CRYPTOGRAPHY
Tuesday, 27 March, 2018
What is Cryptography?
Cryptography is associated with the process of converting ordinary plain text (called plaintext) into unintelligible text (called ciphertext). Decryption is the reverse, in other words, moving from the unintelligible ciphertext back to plaintext. A cipher (or cypher) is a pair of algorithms that 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". It is a method of storing and transmitting data in a particular form so that only those for whom it is intended can read and process it. Cryptography not only protects data from theft or alteration, but can also be used for user authentication.
Alphabet shift ciphers are believed to have been used by Julius Caesar over 2,000 years ago. This is an example with k=3. In other words, the letters in the alphabet are shifted three in one direction to encrypt and three in the other direction to decrypt.
Earlier cryptography was effectively synonymous with encryption but nowadays cryptography is mainly based on mathematical theory and computer science practice.
Modern cryptography concerns with:
1. Confidentiality – Information cannot be understood by anyone
2. Integrity - Information cannot be altered.
3. Non-repudiation - Sender cannot deny his/her intentions in the transmission of the information at a later stage
4. Authentication - Sender and receiver can confirm each
Cryptography is used in many applications like banking transactions cards, computer passwords, and e- commerce transactions.
Three types of cryptographic techniques used in general.
1. Symmetric-key cryptography
2. Hash functions.
3. Public-key cryptography
Symmetric-key Cryptography: Both the sender and receiver share a single key. The sender uses this key to encrypt plaintext and send the cipher text to the receiver. On the other side the receiver applies the same key to decrypt the message and recover the plain text.
Public-Key Cryptography: This is the most revolutionary concept in the last 300-400 years. In Public-Key Cryptography two related keys (public and private key) are used. Public key may be freely distributed, while its paired private key, remains a secret. The public key is used for encryption and for decryption private key is used.
Hash Functions: No key is used in this algorithm. A fixed-length hash value is computed as per the plain text that makes it impossible for the contents of the plain text to be recovered. Hash functions are also used by many operating systems to encrypt passwords.
Attacks on Cryptography
We now briefly mention some attacks one could launch on a cryptographic protocol with the goal to learn the decryption key or in some cases the plaintext that was input to the protocol. We assume that Alice and Bob talk using the cryptographic protocol. Eve is the enemy that can passively observe encrypted messages but cannot change them, while Mallory is the enemy that sits on the path of the messages and can modify them at will.
1. Cipher text-only attack
In this attack Eve observes cipher texts and uses them to guess plaintext and the decryption key. Such attack e.g., would be possible on monoalphabetic cipher, that replaces each letter in the alphabet with another letter, according to a map. Because the frequency of the letters does not change with this cipher Eve can use frequency analysis to break it. How does Eve know that she succeeded? The assumption here is that the plaintext message is in a language Eve can recognize - thus she can differentiate a useful message from a garbage. What if the message were short?
2. Known-plaintext attack
In this attack Eve knows some portions of the plaintext. How does she know this? Perhaps messages are exchanged using a known protocol, that has predictable fields such as timestamp, a keyword chosen from a small set, etc. This way Eve can collect plaintext-cipher text pairs and use them to obtain the key if the cipher is simple.
3. Chosen-plaintext attack
In this attack Mallory can inject some messages to be encrypted and observe the output. Doing this she can generate plaintext-cipher text pairs at will answer use them to obtain the key.
4. Man-in-the-middle attack
In this attack Mallory sits on the path of the messages and can do whatever she likes with them. She can modify them, inject new messages, drop the existing ones or replay messages. MITM attacks are notoriously hard to detect and defend from.
5. Brute-force attack
In this attack Eve will simply try all possible keys until she finds the one that works. Again, there is an assumption that she can tell which one has worked because decryption of a cipher text will result in something intelligible in a chosen language. This attack can be made arbitrarily hard if the key is long. What if the decryption key depends on a password in some known way, how could Eve use this to break the encryption?
CONCLUSION
This article described how digital cryptography works. The reader must beware, however, that there are a number of ways to attack every one of these systems; cryptanalysis and attacks on cryptosystems, however, are well beyond the scope of this paper. In the words of Sherlock Holmes, "What one man can invent, another can discover"
There are a lot of topics that have been discussed above that will be big issues going forward in cryptography. As compute power increases, attackers can go after bigger keys and local devices can process more complex algorithms. Some of these issues include the size of public keys, the ability to forge public key certificates, which hash function(s) to use, and the trust that we will have in random number generators.
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