Randomize signing contexts

Randomize signing contexts on creation if `rand-std` feature is enabled.

Author: tcharding

src/context.rs

@@ -191,16 +191,32 @@ mod alloc_only {
     }
 
     impl Secp256k1<All> {
-        /// Creates a new Secp256k1 context with all capabilities
+        /// Creates a new Secp256k1 context with all capabilities.
+        ///
+        /// If `rand-std` feature is enabled, context will have been randomized using `thread_rng`.
+        #[allow(unused_mut)]    // Unused when `rand-std` is not enabled.
         pub fn new() -> Secp256k1<All> {
-            Secp256k1::gen_new()
+            let mut ctx = Secp256k1::gen_new();
+            #[cfg(feature = "rand-std")]
+            {
+                ctx.randomize(&mut rand::thread_rng());
+            }
+            ctx
         }
     }
 
     impl Secp256k1<SignOnly> {
         /// Creates a new Secp256k1 context that can only be used for signing
+        ///
+        /// If `rand-std` feature is enabled, context will have been randomized using `thread_rng`.
+        #[allow(unused_mut)]    // Unused when `rand-std` is not enabled.
         pub fn signing_only() -> Secp256k1<SignOnly> {
-            Secp256k1::gen_new()
+            let mut ctx = Secp256k1::gen_new();
+            #[cfg(feature = "rand-std")]
+            {
+                ctx.randomize(&mut rand::thread_rng());
+            }
+            ctx
         }
     }