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Cryptography refers to the study and practice of secure communication constructed by third parties with the intention of hiding information. Cryptography focuses on the construction and analyzation of protocols that help promote information security, such as authentication, confidentiality, data integrity, and non-repudiation. It incorporates a wide array of academic disciplines, including mathematics, computer science, and electrical engineering. Cryptographic technology exists everywhere in society, including the Internet, ATM machines, and claims processing centers.

In essence, cryptography keeps unwanted persons from reading sensitive data by converting readable information into an encrypted string of apparent nonsense. Decryption focuses on unraveling encoded information to a readable format. Cryptography-related technology has stirred controversy, especially among law enforcement. Encrypted communication can impede the investigation and interrogation process, which has prompted stern responses. Currently, law enforcement cannot intercept or monitor the communications of a suspected individual or persons without a court subpoena. Privacy advocates have challenged recent developments surrounding electronic surveillance by citing violations of the Fifth Amendment. Despite these efforts, widespread surveillance continues to expand with the intention of preserving national security.

**Classic Cryptography**

Before the technological age, cryptography was primarily concerned with the confidentiality of messages. Messages were converted from their original comprehensible form to an apparent garbled mess in order to preserve secrecy of the content within the message while still making it readable to its intended recipient. Earlier forms of cryptography involved rearranging or substituting letters and numbers in a message to discombobulate unwanted recipients. For example, the phrase "hello world" would translate to "ehlol owrdl" in a simple rearrangement format. The phrase "fly at once" would translate to "gmz bu podf" in a simple substitution scheme. The Caesar cipher, one of the earliest substitution ciphers in recorded history, focused on replacing a letter with another down the alphabet. It is reported that Julius Caesar commanded this type of encryption with a shift of three letters in order to communicate with his generals. The Spartan military used scytale transposition to communicate with one another. The Kama Sutra encourages encryption between lovers to keep their relationship discreet.

Steganography, the art of concealing a message, was also developed during ancient times. Modern forms of steganography include the use of digital watermarks, microdots, and invisible inks. Prisoners have been known to use steganography to communicate with other inmates. Cipher-texts reveal statistical information about the plain-text. Al-Kindi, an Arab mathematician, exposed this weakness through frequency analysis during the 9^{th} century. This made nearly all ciphers breakable by informed decoders. Al-Kindi wrote a manuscript that described the first cryptanalysis techniques. The frequency analysis technique increased vulnerability of all ciphers until the development of the poly-alphabetic cipher around 1467. Leon Battista Alberti is credited for the creation of the poly-alphabetic cipher, a substitution method using different ciphers in various parts of a message. He also created an automatic cipher device. The poly-alphabetic Vigenere cipher uses a key word to control letter substitution. The Vigenere cipher became vulnerable to Kasiski examination during the mid-19^{th} century. Frequency analysis still remains a powerful technique against many ciphers.

- Classical Encryption Techniques (PDF)
- Classical Cryptography (PDF)
- An Introduction to Classical Cryptography (PDF)
- Basic Concepts in Data Encryption: Classic Cryptography
- Introduction to Cryptography

**Modern Cryptography**

Modern encryption methods incorporate a series of algorithms that have a key to encrypt and decrypt data. These keys convert the messages into a digital form of apparent nonsense then return them back to their original form. The longer the key, the harder it is to crack the code. An attacker trying to decipher a long key would have to attempt every possible key, which will likely lead to a great deal of frustration. To put this into perspective, each bit of information has a value of zero or one. An 8-bit key would have 256 possible keys, whereas a 56-bit key would have 72 quadrillion. This makes it almost impossible to crack without modern technology. Modern technology has made it easier to decipher encrypted data; however, the quality of encryption has also advanced at the same pace as modern technology.

- Public Key Cryptography: What is It? (Video)
- Understanding Cryptography in Modern Military Communications
- Cryptography and State-of-the-Art Techniques (PDF)
- On the Foundations of Modern Cryptography (PDF)

**Symmetric-Key Cryptography**

Symmetric-key cryptanalysis involves encryption technology whereby the sender and recipient share the same key. The general public knew of no other type of encryption other than this one until June of 1976. Symmetric-key cryptography implements block ciphers or stream ciphers. A block cipher replaces the plain-text in blocks as opposed to individual characters. A stream cipher does the exact opposite by replacing the plain-text character-by-character. The United States government designated the Data Encryption Standard (DES) and the Advanced Encryption Standard (AES) as its core, block cipher encryption methods. The Data Encryption Standard (DES) remains in use across a wide range of civilian applications that promote e-mail privacy, ATM security, and secure remote access. Other forms of block ciphers have been designed; however, not quite as successful as DES and AES. RC4, a popular stream cipher, replaces the plain-text bit-by-bit.

Cryptographic hash functions convert a message of any length to a short, fixed hash. Hash functions make it difficult for attackers to find two messages that produce the same hash. MD4 and MD5 are both long-used hash functions. The United States National Security Agency (NSA) created the Secure Hash Algorithm Series that mirror MD5 hash functions. Message authentication codes (MACs) act like hash functions, except they incorporate a secret key to unlock the hash code through authentication upon receipt.

- Microsoft Support: Description of Symmetric and Asymmetric Encryption
- A Symmetric Key Cryptographic Algorithm (PDF)
- Symmetric-Key Cryptography
- HowStuffWorks: Symmetric-Key Cryptography
- Cornell University: Symmetric-Key Cryptography

**Public-Key Cryptography**

Public-key cryptography requires two separate keys, including one private and one public, to exchange encrypted information. Both keys are mathematically linked despite their differences. The public key is used to encrypt plain-text, such as a digital signature. The private key is used to decrypt cipher-texts or create a digital signature. Public-key cryptography is seen in many cryptosystems, applications, standards and protocols, such as PGP, GPG, and Transport Layer Security (TLS). A percentage of public-key algorithms provide key secrecy and distribution, digital signatures, and some provide both. All public-key algorithms are popular and well-trusted.

- Introduction to Public-Key Cryptography
- The Alternative History of Public-Key Cryptography
- Temple University: Introduction to Public-Key Cryptography
- Public Key Cryptography (PDF)

**Cryptanalysis**

Cryptanalysis refers to the act of analyzing information systems in order to discover hidden aspects of those systems. The application of cryptanalysis involves breaching security systems in order to gain access to encrypted data. This may be achieved even if the breacher does not know the key. The goal of cryptanalysis has always been the same; however, the methods of gaining access to encrypted data have changed over time to keep pace with ever-evolving cryptographic technology. Modern methods for breaking into cryptosystems involve pure mathematics, especially integer factorization.

- What's Cryptanalysis?
- Basic Cryptanalysis Modules
- Cryptanalysis and Attacks on Cryptosystems
- Basic Cryptanalysis Techniques (PDF)

**Cryptographic Primitives**

Cryptographic primitives, or algorithms containing basic cryptographic properties, are used to build computer security systems. The basis of cryptography surrounds cryptographic primitives and other problems related to it. In essence, cryptographic primitives are the building blocks for more complicated tools to solve cryptographic problems. They help build cryptosystems and cryptographic protocols. This guarantees high-level security for those wishing to protect their data.

- A Study of the Relationships Among Cryptographic Primitives
- Cryptographic Primitives (PDF)
- Cryptographic Mathematics for Dummies
- A Survey of Cryptographic Primitives and Implementations (PDF)

**Cryptosystems**

Cryptosystems consist of one or more cryptographic primitives used to create a complex algorithm. Cryptosystems provide a specific functionality while guaranteeing their security properties. Cryptosystems use the underlying cryptographic primitives to support the system's security properties. A sophisticated cryptosystem may consist of one or more primitive cryptosystems. A cryptosystem functions by providing space or time for communication between one or more parties. Some commonly known cryptosystems are Schnorr signature, El-Gamal encryption, PGP, RSA encryption, signcryption and electronic cash systems.

- Modern Cryptography: Secure Cryptosystems
- Cryptography As a Teaching Tool
- What is a Cryptosystem? (PDF)
- Simple Cryptosystems: Definition of Cryptosystem (PDF)

**Cryptography/Cryptology Tools**

Here is a list of some cryptography/cryptology tools related sites that provide information and help in how to learn and use cryptography.

- The CrypTool Portal
- Microsoft Cryptography Tools
- Kerberos Reference Page
- The GNU Crypto project
- MD5 Homepage (unofficial)

*Email Emma Kavanagh*