When The NSA Made Crypto Better

In 2013, Edward Snowden outlined how the NSA had pushed a specific elliptic curve method as a standard — Dual_EC_DRBG (Dual Elliptic Curve Deterministic Random Bit Generator) — and which they knew that they could crack. There is thus always a challenge for law enforcement agencies in creating backdoors in cryptography, in that they will provide them with an advantage over their adversaries. But, there is a case from the 1990s of the NSA actually improving a cryptography method, in order to stop it from being vulnerable.

Differential cryptanalysis method

A differential attack on a block cipher is where we analyse the change between one plaintext value and another, and the change that it makes on the output ciphers. In many cases we change one bit in the input, and observe one bit change on the input and observe the change in the output. A well designed cipher will cause an average of 50% of the bits to change. If the result is non-random, it gives an attacker an advantage in cracking the block cipher.

The differential cryptanalysis method was created in the 1990s and where it was possible to change a single bit in plaintext (P and P’) and then observe the change in the output ciphertext (C and C’):