diff --git a/src/constants.rs b/src/constants.rs
new file mode 100644
index 0000000..91d65f0
--- /dev/null
+++ b/src/constants.rs
@@ -0,0 +1,3 @@
+pub const BLOCK_SIZE: usize = 16;
+pub const NONCE_SIZE: usize = 12;
+pub const TAG_SIZE: usize = 16;
diff --git a/src/ctr.rs b/src/ctr.rs
index 84b0268..1369d77 100644
--- a/src/ctr.rs
+++ b/src/ctr.rs
@@ -1,6 +1,6 @@
+use crate::constants::{BLOCK_SIZE, NONCE_SIZE};
 use crate::error::Error;
 use crate::types::{Bytes, Key, Nonce, Result};
-use aes::cipher::typenum::Unsigned;
 use aes::cipher::KeyInit;
 use aes::cipher::{InnerIvInit, StreamCipher, StreamCipherSeek};
 use aes::Aes256;
@@ -8,28 +8,24 @@ use aes::Aes256;
 use crate::types::CTRInitializer;
 pub struct Aes256Ctr32(ctr::Ctr32BE<Aes256>);
 
-const _BLOCK_SIZE: usize =
-    <Aes256 as aes::cipher::BlockSizeUser>::BlockSize::USIZE;
-const _NONCE_SIZE: usize = _BLOCK_SIZE - 4;
-
 impl Aes256Ctr32 {
     pub fn new(
         algo: Aes256,
         nonce: &Nonce,
         initializer: CTRInitializer,
     ) -> Result<Self> {
-        if !is_valid_nonce_size(nonce, _NONCE_SIZE) {
+        if !is_valid_nonce_size(nonce, NONCE_SIZE) {
             return Err(Error::InvalidNonceSize {
-                expected_size: _NONCE_SIZE,
+                expected_size: NONCE_SIZE,
             });
         }
-        let mut _nonce_block = [0u8; _BLOCK_SIZE];
-        _nonce_block[0.._NONCE_SIZE].copy_from_slice(nonce);
+        let mut _nonce_block = [0u8; BLOCK_SIZE];
+        _nonce_block[0..NONCE_SIZE].copy_from_slice(nonce);
 
         let mut ctr = ctr::Ctr32BE::from_core(
             ctr::CtrCore::inner_iv_init(algo, &_nonce_block.into()),
         );
-        ctr.seek(_BLOCK_SIZE * (initializer as usize));
+        ctr.seek(BLOCK_SIZE * (initializer as usize));
         Ok(Self(ctr))
     }
 
diff --git a/src/gcm.rs b/src/gcm.rs
index 293b148..c7b1b32 100644
--- a/src/gcm.rs
+++ b/src/gcm.rs
@@ -1,3 +1,4 @@
+use crate::constants::{NONCE_SIZE, TAG_SIZE};
 use crate::ctr::Aes256Ctr32;
 use crate::error::Error;
 use crate::types::{Bytes, Key, Nonce, Result};
@@ -7,9 +8,6 @@ use aes::Aes256;
 use ghash::universal_hash::UniversalHash;
 use ghash::GHash;
 
-pub const TAG_SIZE: usize = 16;
-pub const NONCE_SIZE: usize = 12;
-
 #[derive(Clone)]
 pub struct GcmGhash {
     ghash: GHash,
diff --git a/src/lib.rs b/src/lib.rs
index 20a5e63..0ccf9d7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,11 +1,13 @@
+pub mod constants;
 pub mod ctr;
 pub mod error;
 pub mod gcm;
 pub mod types;
 
+use crate::constants::TAG_SIZE;
 use ctr::Aes256Ctr32;
 use error::Error;
-use gcm::{setup as setup_gcm, GcmGhash, TAG_SIZE};
+use gcm::{setup as setup_gcm, GcmGhash};
 use subtle::ConstantTimeEq;
 use types::{Bytes, Key, Nonce, Result};