Whether the algorithms and
For symmetric key algorithms used to encrypt streams or blocks of data, if the key is not changed at intervals, or is a short length key, a search of all possible keys in the key space can be used to decrypt intercepted communications. This type of attack is very time- and CPU-
Exercise 6.01 using L0phtCrack can be run as a brute force attack by removing the reference to a dictionary file from the command line. The LANMAN and NT password hash algorithms are commonly subjected to brute force attacks.
A ciphertext-only attack involves capturing samples of ciphertext and analyzing it to determine the key. This type of attack relies on the statistical repetition of some patterns in the plaintext being visible or discernible in the ciphertext. For example, if the encrypted data utilizes a block cipher in ECB mode, repeated plaintext data will be discernable in the ciphertext.
In some cases, it may be possible for the attacker to determine what plaintext was encrypted and sent (for example, a message containing only the word
with a standard message header, subject, or footer). In such cases, the key can be "reverse-engineered" by
A chosen plaintext attack involves encryption of known plaintext messages and analyzing and comparing the resulting ciphertext to search for the key. This attack is conditional upon the ability to present various plaintexts to the encryption engine. This
Such an attack might be used if dealing with a black box-type of device, where control of the input was available as well as capturing the output. The attacker might feed in data such as "The quick brown fox jumped over the lazy dog" and mathematically compare this and other input with the output, which might be "Uifrvjdlcspxogpykvoqfepwdsuidmbazeph."
As an example of a Man-in-the-middle (MITM)-type of attack
consider that someone called Al is performing a standard Diffie-Hellman key exchange with Charlie for the very first time, while Beth is in a position such that all traffic between Al and Charlie
When Al transmits a message to Charlie, he will encrypt it using Beth's public key. Beth will intercept the message and decrypt it using her private key. Once Beth has read the message, she encrypts it again using Charlie's public key and transmits the message on to Charlie. She may even modify the message contents if she so desires. Charlie then receives Beth's modified message, believing it to come from Al. He replies to Al and encrypts the message using Beth's public key. Beth again intercepts the message, decrypts it with her private key, and modifies it. Then she encrypts the new message with Al's public key and sends it on to Al, who receives it and believes it to be from Charlie.
Clearly, this type of communication is undesirable because a third party not only has access to confidential information, but she can also modify it at will. In this type of attack, no encryption is broken because Beth does not know either Al or Charlie's private keys, so the Diffie-Hellman algorithm is not really at fault. Beware of the key exchange mechanism used by any public key encryption system. If the key exchange protocol does not authenticate at least one and preferably both sides of the connection, it may be vulnerable to MITM-type attacks. Authentication systems
Be sure to know what sort of attacks might or might not be