Cryptography: An introduction

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AH authenticates the entire packet transmitted on the network whereas ESP only covers a portion of the packet transmitted on the network the higher layer data in transport mode and the entire original packet in tunnel mode. But the ramifications are significant. The third component of IPsec is the establishment of security associations and key management.

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These tasks can be accomplished in one of two ways. The simplest form of SA and key management is manual management. In this method, a security administer or other individual manually configures each system with the key and SA management data necessary for secure communication with other systems. Manual techniques are practical for small, reasonably static environments but they do not scale well.

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Several protocols have defined for these functions:. HMAC uses a shared secret key between two parties rather than public key methods for message authentication. In HMAC, both parties share a secret key. The secret key will be employed with the hash algorithm in a way that provides mutual authentication without transmitting the key on the line. IPsec key management procedures will be used to manage key exchange between the two parties. Recall that hash functions operate on a fixed-size block of input at one time; MD5 and SHA-1, for example, work on 64 byte blocks. These functions then generate a fixed-size hash value; MD5 and SHA-1, in particular, produce 16 byte bit and 20 byte bit output strings, respectively.

The client and server then agree upon an encryption scheme. SSL v2. SSL v3. In , SSL v3 was found to be breakable. In , the theoretical became practical when a CBC proof-of-concept exploit was released. Meanwhile, TLS v1. In , TLS v1. The client i. The communication between the client and server comprises the TLS protocol handshake Figure 18 , which has three phases, followed by actual data exchange.

Introduction to Cryptography in .NET | Pluralsight

The first phase of the protocol handshake is Key Exchange , used to establish the shared key and select the encryption method. This is the only phase of TLS communication that is not encrypted. During this phase:.

From this point forward, all communication is encrypted. The second step of the protocol handshake is the Server Parameters phase, where the server specifies other, additional handshake parameters.

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The server accomplishes this task by the use of two messages:. The third, and final phase, of the TLS protocol handshake is Authentication , during which the server is authenticated and, optionally, the client , keys are confirmed, and the integrity of the handshake assured. The messages exchanged during this phase include:. During this phase, the server sends its authentication messages followed by the client sending its authentication messages.

Once the Finished messages have been exchanged, the protocol handshake is complete, and the client and server now start to exchange encrypted Application Data. Most of us have used SSL to engage in a secure, private transaction with some vendor.

Lecture 2: Modular Arithmetic and Historical Ciphers by Christof Paar

The steps are something like this. During the SSL exchange with the vendor's secure server, the server sends its certificate to our client software. The certificate includes the vendor's public key and a validation of some sort from the CA that issued the vendor's certificate signed with the CA's private key. Our browser software is shipped with the major CAs' certificates containing their public keys; in that way, the client software can authenticate the server's certificate.

Note that the server generally does not use a certificate to authenticate the client. Instead, purchasers are generally authenticated when a credit card number is provided; the server checks to see if the card purchase will be authorized by the credit card company and, if so, considers us valid and authenticated! The reason that only the server is authenticated is rooted in history. SSL was developed to support e-commerce by providing a trust mechanism so that customers could have faith in a merchant.

In the real world, you "trust" a store because you can walk into a brick-and-mortar structure. The store doesn't know who the customer is; they check to see if the credit card is valid and, if so, a purchase goes through. In addition, how many people would have been willing to purchase an individual certificate and install it on their browser merely so that they shop online?

This latter requirement, if implemented, could have killed e-commerce before it ever got started. See E.

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For several decades, it had been illegal to generally export products from the U. By the lates, products using strong SKC has been approved for the worldwide financial community. As mentioned earlier, SSL was designed to provide application-independent transaction security for the Internet. DTLS v1. Known as Heartbleed , this vulnerability had apparently been introduced into OpenSSL in late with the introduction of a feature called heartbeat.

Heartbleed exploited an implementation flaw in order to exfiltrate keying material from an SSL server or some SSL clients, in what is known at reverse Heartbleed ; the flaw allowed an attacker to grab 64 KB blocks from RAM. Heartbleed is known to only affect OpenSSL v1. In addition, the OpenSSL 0. Note also that Heartbleed affects some versions of the Android operating system , notably v4.

But that wasn't the only problem with SSL. Weeks later, an SSL vulnerability in the bash Unix command shell was discovered, aptly named Shellshock. Here's a nice overview of the SSL problems! You might have read above that SSLv2 fell out of use by the early s and was formally deprecated in This is true.

In general, public key cryptography systems use hard-to-solve problems as the basis of the algorithm. The most predominant algorithm today for public key cryptography is RSA, based on the prime factors of very large integers. While RSA can be successfully attacked, the mathematics of the algorithm have not been compromised, per se; instead, computational brute-force has broken the keys. Elliptic curves combine number theory and algebraic geometry. These curves can be defined over any field of numbers i. An elliptic curve consists of the set of real numbers x,y that satisfies the equation:.

The set of all of the solutions to the equation forms the elliptic curve. Changing a and b changes the shape of the curve, and small changes in these parameters can result in major changes in the set of x,y solutions. Figure 19 shows the addition of two points on an elliptic curve. Elliptic curves have the interesting property that adding two points on the elliptic curve yields a third point on the curve.

Therefore, adding two points, P and Q, gets us to point R, also on the curve. Small changes in P or Q can cause a large change in the position of R. So let's go back to the original problem statement from above. An attacker might know P and Q but finding the integer, n , is a difficult problem to solve. RFC proposes a set of elliptic curve domain parameters over finite prime fields for use in these cryptographic applications and RFC proposes additional elliptic curves for use with OpenPGP.

ECC, however, is emerging as a replacement in some environments because it provides similar levels of security compared to RSA but with significantly reduced key sizes. Since the ECC key sizes are so much shorter than comparable RSA keys, the length of the public key and private key is much shorter in elliptic curve cryptosystems. This results into faster processing times, and lower demands on memory and bandwidth; some studies have found that ECC is faster than RSA for signing and decryption, but slower for signature verification and encryption.

In September of that year, they put out a formal Call for Algorithms and in August announced that 15 candidate algorithms were being considered Round 1. The remarkable thing about this entire process has been the openness as well as the international nature of the "competition. Their Overview of the AES Development Effort has full details of the process, algorithms, and comments so I will not repeat everything here. With the report came the recommendation that Rijndael be named as the AES standard. AES contains a subset of Rijndael's capabilities e. The day comment period ended on May 29, and the U.

Rijndael pronounced as in "rain doll" or "rhine dahl" is a block cipher designed by Joan Daemen and Vincent Rijmen, both cryptographers in Belgium.


Rijndael can operate over a variable-length block using variable-length keys; the specification submitted to NIST describes use of a , , or bit key to encrypt data blocks that are , , or bits long; note that all nine combinations of key length and block length are possible. The design of Rijndael was strongly influenced by the block cipher called Square , also designed by Daemen and Rijmen. Rijndael is an iterated block cipher, meaning that the initial input block and cipher key undergoes multiple rounds of transformation before producing the output.

Each intermediate cipher result is called a State. For ease of description, the block and cipher key are often represented as an array of columns where each array has 4 rows and each column represents a single byte 8 bits. An array representing a State will have Nb columns, where Nb values of 4, 6, and 8 correspond to a , , and bit block, respectively. Similarly, an array representing a Cipher Key will have Nk columns, where Nk values of 4, 6, and 8 correspond to a , , and bit key, respectively.