net: add low-entropy, shared secret authentication protocol.

crypto/p224.h

Index: crypto/p224.h
diff --git a/crypto/p224.h b/crypto/p224.h
index 0b2aca90012b14dd42b7ad45f631bd594a4fa236..412ca9990583046b545e8d2b47b6f3186260e67a 100644
--- a/crypto/p224.h
+++ b/crypto/p224.h
@@ -36,6 +36,10 @@ struct CRYPTO_EXPORT Point {
   FieldElement x, y, z;
 };
 
+// kScalarBytes is the number of bytes needed to represent an element of the
+// P224 field.
+static const size_t kScalarBytes = 28;
+
 // ScalarMult computes *out = in*scalar where scalar is a 28-byte, big-endian
 // number.
 void CRYPTO_EXPORT ScalarMult(const Point& in, const uint8* scalar, Point* out);