From cfbcb6ca6167f2f90ffdd34394050e48715d1dd5 Mon Sep 17 00:00:00 2001
From: Isis Lovecruft <isis@patternsinthevoid.net>
Date: Mon, 30 Dec 2019 22:05:01 +0000
Subject: [PATCH] Implement conversions between basepoint table sizes.

This is useful for programs/protocol which can do some heuristics or
learning-based approach towards optimising the table size based on the number of
uses of e.g. a public key, the second basepoint in a Pedersen commitment, etc.,
i.e. the first time a public key is used to verify a signature, the usual
variable-time basepoint multiscalar multiplication is used, however after 1000
verifications, the table size is upgraded, and again after 10000 verifications,
etc.
---
 src/edwards.rs | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/src/edwards.rs b/src/edwards.rs
index 40b2b5e..b64e4a3 100644
--- a/src/edwards.rs
+++ b/src/edwards.rs
@@ -911,6 +911,29 @@ impl_basepoint_table! {Name = EdwardsBasepointTableRadix256, LookupTable = Looku
 // it.
 pub type EdwardsBasepointTableRadix16 = EdwardsBasepointTable;
 
+macro_rules! impl_basepoint_table_conversions {
+    (LHS = $lhs:ty, RHS = $rhs:ty) => {
+        impl<'a> From<&'a $lhs> for $rhs {
+            fn from(table: &'a $lhs) -> $rhs {
+                <$rhs>::create(&table.basepoint())
+            }
+        }
+
+        impl<'a> From<&'a $rhs> for $lhs {
+            fn from(table: &'a $rhs) -> $lhs {
+                <$lhs>::create(&table.basepoint())
+            }
+        }
+    }
+}
+
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix16, RHS = EdwardsBasepointTableRadix64}
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix16, RHS = EdwardsBasepointTableRadix128}
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix16, RHS = EdwardsBasepointTableRadix256}
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix64, RHS = EdwardsBasepointTableRadix128}
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix64, RHS = EdwardsBasepointTableRadix256}
+impl_basepoint_table_conversions!{LHS = EdwardsBasepointTableRadix128, RHS = EdwardsBasepointTableRadix256}
+
 impl EdwardsPoint {
     /// Multiply by the cofactor: return \\([8]P\\).
     pub fn mul_by_cofactor(&self) -> EdwardsPoint {