Symmetric key encryption
Single shared key encryption and decryption.
5-mark Exam Answer
Symmetric key cryptography, also called secret key cryptography, is a method of encryption in which both the sender and the receiver use the same key to encrypt and decrypt the message. Laudon and Traver describe its strengths and three key flaws.
- 1.Same key for encrypt and decrypt
- 2.Flaw 1: ancient ciphers breakable by modern computers
- 3.Flaw 2: key must be shared over insecure medium
- 4.Flaw 3: n(n-1) keys needed for large populations
- 5.Modern algorithms: DES, Triple DES, AES
In symmetric key cryptography, both the sender and the receiver use the same key to encrypt and decrypt the message. The receiver must know the secret cipher used to encrypt the plain text. Symmetric key cryptography was used extensively throughout World War II and is still a part of Internet cryptography.
The possibilities for simple substitution and transposition ciphers are endless, but they all suffer from common flaws. First, in the digital age, computers are so powerful and fast that these ancient means of encryption can be broken quickly. Second, symmetric key cryptography requires that both parties share the same key; they must send the key over a presumably insecure medium where it could be stolen and used to decipher messages. If the secret key is lost or stolen, the entire encryption system fails.
Third, in commercial use, you would need a secret key for each of the parties with whom you transacted — one key for the bank, another for the department store, another for the government. In a population of millions of Internet users, this could result in as many as n(n-1) keys — potentially 177² million keys for 177 million U.S. e-commerce customers. This is too unwieldy to work in practice.
Modern symmetric key systems use long binary keys. The Data Encryption Standard (DES), developed by the NSA and IBM in the 1950s, uses a 56-bit key. Triple DES (TDEA) encrypts the message three times with separate keys. Today the most widely used symmetric key algorithm is the Advanced Encryption Standard (AES), which offers key sizes of 128, 192, and 256 bits.
If 177 million U.S. e-commerce customers each needed a unique symmetric key with every merchant, potentially 177² million keys would be needed — clearly too unwieldy. This key-distribution problem motivates public key cryptography and digital envelopes.
Symmetric key encryption is fast and widely used (DES, AES), but its key-distribution problem makes it impractical alone for large-scale e-commerce. It is typically combined with public key encryption through digital envelopes.
The exam interface follows the university paper pattern: Section A & B carry 5-mark questions; Section C carries objective questions.