User talk:Sandy Harris/Sandbox

Principles of cryptosystems

In encryption and decryption, a key is one or more unique values used by an encryption or decryption algorithm. Encryption algorithms take as input a key and plaintext, producing ciphertext output. For decryption, the process is reversed to turn ciphertext into plaintext. In a secret key system, decryption requires the same key as was used for encryption. In a public key system, the two keys are different.

The ciphertext produced by an encryption algorithm should bear no resemblance to the original message. Ideally, it should be indistinguishable from a random string of symbols.

The system should also be secure against an attacker who knows all its details except the key; this is known as Kerckhoffs' Principle.

Methods of defeating cryptosystems have a long history and an extensive literature; see cryptanalysis. Anyone designing or deploying a cyptosystem must take cryptanalytic results into account.

Secret key systems

Until the 1970s, all (publicly known) cryptosystems used secret key or symmetric key cryptography methods. In such a system, there is only one secret key for a message; that key can be used either to encrypt or decrypt the message. Both the sender and receiver must have the key, and third parties (potential intruders) must be prevented from obtaining the key. Symmetric key encryption may also be called traditional, shared-secret, secret-key, or conventional encryption.

Historically, ciphers worked at the level of letters; see history of cryptography for details. Attacks on them used techniques based largely on linguistic analysis, such as frequency counting; see cryptanalysis.

On computers, there are two main types of symmetric encryption algorithm:

A block cipher breaks the data up into fixed-size blocks and encrypt each block under control of the key. Since the message length will rarely be an integer number of blocks, there will usually need to be some form of "padding" to make the final block long enough. The block cipher itself defines how a single block is encrypted; modes of operation specify how these operations are combined to achieve some larger goal.

A stream cipher encrypts a stream of input data by combining it with a pseudo-random stream of data; the pseudo-random stream is generated under control of the encryption key.

A one-time pad works much like a stream cipher, but it does not need to generate a pseudo-random stream because its key is a truly random stream as long as the message. This is the only known cipher which is provably secure, but it is impractical for most applications because managing such keys is too difficult.

More generally, key management is a problem for any secret key system. It is critically important to protect keys from unauthorised access; if an enemy obtains the key, then he or she can read all messages ever sent with that key. It is also necessary to change keys periodically, both to limit the damage if an attacker does get a key and to prevent various attacks which become possible if the enemy can collect a large sample of data encrypted with a single key. On the other hand, it is necessary to communicate the keys; without a copy of the identical key, the intended receiver cannot decrypt the message.

One problem is where, and how, to safely store the key. In a manual system, you need a key that is long and hard to guess because keys that are short or guessable provide little security. However, such keys are hard to remember and if the user writes them down, then you have to worry about someone looking over his shoulder, or breaking in a copying the key, the writing making an impression on the next page of a pad, and so on.

On a computer, keys must be protected so that enemies cannot obtain them. Simply storing the key unencrypted in a file or database is a poor strategy. A better method is to encrypt the key and store it in a file that is protected by the file system; this way, only authorized users of the system should be able to read the file. But then, where should one store the key used to encrypt the secret key? It becomes a recursive problem. Also, what about an attacker that can defeat the file system protection? If the key is stored encrypted but you have a program that decrypts and uses it, can an attacker obtain the key via a memory dump or a debugging tool? If a network is involved, can an attacker get keys by intercepting network packets? Can an attacker put a keystroke logger on the machine; if so, he can get everything you type, possibly including keys or passwords.

Communicating keys is an even harder problem. With secret key encryption alone, it would not be possible to open up a new secure connection on the internet, because there would be no safe way initially to transmit the shared key to the other end of the connection without intruders being able to intercept it.

Public key systems

The second method is called public key or asymmetric key cryptography; in this, keys are created in pairs, such that when one is used to encrypt, the other must be used to decrypt. One key is the private key; this is kept secret from everyone. The other is the public key; this can be published anywhere; on the net, in the phonebook, on business cards.

Practical use of asymmetric cryptography, on any sizable basis, requires a public key infrastructure (PKI). A public key will normally be embedded in a digital certificate that is issued by a certification authority. In the event of compromise of the private key, the certification authority can revoke the key by adding it to a certificate revocation list. Digital certificates, like passports or other identification documents, usually have expiration dates, and a means of verifying both the validity of the certificate and of the certificate issuer.

Public key exchanges are used to open up secure secret key channels between strangers across the internet.

In contrast with symmetric encryption, asymmetric encryption, a user has their computer produce two different keys, related mathematically so that data encrypted with one can only be decrypted with the other. One key is the public key and can be published. The other is the private key and is kept secret, never leaving the user's computer. It was first proposed, in public, in 1976 by Whitfield Diffie and Martin Hellman..

A public key cryptography system is constructed so that calculation of the private key is computationally infeasible from knowledge of the public key, even though they are necessarily related. Instead, both keys are generated secretly, as an interrelated pair. The historian David Kahn described public-key cryptography as "the most revolutionary new concept in the field since polyalphabetic substitution emerged in the Renaissance".

When used for content confidentiality, the public key is typically used for encryption, while the private or secret key is used for decryption. There needs to be a separate public-private pair in each direction of communication. The early asymmetric techniques were vulnerable to some forms of cryptanalysis, until Diffie and Hellman showed that public-key cryptography was practucak by presenting the Diffie-Hellman key exchange protocol. In 1978, Ronald Rivest, Adi Shamir, and Len Adleman invented Rivest-Shamir-Adleman (RSA), another public-key system Previously released as an MIT "Technical Memo" in April 1977, and published in Martin Gardner's Scientific American "Mathematical Recreations" column</ref>. In 1997, it finally became publicly known that asymmetric cryptography had been invented by James H. Ellis at GCHQ, a British intelligence organization, in the early 1970s, and that both the Diffie-Hellman and RSA algorithms had been previously developed (by Malcolm J. Williamson and Clifford Cocks, respectively).

Hybrid cryptosystems

Generating session keys

The primary usage of public-key encryption is in hybrid systems where a symmetric algorithm does the bulk data encryption while the public key algorithm provides other services. Public-key encryption is slower than conventional symmetric encryption.For example, in Pretty Good Privacy (PGP) email encryption the sender generates a random key for the symmetric bulk encryption and uses public key techniques to securely deliver it to the receiver. In the Diffie-Hellman key agreement protocol, used in IPsec and other systems, public key techniques provide authentication.

An advantage of asymmetric over symmetric cryptosystems is that all symmetric system keys must be kept secret, and the logistics of key management become complex. Each distinct pair of communicating parties must share a different key. The number of keys required increases as the square of the number of network members, which requires very complex key management schemes in large networks. The difficulty of establishing a secret key between two communicating parties when a secure channel doesn't already exist between them also presents a chicken-and-egg problem which is a considerably practical obstacle for cryptography users in the real world.

Digital signatures

In addition to encryption, public-key cryptography can be used to implement digital signature schemes. A digital signature is somewhat like an ordinary signature; they have the characteristic that they are easy for a user to produce, but difficult for anyone else to forge. Digital signatures can also be permanently tied to the content of the message being signed; they cannot be 'moved' from one document to another, for any attempt will be detectable. In digital signature schemes, there are two algorithms: one for signing, in which a secret key is used to process the message (or a hash of the message or both), and one for verification, in which the matching public key is used with the message to check the validity of the signature. RSA and DSA are two of the most popular digital signature schemes. Digital signatures are central to the operation of public key infrastructures and to many network security schemes (Transport Layer Security (TLS), also Secure Sockets Layer (SSL), many VPNs, etc).

One-way encryption

There are a substantial number of applications where it is not necessary to be able to reconstruct the plaintext from the ciphertext, but merely to be able to prove that some piece of information could be generated only from the original plaintext. See one-way encryption for the techniques; some applications are presented here.

Protecting stored passwords

When passwords are stored on a computer, it is essential that they be kept secret. Thus it is recommended practice to encrypt the passwords before writing them to disk, and furthermore to prevent anyone who might find them from decrypting them. One-way encryption involves storing an encrypted string which cannot be decrypted. When a user later enters their password, the newly enter password is first encrypted, and then is compared to the encrypted stored string.

The password is usually encrypted as a message digest or hash, a large number generated by scrambling and condensing plain text letters. An example of a hash digest is SHA-1, which dates from 1994. The SHA-1 algorithm takes a string as input. The algorithm is a digest because the result is a fixed-size number. The SHA-1 algorithm always outputs a 160-bit number (20 bytes of storage). 48 decimal digits would be required to express this number, and it is usually displayed to humans as a 28-character, base-64 encoded string. Here are some examples:

Hello World   z7R8yBtZz0+eqead7UEYzPvVFjw=
VB            L1SHP0uzuGbMUpT4z0zlAdEzfPE=
vb            eOcnhoZRmuoC/Ed5iRrW7IxlCDw=
Vb            e3PaiF6tMmhPGUfGg1nrfdV3I+I=
vB            gzt6my3YIrzJiTiucvqBTgM6LtM=