Two other upstream crypto libraries are using zeroize to clear memory used for storing secrets (see https://docs.rs/ed25519-dalek/1.0.1/src/ed25519_dalek/secret.rs.html#43 and https://docs.rs/schnorrkel/0.9.1/src/schnorrkel/keys.rs.html#680), but libsecp256k1 apparently does not. This will normally not be a severe issue in substrate as this algorithm is not used much, but it might cause all secrets to memory leak in the Signer, which depends on sp-core and uses all 3 crypto algorithms to maximize future compatibility.

As discussed with @kirushik and @burdges in PM, so tagging them here as well.