Add definitions for keys and signature

include/relay/CryptoPrimitives.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <stdint.h>
+
+#pragma pack(push, 1)
+
+/**
+ * @brief A private key for asymmetric cryptography
+ */
+typedef struct {
+
+    /// @brief The size of a private key
+    static const int size = 32;
+
+    uint8_t bytes[size];
+
+} PrivateKey;
+
+/**
+ * @brief A public key for asymmetric cryptography
+ */
+typedef struct {
+
+    /// @brief The size of a public key
+    static const int size = 32;
+
+    uint8_t bytes[size];
+
+} PublicKey;
+
+/**
+ * @brief A signature of some data using a private key
+ */
+typedef struct {
+
+    /// @brief The size of a message signature
+    static const int size = 64;
+
+    uint8_t bytes[size];
+
+} Signature;
+
+#pragma pack(pop)