diff --git a/cipher-core/src/error.rs b/cipher-core/src/error.rs
new file mode 100644
index 0000000..5819437
--- /dev/null
+++ b/cipher-core/src/error.rs
@@ -0,0 +1,87 @@
+use thiserror::Error;
+
+#[derive(Debug, Error, Clone, Copy, PartialEq, Eq)]
+pub enum CryptoError {
+    /// Invalid key size for the cipher
+    #[error("invalid key size: expected {expected} bytes, got {actual}")]
+    InvalidKeySize { expected: usize, actual: usize },
+
+    /// Input data doesn't match the cipher's block size
+    #[error("invalid block size: expected {expected} bytes, got {actual}")]
+    InvalidBlockSize { expected: usize, actual: usize },
+
+    /// Decryption detected invalid padding
+    #[error("invalid padding detected during decryption")]
+    InvalidPadding,
+
+    /// Input length not valid for unpadded operation
+    #[error("invalid plaintext length: {actual} bytes (must be multiple of block size)")]
+    InvalidPlaintextLength { actual: usize },
+
+    /// General size validation failure
+    #[error("size mismatch: expected {expected} bytes, got {actual}")]
+    InvalidSize { expected: usize, actual: usize },
+}
+
+impl CryptoError {
+    /// Creates a key size error
+    pub fn invalid_key_size(expected: usize, actual: usize) -> Self {
+        CryptoError::InvalidKeySize { expected, actual }
+    }
+
+    /// Creates a block size error  
+    pub fn invalid_block_size(expected: usize, actual: usize) -> Self {
+        CryptoError::InvalidBlockSize { expected, actual }
+    }
+
+    /// Creates an invalid padding error
+    pub fn invalid_padding() -> Self {
+        CryptoError::InvalidPadding
+    }
+
+    /// Creates a plaintext length error
+    pub fn invalid_plaintext_length(actual: usize) -> Self {
+        CryptoError::InvalidPlaintextLength { actual }
+    }
+
+    /// Returns true if this is a key size error
+    pub fn is_key_error(&self) -> bool {
+        matches!(self, CryptoError::InvalidKeySize { .. })
+    }
+
+    /// Returns true if this is a block size error
+    pub fn is_block_error(&self) -> bool {
+        matches!(self, CryptoError::InvalidBlockSize { .. })
+    }
+
+    /// Returns true if this is a size-related error
+    pub fn is_size_error(&self) -> bool {
+        self.is_key_error()
+            || self.is_block_error()
+            || matches!(self, CryptoError::InvalidSize { .. })
+    }
+
+    /// Returns the expected size for size-related errors
+    pub fn expected_size(&self) -> Option<usize> {
+        match self {
+            CryptoError::InvalidKeySize { expected, .. } => Some(*expected),
+            CryptoError::InvalidBlockSize { expected, .. } => Some(*expected),
+            CryptoError::InvalidSize { expected, .. } => Some(*expected),
+            _ => None,
+        }
+    }
+
+    /// Returns the actual size for size-related errors
+    pub fn actual_size(&self) -> Option<usize> {
+        match self {
+            CryptoError::InvalidKeySize { actual, .. } => Some(*actual),
+            CryptoError::InvalidBlockSize { actual, .. } => Some(*actual),
+            CryptoError::InvalidSize { actual, .. } => Some(*actual),
+            CryptoError::InvalidPlaintextLength { actual } => Some(*actual),
+            _ => None,
+        }
+    }
+}
+
+/// Type alias for clean Result types
+pub type CryptoResult<T> = core::result::Result<T, CryptoError>;
diff --git a/cipher-core/src/lib.rs b/cipher-core/src/lib.rs
index b93cf3f..d102e1b 100644
--- a/cipher-core/src/lib.rs
+++ b/cipher-core/src/lib.rs
@@ -1,14 +1,3 @@
-pub fn add(left: u64, right: u64) -> u64 {
-    left + right
-}
+mod error;
 
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
-    }
-}
+pub use error::{CryptoError, CryptoResult};