DES uses a 56-bit key with 64-bit blocks over 16 Feistel rounds — it's insecure because 2^56 keys can be brute-forced in hours with modern hardware.

DES parameters:

History:

• Developed by IBM, adopted as a US federal standard (FIPS) by the National Bureau of Standards (now NIST) in 1977
• IBM's original "Lucifer" design used a longer key; the published standard fixed it at 56 bits — a shortening widely attributed to NSA influence and controversial at the time
• In 1999, the EFF's "Deep Crack" machine (working with distributed.net) recovered a DES key in about 22 hours

Why 56 bits is too short:

• 2^56 = 7.2 × 10^16 possible keys
• Modern hardware (FPGAs, ASICs, cloud computing) can search this space quickly
• DES was officially withdrawn as a standard in 2005

The algorithm itself is well-designed — the weakness is purely the short key length. This is why 3DES was created as a stopgap.

Go deeper:

• Data Encryption Standard (Wikipedia) — the 56-bit key controversy, Deep Crack, and withdrawal.

False — RSA is highly vulnerable to side-channel attacks, particularly power analysis (SPA/DPA) during the square-and-multiply computation of $m^d \mod N$.

* Every 1-bit of the private exponent triggers an extra multiply, so its larger power/timing signature lets an attacker read d straight off the trace. *

Why RSA is vulnerable:

• Decryption/signing computes $m^d \mod N$ using SAM (Square-and-Multiply)
• For each bit of $d$: a "1" bit causes a MULTIPLY + SQUARE, a "0" bit causes only a SQUARE
• These operations have different power consumption and different timing
• An attacker monitoring power traces can literally read the private key $d$ bit by bit

Known side-channel attacks on RSA:

• Simple Power Analysis (SPA): Directly read the key from one power trace
• Differential Power Analysis (DPA): Statistical analysis over many traces
• Timing attacks: Different keys cause different computation times
• Electromagnetic emissions: EM radiation leaks key information

Countermeasures:

• Blinding: Randomize the computation using RSA's multiplicative property — multiply $m$ by a random $r^e$ before decryption, then divide by $r$ afterward
• Constant-time implementations: Ensure every bit of the key takes the same time
• Hardware shielding: EM and power filtering

Tip: The multiplicative property of RSA is both a weakness (malleability) AND a tool for defending against side channels (blinding). Context determines whether it's friend or foe.

Go deeper:

• Power analysis — SPA & DPA (Wikipedia) — reading a key straight from the power trace.
• Side-channel attack (Wikipedia) — the whole family: timing, power, EM, cache.
• Blinding (cryptography) (Wikipedia) — the randomisation countermeasure named in the tip.