Symmetric key encryption
Single shared key encryption and decryption.
Learning Objectives
- Define symmetric key cryptography.
- Explain the three main flaws of symmetric key encryption.
- Identify modern symmetric key algorithms (DES, AES).
- Understand the key-distribution problem.
Explanation
Symmetric key encryption uses the same shared key to encrypt and decrypt messages. It's fast but has flaws: the same key must be shared (and could be stolen), computers can break ancient ciphers quickly, and you'd need a separate key for every party you transact with.
In order to decipher (decrypt) encrypted messages, the receiver must know the secret cipher that was used to encrypt the plain text. This is called symmetric key cryptography or secret key cryptography: both the sender and the receiver use the same key to encrypt and decrypt the message. 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; in order to 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, where we are not all part of the same team, 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 — for about 177 million U.S. e-commerce customers, potentially 177² million different keys would be needed. Clearly this situation would be too unwieldy to work in practice.
Modern symmetric key systems use long binary keys to address the first flaw. The Data Encryption Standard (DES), developed by the NSA and IBM in the 1950s, uses a 56-bit encryption 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. However, the key-distribution problem remains — and is solved by public key cryptography and digital envelopes.
Key Points & Important Terms
Key Points
- •Symmetric key cryptography uses the same key for encrypt and decrypt.
- •Also called secret key cryptography; used in WWII and still in Internet crypto.
- •Three flaws: ancient ciphers breakable, key must be shared, n(n-1) keys needed.
- •DES uses 56-bit keys; Triple DES encrypts three times.
- •AES is the most widely used modern symmetric algorithm (128/192/256 bits).
- •Key-distribution problem is solved by public key cryptography and digital envelopes.
Important Terms
- Symmetric key cryptography
- Both the sender and the receiver use the same key to encrypt and decrypt the message (also called secret key cryptography).
- Data Encryption Standard (DES)
- Symmetric key algorithm developed by the NSA and IBM in the 1950s, using a 56-bit encryption key.
- Advanced Encryption Standard (AES)
- The most widely used symmetric key algorithm today, offering 128-, 192-, and 256-bit keys.
- Triple DES (TDEA)
- An improvement to DES that encrypts the message three times, each with a separate key.
- Key-distribution problem
- The challenge of securely sharing the same secret key between sender and receiver.