diff --git a/src/crypto.zig b/src/crypto.zig
index c1500fd..5027da2 100644
--- a/src/crypto.zig
+++ b/src/crypto.zig
@@ -6,17 +6,55 @@
 //
 
 const AES = @import("crypto/aes.zig");
+const ED25519 = @import("crypto/ed25519.zig");
+const HKDF = @import("crypto/hkdf.zig");
+const HMAC = @import("crypto/hmac.zig");
+const SHA256 = @import("crypto/sha256.zig");
+const SHA512 = @import("crypto/sha512.zig");
+const X25519= @import("crypto/x25519.zig");
 
 
-///////////////// Public functions
+///////////////// Structs
 //
 
-pub fn CryptoEngine(comptime Provider: anytype) type {
-    return struct {
-        const Self = @This();
+pub const PartialEngine = struct {
+    aes_cbc256: ? AES.CBC256_Implementation = null,
+    ed25519:    ? ED25519.Implementation    = null,
+    hkdf:       ? HKDF.Implementation       = null,
+    hmac:       ? HMAC.Implementation       = null,
+    sha256:     ? SHA256.Implementation     = null,
+    sha512:     ? SHA512.Implementation     = null,
+    x25519:     ? X25519.Implementation     = null,
+};
 
-        pub const Aes = AES.AesEngine(Provider);
+pub const Engine = struct {
+    aes_cbc256: AES.CBC256_Implementation,
+    ed25519:    ED25519.Implementation,
+    hkdf:       HKDF.Implementation,
+    hmac:       HMAC.Implementation,
+    sha256:     SHA256.Implementation,
+    sha512:     SHA512.Implementation,
+    x25519:     X25519.Implementation,
+};
+
+
+///////////////// Functions
+//
+
+// Initializes the cyrpto engine with the functions given inside Provider
+// These functions must follow the same structure as the struct returned by this
+// function and its sub-structs.
+pub fn resolveEngine(comptime provider: PartialEngine) Engine {
+    return .{
+        .aes_cbc256 = provider.aes_cbc256 orelse AES.CBC256_DefaultImplementation,
+        .ed25519    = provider.ed25519    orelse ED25519.DefaultImplementation,
+        .hkdf       = provider.hkdf       orelse HKDF.DefaultImplementation,
+        .hmac       = provider.hmac       orelse HMAC.DefaultImplementation,
+        .sha256     = provider.sha256     orelse SHA256.DefaultImplementation,
+        .sha512     = provider.sha512     orelse SHA512.DefaultImplementation,
+        .x25519     = provider.x25519     orelse X25519.DefaultImplementation,
     };
+
 }
 
 
@@ -27,7 +65,7 @@ const std = @import("std");
 
 test "AES encryption defaults" {
     
-    const crypto = CryptoEngine();
+    const crypto = resolveEngine(.{});
 
     var data = [_]u8{ 1, 2, 3, 4 };
     const key = [_]u8{ 1, 2, 3, 4 };
diff --git a/src/crypto/aes.zig b/src/crypto/aes.zig
index 4facf45..ec77f0c 100644
--- a/src/crypto/aes.zig
+++ b/src/crypto/aes.zig
@@ -1,41 +1,23 @@
-// WARNING NOT FUNCTIONAL !!!
-// This is just a draft
-// TODO
+pub const CBC256_Implementation = struct {
 
-pub const DefaultEngine = struct {
-    pub fn encrypt(data: []u8) void {
+    pub const BLOCK_SIZE = 16;
+    pub const KEY_SIZE = 32;
+
+    // NOTE: data should be padded to make its length be a multiple of BLOCK_SIZE
+    encypt:  *const fn (key: *const [KEY_SIZE]u8, iv: *const [BLOCK_SIZE]u8, data: *const []u8, ) void,
+    decrypt: *const fn (key: *const [KEY_SIZE]u8, iv: *const [BLOCK_SIZE]u8, data: *const []u8, ) void,
+};
+
+// TODO
+pub const CBC265_DefaultImplementation = struct {
+    pub fn encrypt( key: *const [32]u8, iv: *const [16]u8, data: *const []u8 ) void {
+        _ = key;
+        _ = iv;
+        _ = data;
+    }
+    pub fn decrypt( key: *const [32]u8, iv: *const [16]u8, data: *const []u8 ) void {
+        _ = key;
+        _ = iv;
         _ = data;
     }
 };
-
-pub fn AesEngine(comptime Provider: anytype) type {
-
-    _ = Provider;
-
-    return struct {
-        const Self = @This();
-
-        pub const CBC_265 = struct {
-            const key_length = 256;
-
-            // Encrypts the data block using AES 256 CBC
-            pub fn encrypt(data: []u8, key: [key_length]u8, iv: []u8) void {
-                _ = data;
-                _ = key;
-                _ = iv;
-            }
-        };
-
-        pub const CBC_128 = struct {
-            const key_length = 128;
-
-            // Encrypts the data block using AES 128 CBC
-            pub fn encrypt(data: []u8, key: [key_length]u8, iv: []u8) void {
-                _ = data;
-                _ = key;
-                _ = iv;
-            }
-        };
-
-    };
-}
diff --git a/src/crypto/ed25519.zig b/src/crypto/ed25519.zig
index e69de29..5ab0ca1 100644
--- a/src/crypto/ed25519.zig
+++ b/src/crypto/ed25519.zig
@@ -0,0 +1,21 @@
+pub const Implementation = struct {
+    pub const KEY_SIZE = 32;
+    pub const SIGNATURE_SIZE = 64;
+    sign: *const fn (key: *const [KEY_SIZE]u8, data: *const []const u8, signature_out: *const [SIGNATURE_SIZE]u8) void,
+    verify: *const fn (key: *const [KEY_SIZE]u8, data: *const []const u8, signature: *const [SIGNATURE_SIZE]u8) void,
+};
+
+// TODO
+pub const defaultImplementation = struct {
+    fn sign(key: *const [32]u8, data: *const []const u8, signature_out: *const [64]u8) void {
+        _ = key;
+        _ = data;
+        _ = signature_out;
+        
+    }
+    fn verify(signature: *const [64]u8, data: *const []const u8) void {
+        _ = signature;
+        _ = data;
+    }
+    
+};
diff --git a/src/crypto/hkdf.zig b/src/crypto/hkdf.zig
index e69de29..5344700 100644
--- a/src/crypto/hkdf.zig
+++ b/src/crypto/hkdf.zig
@@ -0,0 +1,7 @@
+pub const Implementation = struct {
+    
+};
+
+pub const defaultImplementation = struct {
+    
+};
diff --git a/src/crypto/hmac.zig b/src/crypto/hmac.zig
index e69de29..5344700 100644
--- a/src/crypto/hmac.zig
+++ b/src/crypto/hmac.zig
@@ -0,0 +1,7 @@
+pub const Implementation = struct {
+    
+};
+
+pub const defaultImplementation = struct {
+    
+};
diff --git a/src/crypto/sha256.zig b/src/crypto/sha256.zig
index e69de29..5344700 100644
--- a/src/crypto/sha256.zig
+++ b/src/crypto/sha256.zig
@@ -0,0 +1,7 @@
+pub const Implementation = struct {
+    
+};
+
+pub const defaultImplementation = struct {
+    
+};
diff --git a/src/crypto/sha512.zig b/src/crypto/sha512.zig
index e69de29..5344700 100644
--- a/src/crypto/sha512.zig
+++ b/src/crypto/sha512.zig
@@ -0,0 +1,7 @@
+pub const Implementation = struct {
+    
+};
+
+pub const defaultImplementation = struct {
+    
+};
diff --git a/src/crypto/x25519.zig b/src/crypto/x25519.zig
index e69de29..5344700 100644
--- a/src/crypto/x25519.zig
+++ b/src/crypto/x25519.zig
@@ -0,0 +1,7 @@
+pub const Implementation = struct {
+    
+};
+
+pub const defaultImplementation = struct {
+    
+};
diff --git a/src/root.zig b/src/root.zig
index 95a57a1..9ab6784 100644
--- a/src/root.zig
+++ b/src/root.zig
@@ -1,2 +1,13 @@
-pub const packet = @import("packet.zig");
-// pub const crypto = @import("crypto.zig");
+const crypto = @import("crypto.zig");
+
+// This function is used to initialize the library
+// You can pass your own cryptographic functions to take advantage of your
+//    platform or use the library's software implementations for compatibility
+// Use its return value to access the library modules
+pub fn RnsZero(comptime cryptoProvider: crypto.PartialEngine) type {
+    const cryptoEngine = crypto.resolveEngine(cryptoProvider);
+    _ = cryptoEngine;
+    return struct {
+        pub const packet = @import("packet.zig");
+    };
+}