new exercise diffie-hellman

diffie-hellman.md

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+Alice and Bob use Diffie-Hellman key exchange to share secrets.  They start with prime numbers, pick private keys, generate and share public keys, and then generate a shared secret key.
+
+## Step 0
+
+The test program supplies prime numbers p and g.
+
+## Step 1
+
+Alice picks a private key, a, greater than 1 and less than p.  Bob does the same to pick a private key b.
+
+## Step 2
+
+Alice calcuates a public key A.
+
+    A = g**a mod p
+
+Using the same p and g, Bob similarly calculates a public key B from his private key b.
+
+## Step 3
+
+Alice and Bob exchange public keys.  Alice calculates secret key s.
+
+    s = B**a mod p
+
+Bob calculates
+
+    s = A**b mod p
+
+The calculations produce the same result!  Alice and Bob now share secret s.

diffie-hellman.yml

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+---
+blurb: "Diffie-Hellman key exchange."
+source: "Wikipedia, 1024 bit key from www.cryptopp.com/wiki."
+source_url: "http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange"