.:: || Modern Cryptography || ::.

Modern Cryptograpy Algorithm

Most modern ciphers use a sequence of binary digits (bits), that is, zeros and ones such as ASCII.

This bit sequence representing the plaintext is then encrypted to give the ciphertext as a bit sequence.

Stream Ciphers



A Stream Cipher is a symmetric key cipher where plaintext bits are combined with a pseudorandom cipher bit stream (keystream), typically by an exclusive-or (xor) operation. In a stream cipher the plaintext digits are encrypted one at a time, and the transformation of successive digits varies during the encryption. An alternative name is a state cipher, as the encryption of each digit is dependent on the current state. In practice, the digits are typically single bits or bytes.

Stream ciphers represent a different approach to symmetric encryption from block ciphers. Block ciphers operate on large blocks of digits with a fixed, unvarying transformation. This distinction is not always clear-cut: in some modes of operation, a block cipher primitive is used in such a way that it acts effectively as a stream cipher. Stream ciphers typically execute at a higher speed than block ciphers and have lower hardware complexity. However, stream ciphers can be susceptible to serious security problems if used incorrectly: see stream cipher attacks — in particular, the same starting state must never be used twice.

Type of Stream Ciphers

1.Synchronous stream ciphers

In a synchronous stream cipher a stream of pseudo-random digits is generated independently of the plaintext and ciphertext messages, and then combined with the plaintext (to encrypt) or the ciphertext (to decrypt). In the most common form, binary digits are used (bits), and the keystream is combined with the plaintext using the exclusive or operation (XOR). This is termed a binary additive stream cipher.

In a synchronous stream cipher, the sender and receiver must be exactly in step for decryption to be successful. If digits are added or removed from the message during transmission, synchronisation is lost. To restore synchronisation, various offsets can be tried systematically to obtain the correct decryption. Another approach is to tag the ciphertext with markers at regular points in the output.

2.Self-synchronizing stream ciphers

This is a another approach uses several of the previous N ciphertext digits to compute the keystream. Such schemes are known as self-synchronizing stream ciphers, asynchronous stream ciphers or ciphertext autokey (CTAK). The idea of self-synchronization was patented in 1946, and has the advantage that the receiver will automatically synchronise with the keystream generator after receiving N ciphertext digits, making it easier to recover if digits are dropped or added to the message stream. Single-digit errors are limited in their effect, affecting only up to N plaintext digits.

An example of a self-synchronising stream cipher is a block cipher in cipher-feedback mode (CFB).

Block Ciphers

A block cipher is a symmetric key cipher operating on fixed-length groups of bits, termed blocks, with an unvarying transformation. A block cipher encryption algorithm might take (for example) a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. The exact transformation is controlled using a second input — the secret key. Decryption is similar: the decryption algorithm takes, in this example, a 128-bit block of ciphertext together with the secret key, and yields the original 128-bit block of plaintext.

To encrypt messages longer than the block size (128 bits in the above example), a mode of operation is used.

Block ciphers can be contrasted with stream ciphers; a stream cipher operates on individual digits one at a time, and the transformation varies during the encryption. The distinction between the two types is not always clear-cut: a block cipher, when used in certain modes of operation, acts effectively as a stream cipher.

An early and highly influential block cipher design was the Data Encryption Standard (DES), developed at IBM and published as a standard in 1977. A successor to DES, the Advanced Encryption Standard (AES)

A block cipher consists of 2 paired algorithms, one for encryption, E, and another for decryption, E-1. Both algorithms accept two inputs: an input block of size n bits and a key of size k bits, yielding an n-bit output block. For any one fixed key, decryption is the inverse function of encryption, so that

E_K^{-1}(E_K(M))=M

for any block M and key K.

Data Encryption Standard(DES)



The Data Encryption Standard (DES) is a block cipher (a form of shared secret encryption) that was selected by the National Bureau of Standards as an official Federal Information Processing Standard (FIPS) for the United States in 1976 and which has subsequently enjoyed widespread use internationally. It is based on a symmetric-key algorithm that uses a 56-bit key. The algorithm was initially controversial with classified design elements, a relatively short key length, and suspicions about a National Security Agency (NSA) backdoor. DES consequently came under intense academic scrutiny which motivated the modern understanding of block ciphers and their cryptanalysis.



DES is now considered to be insecure for many applications. There are also some analytical results which demonstrate theoretical weaknesses in the cipher, although they are unfeasible to mount in practice. The algorithm is believed to be practically secure in the form of Triple DES, although there are theoretical attacks. In recent years, the cipher has been superseded by the Advanced Encryption Standard (AES).

Advanced Encryption Standard

the Advanced Encryption Standard (AES) is an encryption standard adopted by the U.S. government. The standard comprises three block ciphers, AES-128, AES-192 and AES-256, adopted from a larger collection originally published as Rijndael. Each AES cipher has a 128-bit block size, with key sizes of 128, 192 and 256 bits, respectively. The AES ciphers have been analyzed extensively and are now used worldwide, as was the case with its predecessor,[3] the Data Encryption Standard (DES).

AES was announced by National Institute of Standards and Technology (NIST) as U.S. FIPS PUB 197 (FIPS 197) on November 26, 2001 after a 5-year standardization process in which fifteen competing designs were presented and evaluated before Rijndael was selected as the most suitable. It became effective as a standard May 26, 2002. As of 2009[update], AES is one of the most popular algorithms used in symmetric key cryptography.[citation needed] It is available in many different encryption packages. AES is the first publicly accessible and open cipher approved by the NSA for top secret information.

High-level description of the algorithm

• KeyExpansion using Rijndael's key schedule


• Initial Round

1. AddRoundKey

• Rounds

1. SubBytes — a non-linear substitution step where each byte is replaced with another according to a lookup table.
2. ShiftRows — a transposition step where each row of the state is shifted cyclically a certain number of steps.
3. MixColumns — a mixing operation which operates on the columns of the state, combining the four bytes in each column
4. AddRoundKey — each byte of the state is combined with the round key; each round key is derived from the cipher key using a key schedule.

• Final Round (no MixColumns)


• 1. SubBytes


• 2. ShiftRows


• 3. AddRoundKey

The SubBytes step

In the SubBytes step, each byte in the array is updated using an 8-bit substitution box, the Rijndael S-box. This operation provides the non-linearity in the cipher. The S-box used is derived from the multiplicative inverse over GF(28), known to have good non-linearity properties. To avoid attacks based on simple algebraic properties, the S-box is constructed by combining the inverse function with an invertible affine transformation. The S-box is also chosen to avoid any fixed points (and so is a derangement), and also any opposite fixed points.

The ShiftRows step

The ShiftRows step operates on the rows of the state; it cyclically shifts the bytes in each row by a certain offset. For AES, the first row is left unchanged. Each byte of the second row is shifted one to the left. Similarly, the third and fourth rows are shifted by offsets of two and three respectively. For the block of size 128 bits and 192 bits the shifting pattern is the same. In this way, each column of the output state of the ShiftRows step is composed of bytes from each column of the input state. (Rijndael variants with a larger block size have slightly different offsets). In the case of the 256-bit block, the first row is unchanged and the shifting for second, third and fourth row is 1 byte, 3 bytes and 4 bytes respectively - this change only applies for the Rijndael cipher when used with a 256-bit block, as AES does not use 256-bit blocks.

The MixColumns step

In the MixColumns step, the four bytes of each column of the state are combined using an invertible linear transformation. The MixColumns function takes four bytes as input and outputs four bytes, where each input byte affects all four output bytes. Together with ShiftRows, MixColumns provides diffusion in the cipher. Each column is treated as a polynomial over GF(28) and is then multiplied modulo x4 + 1 with a fixed polynomial c(x) = 3x3 + x2 + x + 2. The MixColumns step can also be viewed as a multiplication by a particular MDS matrix in Finite field. This process is described further in the article Rijndael mix columns.

The AddRoundKey step

In the AddRoundKey step, the subkey is combined with the state. For each round, a subkey is derived from the main key using Rijndael's key schedule; each subkey is the same size as the state. The subkey is added by combining each byte of the state with the corresponding byte of the subkey using bitwise XOR.

Message Authentication Code

A message authentication code (often MAC) is a short piece of information used to authenticate a message.

A MAC algorithm, sometimes called a keyed (cryptographic) hash function, accepts as input a secret key and an arbitrary-length message to be authenticated, and outputs a MAC (sometimes known as a tag). The MAC value protects both a message's data integrity as well as its authenticity, by allowing verifiers (who also possess the secret key) to detect any changes to the message content.

Message Authentication Code Flow



the sender of a message runs it through a MAC algorithm to produce a MAC data tag. The message and the MAC tag are then sent to the receiver. The receiver in turn runs the message portion of the transmission through the same MAC algorithm using the same key, producing a second MAC data tag. The receiver then compares the first MAC tag received in the transmission to the second generated MAC tag. If they are identical, the receiver can safely assume that the integrity of the message was not compromised, and the message was not altered or tampered with during transmission.

Hash Function

A hash function is a deterministic procedure that takes an arbitrary block of data and returns a fixed-size bit string, the (cryptographic) hash value, such that an accidental or intentional change to the data will change the hash value. The data to be encoded is often called the "message", and the hash value is sometimes called the message digest or simply digest.

The ideal cryptographic hash function has four main properties:

• it is easy to compute the hash value for any given message,


• it is infeasible to find a message that has a given hash,


• it is infeasible to modify a message without changing its hash,


• it is infeasible to find two different messages with the same hash.

A cryptographic hash function (specifically, SHA-1) at work. Note that even small changes in the source input drastically change the resulting output, by the so-called avalanche effect.

RSA( Rivest, Shamir & Adleman)

RSA (which stands for Rivest, Shamir and Adleman who first publicly described it) is an algorithm for public-key cryptography. It is the first algorithm known to be suitable for signing as well as encryption, and one of the first great advances in public key cryptography. RSA is widely used in electronic commerce protocols, and is believed to be secure given sufficiently long keys and the use of up-to-date implementations.

RSA Key Setup Example :

• 1.Select primes: p=17 & q=11


• 2.Compute n = pq =17 x 11=187


• 3.Compute ø(n)=(p–1)(q-1)=16 x 10=160


• 4.Select e: gcd(e,160)=1; choose e=7
• 
  
• 5.Determine d: de=1 mod 160 and d < 160 Value is d=23


• 6.Publish public key PU={7,187}


• 7.Keep secret private key PR={23,187}

RSA Key Generation

users of RSA must:
determine two primes at random - p, q
select either e or d and compute the other

primes p,q must not be easily derived from modulus n=p*q
means must be sufficiently large
typically guess and use probabilistic test

exponents e, d are inverses, so use Inverse algorithm to compute the other

.:: || Basic Cryptography || ::.

Cryptograpy Definition

Cryptography (or cryptology; from Greek κρυπτός, kryptos, "hidden, secret"; and γράφω, gráphō, "I write", or -λογία, -logia, respectively)[1] is the practice and study of hiding information. Modern cryptography intersects the disciplines of mathematics, computer science, and engineering. Applications of cryptography include ATM cards, computer passwords, and electronic commerce.(from //en.wikipedia.org/wiki/Cryptography).

Cryptograpy Concept

• scramble data in storage


• transmit data in to the Internet


• user can get the data but they can't read

Art of secret writing



i) Steganography

Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity. The word steganography is of Greek origin and means "concealed writing". The first recorded use of the term was in 1499 by Johannes Trithemius in his Steganographia, a treatise on cryptography and steganography disguised as a book on magic. Generally, messages will appear to be something else: images, articles, shopping lists, or some other covertext and, classically, the hidden message may be in invisible ink between the visible lines of a private letter.

The advantage of steganography, over cryptography alone, is that messages do not attract attention to themselves. Plainly visible encrypted messages—no matter how unbreakable—will arouse suspicion, and may in themselves be incriminating in countries where encryption is illegal.[1] Therefore, whereas cryptography protects the contents of a message, steganography can be said to protect both messages and communicating parties.

Steganography includes the concealment of information within computer files. In digital steganography, electronic communications may include steganographic coding inside of a transport layer, such as a document file, image file, program or protocol. Media files are ideal for steganographic transmission because of their large size. As a simple example, a sender might start with an innocuous image file and adjust the color of every 100th pixel to correspond to a letter in the alphabet, a change so subtle that someone not specifically looking for it is unlikely to notice it

Within this picture, the letters position of a hidden message are represented by increasing numbers (1 to 20), and a letter value is given by its intersection position in the grid. For instance, the first letter of the hidden message is at the intersection of 1 and 4. So, after a few tries, the first letter of the message seems to be the 14th letter of the alphabet; the last one (number 20) is the 5th letter of the alphabet.

Termonology Cryptograpy

cryptography referred almost exclusively to encryption, which is the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e., ciphertext).[2] Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms which create the encryption and the reversing decryption. The detailed operation of a cipher is controlled both by the algorithm and in each instance by a key. This is a secret parameter (ideally known only to the communicants) for a specific message exchange context. Keys are important, as ciphers without variable keys are trivially breakable and therefore less than useful for most purposes. Historically, ciphers were often used directly for encryption or decryption without additional procedures such as authentication or integrity checks.

plaintext - original message
ciphertext - coded message
cipher - algorithm for transforming plaintext to ciphertext
key - info used in cipher known only to sender/receiver
encipher (encrypt) - converting plaintext to ciphertext
decipher (decrypt) - recovering ciphertext from plaintext
cryptography - study of encryption principles/methods
cryptanalysis (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key
cryptology - field of both cryptography and cryptanalysis

CRYPTANALYSIS

Definition of Crytanalysis

Cryptanalysis (from the Greek kryptós, "hidden", and analýein, "to loosen" or "to untie") is the study of methods for obtaining the meaning of encrypted information, without access to the secret information which is normally required to do so. Typically, this involves knowing how the system works and finding a secret key. In non-technical language, this is the practice of codebreaking or cracking the code, although these phrases also have a specialised technical meaning (see code).

"Cryptanalysis" is also used to refer to any attempt to circumvent the security of other types of cryptographic algorithms and protocols in general, and not just encryption. However, cryptanalysis usually excludes methods of attack that do not primarily target weaknesses in the actual cryptography, such as bribery, physical coercion, burglary, keystroke logging, and social engineering, although these types of attack are an important concern and are often more effective than traditional cryptanalysis.(from )

Symmetric Algorithm

Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption.

The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transformation to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.

Other terms for symmetric-key encryption are secret-key, single-key, shared-key, one-key, and private-key encryption. Use of the last and first terms can create ambiguity with similar terminology used in public-key cryptography.

Example of Symmetric Algorithm

Assymmetric Algorithm

Assymmetric Algorithm cryptography is a cryptographic approach, employed by many cryptographic algorithms and cryptosystems, whose distinguishing characteristic is the use of asymmetric key algorithms instead of or in addition to symmetric key algorithms. Using the techniques of public key-private key cryptography, many methods of protecting communications or authenticating messages formerly unknown have become practical. They do not require a secure initial exchange of one or more secret keys as is required when using symmetric key algorithms. It can also be used to create digital signatures.

Example of Assymmetric Algorithm

Symmetric Algorithm vs Assymmetric Algorithm


This is a short comparisson of this 2 types of algorithm :

Speed of algorithms

Symmetric-key algorithms are generally much less computationally intensive than asymmetric key algorithms. In practice, asymmetric key algorithms are typically hundreds to thousands times slower than symmetric key algorithms.

Key management

One disadvantage of symmetric-key algorithms is the requirement of a shared secret key, with one copy at each end. In order to ensure secure communications between everyone in a population of n people a total of n(n − 1)/2 keys are needed, which is the total number of possible communication channels.[1] To limit the impact of a potential discovery by a cryptographic adversary, they should be changed regularly and kept secure during distribution and in service. The process of selecting, distributing and storing keys is known as key management, and is difficult to achieve reliably and securely.

Hybrid cryptosystem

In modern cryptosystems designs, both asymmetric (public key) and symmetric algorithms are used to take advantage of the virtues of both. Asymmetric algorithms are used to distribute symmetric-keys at the start of a session. Once a symmetric key is known to all parties of the session, faster symmetric-key algorithms using that key can be used to encrypt the remainder of the session. This simplifies the key distribution problem, because asymmetric keys only have to be distributed authentically, whereas symmetric keys need to be distributed in an authentic and confidential manner.

Systems that use such a hybrid approach include SSL, PGP, GPG etc.

Caesar Ciphers



Caesar cipher also known as a 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 to communicate with his generals.

The encryption step performed by a Caesar cipher is often incorporated as part of more complex schemes, such as the Vigenère cipher, and still has modern application in the ROT13 system. As with all single alphabet substitution ciphers, the Caesar cipher is easily broken and in practice offers essentially no communication security.

Examples :

Using this system, the keyword "zebras" gives us the following alphabets:

Plaintext alphabet: abcdefghijklmnopqrstuvwxyz

Ciphertext alphabet: ZEBRASCDFGHIJKLMNOPQTUVWXY

A message of :

flee at once. we are discovered!

enciphers to

SIAA ZQ LKBA. VA ZOA RFPBLUAOAR!

ROT13 is a Caesar cipher, a type of substitution cipher. In ROT13, the alphabet is rotated 13 steps.

Vigenere Ciphers

The Vigenère cipher is a method of encrypting alphabetic text by using a series of different Caesar ciphers based on the letters of a keyword. It is a simple form of polyalphabetic substitution.

The Vigenère (French pronunciation: [viʒnɛːʁ]) cipher has been reinvented many times. The method was originally described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher".

This cipher is well known because while it is easy to understand and implement, it often appears to beginners to be unbreakable; this earned it the description le chiffre indéchiffrable (French for 'the unbreakable cipher'). Consequently, many people have tried to implement encryption schemes that are essentially Vigenère ciphers, only to have them broken.

Vinegere Tableau

For example, suppose that the plaintext to be encrypted is:

ATTACKATDAWN

The person sending the message chooses a keyword and repeats it until it matches the length of the plaintext, for example, the keyword "LEMON":

LEMONLEMONLE

The first letter of the plaintext, A, is enciphered using the alphabet in row L, which is the first letter of the key. This is done by looking at the letter in row L and column A of the Vigenère square, namely L. Similarly, for the second letter of the plaintext, the second letter of the key is used; the letter at row E and column T is X. The rest of the plaintext is enciphered in a similar fashion:

• Plaintext : ATTACKATDAWN


• Key : LEMONLEMONLE


• Ciphertext: LXFOPVEFRNHR

.:: || Introduction Network Security || ::.

Salam semua ...Ok, macam biasa ..kalau dh nama pun 1st class nih kami telah diterangkan secara ringkas mengenai modul Network Security ni. Pensyarah untuk modul ini adalah En. Zaki bin Mas'ud (email : zaki.masud@utem.edu.my)

Modul ini merangkumi 14 tajuk yang merangkumi :

• ¡Introduction to Information Security
• ¡Introduction to Cryptography
• ¡Modern Cryptography
• ¡Access Control and Authentication
• ¡Program Security
• ¡Database Security
• ¡Security in Networks
• ¡Security in Applications
• ¡Firewall
• ¡Intrusion Detection System
• ¡Wireless Security
• ¡Legal and Ethical Issues in Computer Security
• ¡Cyberlaws
• ¡Administering Security

First class juga kami telah diuji dengan 1st quiz dimana En. Zaki ingin menguji tahap pemahaman & apa yang para pelajar tau tentang serba sedikit mengenai Network Security ini.. haha ..

Serba sedikit review tentang pembelajaran yang pertama :

WHAT IS SECURITY ?

• Security is a quality or state of being secure that is to be free from danger.
• To be protected from adversaries.

SECURITY PRINCIPLE ?

It have 3 types including :

• Confidentially - prevention of unauthorized disclosure of information


• Integrity - prevent of unauthorized modification of information


• Availability - unauthorized withholding of information or resources


SECURITY TRENDS ? (refer picture below)


THREAT & ATTACK



• Threat : A potential for violation of security, which exists when there is a circumstance,capability,action, or event that could breach security & cause harm. That is,a threat is a possible danger that might exploit vulnerability



• Attack : An assault on system that derives from an intelligent threat,that is an intelligent act that is a deliberate attempt (especially in the sense of method or technique) to evade security services & violate the security policy of a system.



Passive Attack : eavesdropping on, or monitoring of, transmissions.
The Goal : to obtain information that is being transmitted.
Types : release of message contents & traffic analysis

Release of message contents
eassily understood. A telephone conversation, an electronic mail message, and a transfered file may contain sensitive or confidential information (refer picture below)


Traffic analysis
Is subtler, which suppose that we had a way of masking the contents of message or other information traffic so that opponents, even if they captured the message, could not extract the information from the message. The common technique for masking contents is encryption. If we had encryption protection in place, an opponent might still be able to observe the patern of these message. The opponent could determine the location & identity of communicating hosts and could observe the frequency and length of messages being exchanged(refer picture below)


Active Attacks : involve some modification of the data stream or the creation of a false stream & can be subdivided into 4 categories :masquerade, replay, modification of message, & denial of service.

Masquerade
takes place when one entity pretends to be a diffrent entity.(refer picture below)


Replay
involve the passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect (refer picture below)


Modification of messages
some portion of a legitimate message is altered, or that message are delayed or reordered, to produce an unauthorized effect.(refer picture below)


Denial of service
prevents or inhibits the normal use or management of communication facilities.(refer picture below)




SECURITY SERVICE

X.800 defines a security service as a service provided by a protocol layer of communicating open systems, which ensures adequate security of the system or of data transfers.

RFC 2828 define which provides the following definitions : a processing or communication service that is provided by a system to give a specific kind of protection to system resources.

5 Categories & 14 Specific Services



SECURITY MECHANISM

Any process (or a device incorporating such a process) that is designed to detect, prevent or recover from a security attack.

Specific Security Mechanism



Pervasive Security Mechanism





LAB 1.::Introduction to Virtualization & VMware::.

Virtual Machines (VM) allow you to run any operating system in a new window. This is done by using virtualization, which is the general technology used by virtual machines. Imagine what you could do if you had under your control many computers, possibly connected together. Best is, they are simple to use and have only little performance hit. Well see what those applications can do, how they can help you (test any OS or application while keeping you PC clean), and how to use them. Further, VM make it possible to create testing plateforms for your applications, websites, or virtually anything!



• Task 1 : VMware Workstation Installation


• Task 2 : 1.Creating Disk Image

: 2.To know how to get the virtual machine console

• Task 3 : Installing Windows Server 2003 on Virtual Machine

.:: || Pentingnya Network Security ||::.

Salam sejahtera semua,

Network Security atau dalam bahasa Melayu Keselamatan Rangkaian.Ape yang korang sume paham & tau pasal tajuk yang saya post neh ?

Saya bukan nak cerita pasal kegembiraan tu tapi apa yang saya dapat dari belajar Network Security ni.Rupanya banyak yang saya tak tahu pasal computer/network security ni.Dalam subjek ni kami diberi praktikal camna nak setup network bagi selamat.Ada lab.Camna nak detect penceroboh ngan belajar Linux.

Rupanya Network Security ni memang teramat penting bagi pengguna komputer,tambah pulak macam satu-satu organisasi besar.Mesti nak selamat punya network diorang.Kalau tak senang2 la orang pecah masuk curi data diorang kan?Benda-benda tu suma diajar utk kami.

Anyway,thanks a lot En Zaki,lecture Network Security kami sebab dah ajar byk benda pasal security ni.Bukan ajar pakai buku jer tapi dari pengalaman dia sendiri.Tu yang best tu.Tabik la.Memang expert.

Baiklah,tu je nak bagitau.

p/s : Agak2 network anda guna selamat tak dari kena ceroboh?