History
The Data Encryption Standard (DES) was developed in the early 1970s by IBM and later adopted by the U.S. National Institute of Standards and Technology (NIST) as a federal standard for encrypting non-classified information. The cipher was officially announced in 1977 and became widely used in various applications, including financial transactions, government communications, and secure data storage.
Key Historical Points:
- Development: IBM's research team, led by Horst Feistel, developed DES as part of a project known as Lucifer, which was eventually refined into DES.
- Standardization: DES was adopted by the National Institute of Standards and Technology (NIST) as FIPS PUB 46 in 1977.
- Security Concerns: In the late 1990s, advances in computing power led to concerns about DES's security. By 1998, DES was considered insecure against modern brute-force attacks, prompting the introduction of stronger encryption standards like AES (Advanced Encryption Standard).
Working
DES is a symmetric-key block cipher, meaning the same key is used for both encryption and decryption. It operates on fixed-size blocks of data and performs multiple rounds of processing to provide encryption.
Key Concepts:
- Block Cipher: DES encrypts data in fixed-size blocks of 64 bits.
- Symmetric Key: DES uses a single key of 56 bits to encrypt and decrypt data.
- Rounds: DES performs 16 rounds of complex transformations to ensure security.
Algorithm
1. Key Generation:
- Initial Key: DES uses a 64-bit key, but only 56 bits are used for encryption, with 8 bits used for parity checks.
- Key Schedule: The 56-bit key is divided into two 28-bit halves. These halves are rotated and permuted to generate 16 different subkeys, each used in one of the 16 rounds.
2. Initial Permutation (IP):
The 64-bit plaintext block is subjected to an initial permutation (IP) that rearranges the bits according to a fixed table.
3. Rounds:
- Splitting: The permuted block is divided into two 32-bit halves: the left half (L0) and the right half (R0).
- Feistel Function: Each round involves the Feistel function, which processes the right half and combines it with the left half using the round subkey.
- Expansion: The 32-bit right half is expanded to 48 bits.
- Subkey Mixing: The expanded block is XORed with the round subkey.
- Substitution: The result is substituted using S-boxes, which compress the data from 48 bits to 32 bits.
- Permutation: The substituted block is permuted to form the new right half for the next round.
- The halves are swapped at the end of each round.
4. Final Permutation (IP⁻¹):
After 16 rounds, the final permutation (IP⁻¹) is applied to the block to produce the ciphertext.
Example:
Given plaintext P and key K, the encryption process can be outlined as follows:
- Initial Permutation (IP): Apply the initial permutation to
P.
- Rounds: For each round
i (from 1 to 16):
- Split the block into
L and R.
- Compute the new
R as: Ri = Li-1 ⊕ F(Ri-1, Ki)
- Compute the new
L as: Li = Ri-1
- Swap
L and R.
- Final Permutation (IP⁻¹): Apply the final permutation to the result of the last round.
Decryption
Decryption of DES is simply the reverse of the encryption process:
- Initial Permutation (IP): Apply to the ciphertext.
- Rounds: Perform 16 rounds of Feistel function using the subkeys in reverse order.
- Final Permutation (IP⁻¹): Apply to the result of the last round to get back the original plaintext.
Summary
DES was a widely used symmetric-key algorithm providing strong security at its inception. However, due to its relatively short key length and susceptibility to brute-force attacks, it has largely been replaced by more secure algorithms like AES. Despite this, DES remains a foundational element in the study of cryptography.