diff --git a/aes/src/aes.rs b/aes/src/aes.rs
index b732e36..8fd2ff3 100644
--- a/aes/src/aes.rs
+++ b/aes/src/aes.rs
@@ -1,3 +1,5 @@
+use cipher_core::BlockCipher;
+
 use crate::key::Key;
 
 pub struct Aes {}
@@ -8,10 +10,17 @@ impl Aes {
     }
 }
 
-impl Aes {
+impl BlockCipher for Aes {
     const BLOCK_SIZE: usize = 16;
-
     fn from_key(key: &[u8]) -> Self {
         Self::new(key)
     }
+
+    fn transform_impl(
+        &self,
+        block: &[u8],
+        action: cipher_core::CipherAction,
+    ) -> cipher_core::CipherResult<cipher_core::Output> {
+        todo!()
+    }
 }
diff --git a/aes/src/block/secret_block.rs b/aes/src/block/secret_block.rs
index f70f0bb..eb3432f 100644
--- a/aes/src/block/secret_block.rs
+++ b/aes/src/block/secret_block.rs
@@ -121,9 +121,11 @@ macro_rules! secret_block {
         pub const fn as_u64(&self) -> u64 {
             self.0 as u64
         }
+
+        secret_block!(@conversions_as u128);
     };
     (@conversions_as u128) => {
-        /// Return value as u64
+        /// Return value as u129
         #[allow(dead_code)]
         #[inline]
         #[must_use]
@@ -175,7 +177,7 @@ macro_rules! secret_block {
         secret_block!(@conversions_from u32 $int);
     };
     (@conversions_from u128 $int:tt) => {
-        /// Create value from u64
+        /// Create value from u128
         #[allow(dead_code)]
         #[inline]
         #[must_use]
diff --git a/aes/src/key/mod.rs b/aes/src/key/mod.rs
index 148a4ca..b5aaac9 100644
--- a/aes/src/key/mod.rs
+++ b/aes/src/key/mod.rs
@@ -1,3 +1,7 @@
 mod aes_key;
+mod secret_key;
+mod subkey;
+mod subkeys;
 
+use crate::secret_key;
 pub use aes_key::Key;
diff --git a/aes/src/key/secret_key.rs b/aes/src/key/secret_key.rs
new file mode 100644
index 0000000..51b5f7f
--- /dev/null
+++ b/aes/src/key/secret_key.rs
@@ -0,0 +1,149 @@
+/// Macro to generate a masked, zeroizable integer wrapper type.
+///
+/// Usage:
+/// ```
+/// use des::secret_key;
+/// secret_key! {
+///     /// docs...
+///     pub struct Subkey(u64, 48, 0x0000_FFFF_FFFF_FFFFu64);
+/// }
+/// ```
+#[macro_export]
+macro_rules! secret_key {
+    (
+    $(#[$meta:meta])*
+    $vis:vis struct $name:ident ( $int:tt, $bits:expr, $mask:expr );
+) => {
+        $(#[$meta])*
+        #[derive(::zeroize::ZeroizeOnDrop, Default)]
+        $vis struct $name($int);
+
+        impl $name {
+            /// Mask to restrict the underlying integer to valid bits.
+            pub const MASK: $int = $mask;
+
+            secret_key!(@conversions_as $int);
+            secret_key!(@conversions_from $int $int);
+        }
+
+        impl ::std::fmt::Debug for $name {
+            fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
+                f.write_str(concat!(stringify!($name), "[REDACTED]"))
+            }
+        }
+
+        impl From<$int> for $name {
+            fn from(v: $int) -> Self {
+                Self(v & Self::MASK)
+            }
+        }
+    };
+    // Helper: generate conversions_as based on type
+    (@conversions_as u8) => {
+        /// Return value as u8
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn as_u8(&self) -> u8 {
+            self.0 as u8
+        }
+
+        secret_key!(@conversions_as u16);
+    };
+    (@conversions_as u16) => {
+        /// Return value as u16
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn as_u16(&self) -> u16 {
+            self.0 as u16
+        }
+
+        secret_key!(@conversions_as u32);
+    };
+    (@conversions_as u32) => {
+        /// Return value as u32
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn as_u32(&self) -> u32 {
+            self.0 as u32
+        }
+
+        secret_key!(@conversions_as u64);
+    };
+    (@conversions_as u64) => {
+        /// Return value as u64
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn as_u64(&self) -> u64 {
+            self.0 as u64
+        }
+
+        secret_key!(@conversions_as u128);
+    };
+    (@conversions_as u128) => {
+        /// Return value as u128
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn as_u128(&self) -> u128 {
+            self.0 as u128
+        }
+    };
+    // Helper: generate conversions_from based on type
+    (@conversions_from u8 $int:tt) => {
+        /// Create value from u8
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn from_u8(key: u8) -> Self {
+            Self(key as $int & Self::MASK)
+        }
+    };
+    (@conversions_from u16 $int:tt) => {
+        /// Create value from u16
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn from_u16(key: u16) -> Self {
+            Self(key as $int & Self::MASK)
+        }
+
+        secret_key!(@conversions_from u8 $int);
+    };
+    (@conversions_from u32 $int:tt) => {
+        /// Create value from u32
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn from_u32(key: u32) -> Self {
+            Self(key as $int & Self::MASK)
+        }
+
+        secret_key!(@conversions_from u16 $int);
+    };
+    (@conversions_from u64 $int:tt) => {
+        /// Create value from u64
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn from_u64(key: u64) -> Self {
+            Self(key as $int & Self::MASK)
+        }
+
+        secret_key!(@conversions_from u32 $int);
+    };
+    (@conversions_from u128 $int:tt) => {
+        /// Create value from u128
+        #[allow(dead_code)]
+        #[inline]
+        #[must_use]
+        pub const fn from_u128(key: u128) -> Self {
+            Self(key & Self::MASK)
+        }
+
+        secret_key!(@conversions_from u64 $int);
+    }
+}
diff --git a/aes/src/key/subkey.rs b/aes/src/key/subkey.rs
new file mode 100644
index 0000000..74c9a7a
--- /dev/null
+++ b/aes/src/key/subkey.rs
@@ -0,0 +1,13 @@
+use crate::key::secret_key;
+
+secret_key! {
+    /// A single AES round subkey
+    pub struct Subkey(u32, 32, 0xFFFF_FFFF);
+}
+
+impl Subkey {
+    /// Zero value.
+    pub const fn zero() -> Self {
+        Self(0)
+    }
+}
diff --git a/aes/src/key/subkeys.rs b/aes/src/key/subkeys.rs
new file mode 100644
index 0000000..7b29ca8
--- /dev/null
+++ b/aes/src/key/subkeys.rs
@@ -0,0 +1,132 @@
+use crate::key::{Key, subkey::Subkey};
+use std::{
+    fmt::Debug,
+    iter::Rev,
+    ops::Index,
+    slice::{Iter, IterMut},
+};
+
+// #[derive(Default)]
+pub struct Subkeys([Subkey; 44]);
+
+impl Subkeys {
+    /// Generates 44 round subkeys from the given key.
+    #[must_use]
+    pub fn from_key(key: &Key) -> Self {
+        todo!()
+    }
+
+    /// Returns an iterator over the subkeys.
+    pub fn iter(&self) -> Iter<'_, Subkey> {
+        self.0.iter()
+    }
+
+    /// Returns a reverse iterator over the subkeys.
+    pub fn iter_rev(&self) -> Rev<Iter<'_, Subkey>> {
+        self.0.iter().rev()
+    }
+
+    /// Returns a mutable iterator over the subkeys.
+    pub fn iter_mut(&mut self) -> IterMut<'_, Subkey> {
+        self.0.iter_mut()
+    }
+}
+
+impl<'a> IntoIterator for &'a Subkeys {
+    type Item = &'a Subkey;
+    type IntoIter = Iter<'a, Subkey>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a mut Subkeys {
+    type Item = &'a mut Subkey;
+    type IntoIter = IterMut<'a, Subkey>;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter_mut()
+    }
+}
+
+impl Index<usize> for Subkeys {
+    type Output = Subkey;
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.0[index]
+    }
+}
+
+impl Debug for Subkeys {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("Subkeys[REDACTED]")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_KEY: u128 = 0x0F1571C947D9E8591CB7ADD6AF7F6798;
+
+    impl PartialEq<[[u8; 4]; 44]> for Subkeys {
+        fn eq(&self, other: &[[u8; 4]; 44]) -> bool {
+            self.iter()
+                .zip(other)
+                .all(|(a, &b)| a.as_u32() == u32::from_be_bytes(b))
+        }
+    }
+
+    #[test]
+    fn from_key() {
+        let key = Key::from(TEST_KEY);
+        let subkeys = Subkeys::from_key(&key);
+        assert_eq!(
+            subkeys,
+            [
+                [0x0F, 0x15, 0x71, 0xC9],
+                [0x47, 0xD9, 0xE8, 0x59],
+                [0x1C, 0xB7, 0xAD, 0xD6],
+                [0xAF, 0x7F, 0x67, 0x98],
+                [0xDC, 0x90, 0x37, 0xB0],
+                [0x9B, 0x49, 0xDF, 0xE9],
+                [0x87, 0xFE, 0x72, 0x3F],
+                [0x28, 0x81, 0x15, 0xA7],
+                [0xD2, 0xC9, 0x6B, 0x84],
+                [0x49, 0x80, 0xB4, 0x6D],
+                [0xCE, 0x7E, 0xC6, 0x52],
+                [0xE6, 0xFF, 0xD3, 0xF5],
+                [0xC0, 0xAF, 0x8D, 0x0A],
+                [0x89, 0x2F, 0x39, 0x67],
+                [0x47, 0x51, 0xFF, 0x35],
+                [0xA1, 0xAE, 0x2C, 0xC0],
+                [0x2C, 0xDE, 0x37, 0x38],
+                [0xA5, 0xF1, 0x0E, 0x5F],
+                [0xE2, 0xA0, 0xF1, 0x6A],
+                [0x43, 0x0E, 0xDD, 0xAA],
+                [0x97, 0x1F, 0x9B, 0x22],
+                [0x32, 0xEE, 0x95, 0x7D],
+                [0xD0, 0x4E, 0x64, 0x17],
+                [0x93, 0x40, 0xB9, 0xBD],
+                [0xBE, 0x49, 0xE1, 0xFE],
+                [0x8C, 0xA7, 0x74, 0x83],
+                [0x5C, 0xE9, 0x10, 0x94],
+                [0xCF, 0xA9, 0xA9, 0x29],
+                [0x2D, 0x9A, 0x44, 0x74],
+                [0xA1, 0x3D, 0x30, 0xF7],
+                [0xFD, 0xD4, 0x20, 0x63],
+                [0x32, 0x7D, 0x89, 0x4A],
+                [0x52, 0x3D, 0x92, 0x57],
+                [0xF3, 0x00, 0xA2, 0xA0],
+                [0x0E, 0xD4, 0x82, 0xC3],
+                [0x3C, 0xA9, 0x0B, 0x89],
+                [0x9A, 0x16, 0x35, 0xBC],
+                [0x69, 0x16, 0x97, 0x1C],
+                [0x67, 0xC2, 0x15, 0xDF],
+                [0x5B, 0x6B, 0x1E, 0x56],
+                [0xD3, 0x64, 0x84, 0x85],
+                [0xBA, 0x72, 0x13, 0x99],
+                [0xDD, 0xB0, 0x06, 0x46],
+                [0x86, 0xDB, 0x18, 0x10],
+            ]
+        )
+    }
+}