Block Cipher
DES is a block cipher.
It doesn’t encrypt your whole message at once. It chops it into fixed-size pieces called blocks.
Each block is exactly 64 bits (8 bytes).
Think of it like a machine that only processes one chunk at a time:
- Your message gets split into 64-bit pieces
- Each piece goes through the cipher separately
- The encrypted pieces are joined back together
Why 64 bits?
In the 1970s, 64 bits was a practical tradeoff:
- Small enough to process quickly on slow hardware
- Large enough to hide patterns within each block
Modern ciphers like AES use 128-bit blocks because computers are faster now.
The Key
The key is the secret that controls how the encryption scrambles your data.
- Same key encrypts
- Same key decrypts
- Lose the key, lose your data
The 56-bit Controversy
Here’s where it gets interesting.
The DES key is technically 64 bits long.
But only 56 bits are actually used for encryption.
What happened to the other 8 bits?
Parity Bits
Every 8th bit is a parity bit used for error checking, not encryption.
Each byte contributes 7 bits to the actual key.
The parity bits detect transmission errors. They add zero security.
Why 56 Bits Matters
56 bits means there are possible keys.
That’s about 72 quadrillion combinations:
Sounds impossible to crack, right?
The Math That Killed DES
In 1977, trying all keys would take centuries.
But computers get faster. Exponentially faster.
Moore’s Law roughly states that computing power doubles every two years.
What takes centuries today takes decades in 10 years, years in 20, days in 30…
56 bits bought about 20 years. Then it collapsed.
The Suspicion
IBM’s original Lucifer cipher used a 128-bit key.
The NSA made them cut it to 56 bits.
Some believe the NSA wanted a key size that was:
- Strong enough for businesses to trust
- Weak enough for NSA supercomputers to crack
We still don’t know for sure.
56 bits was the weakness that killed DES.
Today, 128 bits is the minimum. 256 is preferred.