small program that allows the attacker to crypt the source code of a Trojan or any form of malware, A crypter basically jumbles the source of the file to make it undetectable, As I told you before that an antivirus use a signature based detection, the crypter jumbles the source so when an antivirus scans the virus/malware/trojan it cannot be detected.

FUD

FUD stands for “Fully Undetectable” which simply means that a trojan or a virus which cannot be detected by an antivirus. FUD server is very difficult to achieve, you are very lucky if you can find any binders or crypters out there which is FUD. Free Crypters lose their affectivity as antivirus makes or composes a signature for them, however paid crypters are said to be Fully undetectable.

Ultimate Crypter – Ultimate crypter is one of the most commonly used crypters around, Ultimate crypter may not be able to achieve a FUD server but it has a very low detection rate. It has a paid version too which claims to make the server FUD however I haven’t tried it

Yoda’s Crypter – Yoda’s crypter has a lower antivirus detection rate than ultimate crypter, it has a user friendly graphical representation and is very easy to use.

Crypters are not always used for hacking purposes.

A Crypter encrypts and packs other software in a way that makes the actual bytes unreadable. People use a crypter to protect software from reverse engineering, piracy / theft, hacking / tampering. Antivirus positives can be defeated this way as well, though unfortunately this technique of using crypters is also used by hackers to make a virus and other malware undetected by antivirus software.