Obfuscate SharedSecret when printing

Currently printing the `SharedSecret` using `Display` or `Debug` prints
the real secret, this is sub-optimal. We have a solution for other
secrets in the project where printing is obfuscated and we provide a
`display_secret` method for explicitly printing.

Mirror the logic for other secrets and obfuscate the `SharedSecret` when printing.

src/ecdh.rs

@@ -16,12 +16,13 @@
 //! Support for shared secret computations
 //!
 
-use core::ptr;
+use core::{fmt, ptr};
 use core::ops::{FnMut, Deref};
 
 use key::{SecretKey, PublicKey};
 use ffi::{self, CPtr};
 use secp256k1_sys::types::{c_int, c_uchar, c_void};
+use to_hex;
 
 /// The buffer size (in bytes) used by `SharedSecret`.
 const BUF_SIZE: usize = 256;
@@ -48,7 +49,7 @@ pub struct SharedSecret {
     data: [u8; BUF_SIZE],
     len: usize,
 }
-impl_raw_debug!(SharedSecret);
+impl_display_secret!(SharedSecret);
 
 // This implements `From<N>` for all `[u8; N]` arrays from 128bits(16 byte) to 2048bits allowing known hash lengths.
 // Lower than 128 bits isn't resistant to collisions any more.
@@ -88,6 +89,11 @@ impl SharedSecret {
         debug_assert!(len <= self.data.len());
         self.len = len;
     }
+
+    /// Serializes the whole shared secret buffer as byte value.
+    pub(crate) fn serialize_secret(&self) -> [u8; BUF_SIZE] {
+        self.data
+    }
 }
 
 impl PartialEq for SharedSecret {
@@ -183,6 +189,68 @@ impl SharedSecret {
         y.copy_from_slice(&xy[32..]);
         hash_function(x, y)
     }
+
+    /// Formats the explicit byte value of the shared secret kept inside the type as a
+    /// little-endian hexadecimal string using the provided formatter.
+    ///
+    /// This is the only method that outputs the actual shared secret value, and, thus,
+    /// should be used with extreme precaution.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # #[cfg(all(feature = "std", not(feature = "bitcoin_hashes")))] {
+    /// # use std::str::FromStr;
+    /// # use secp256k1::{SecretKey, PublicKey};
+    /// use secp256k1::ecdh::SharedSecret;
+    /// # let pk = PublicKey::from_slice(&[3, 23, 183, 225, 206, 31, 159, 148, 195, 42, 67, 115, 146, 41, 248, 140, 11, 3, 51, 41, 111, 180, 110, 143, 114, 134, 88, 73, 198, 174, 52, 184, 78]).expect("hard coded slice should parse correctly");
+    /// # let sk = SecretKey::from_str("57f0148f94d13095cfda539d0da0d1541304b678d8b36e243980aab4e1b7cead").unwrap();
+    /// let secret = SharedSecret::new(&pk, &sk);
+    /// // Normal display hides the value.
+    /// assert_eq!(format!("{:?}", secret), "SharedSecret(#6b778f6da1cd7d46)");
+    /// // Here we explicitly display the secret value:
+    /// assert_eq!(
+    ///     format!("{}", secret.display_secret()),
+    ///     "cf05ae7da039ddce6d56dd57d3000c6dd91c6f1695eae47e05389f11e24670430000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000");
+    /// // Can explicitly display with `{}` and `{:?}`.
+    /// assert_eq!(
+    ///     format!("{:?}", secret.display_secret()),
+    ///     format!("DisplaySecret(\"{}\")", secret.display_secret()));
+    /// # }
+    /// ```
+    #[inline]
+    pub fn display_secret(&self) -> DisplaySecret {
+        DisplaySecret { secret: self.serialize_secret() }
+    }
+}
+
+/// Helper struct for safely printing a shared secret.
+///
+/// Formats the explicit byte value of the secret as a little-endian hexadecimal string using the
+/// provided formatter.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct DisplaySecret {
+    secret: [u8; BUF_SIZE]
+}
+
+impl fmt::Debug for DisplaySecret {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut slice = [0u8; BUF_SIZE * 2];
+        let hex = to_hex(&self.secret, &mut slice).expect("fixed-size hex serializer failed");
+        f.debug_tuple("DisplaySecret")
+            .field(&hex)
+            .finish()
+    }
+}
+
+impl fmt::Display for DisplaySecret {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        for i in &self.secret {
+            write!(f, "{:02x}", i)?;
+        }
+        Ok(())
+    }
 }
 
 #[cfg(test)]