diff --git a/quiche/src/crypto/boringssl.rs b/quiche/src/crypto/boringssl.rs
index 8eaed89d8a..447538fdef 100644
--- a/quiche/src/crypto/boringssl.rs
+++ b/quiche/src/crypto/boringssl.rs
@@ -1,5 +1,7 @@
 use super::*;
 
+use std::convert::TryFrom;
+
 use std::mem::MaybeUninit;
 
 use libc::c_int;
@@ -195,38 +197,54 @@ impl HeaderProtectionKey {
         }
     }
 
-    pub fn new_mask(&self, sample: &[u8]) -> Result<[u8; 5]> {
-        let mut new_mask = [0_u8; 5];
-
+    pub fn new_mask(&self, sample: &[u8]) -> Result<HeaderProtectionMask> {
         match self {
-            Self::Aes(aes_key) => unsafe {
-                AES_ecb_encrypt(
-                    sample.as_ptr(),
-                    new_mask.as_mut_ptr(),
-                    aes_key as _,
-                    1,
-                );
+            Self::Aes(aes_key) => {
+                let mut block = [0_u8; 16];
+
+                unsafe {
+                    AES_ecb_encrypt(
+                        sample.as_ptr(),
+                        block.as_mut_ptr(),
+                        aes_key as _,
+                        1,
+                    )
+                };
+
+                // Downsize the encrypted block to the size of the header
+                // protection mask.
+                //
+                // The length of the slice will always match the size of
+                // `HeaderProtectionMask` so the `unwrap()` is safe.
+                let new_mask =
+                    HeaderProtectionMask::try_from(&block[..HP_MASK_LEN])
+                        .unwrap();
+                Ok(new_mask)
             },
 
-            Self::ChaCha(key) => unsafe {
-                const PLAINTEXT: &[u8; 5] = &[0_u8; 5];
+            Self::ChaCha(key) => {
+                const PLAINTEXT: &[u8; HP_MASK_LEN] = &[0_u8; HP_MASK_LEN];
+
+                let mut new_mask = HeaderProtectionMask::default();
 
                 let counter = u32::from_le_bytes([
                     sample[0], sample[1], sample[2], sample[3],
                 ]);
 
-                CRYPTO_chacha_20(
-                    new_mask.as_mut_ptr(),
-                    PLAINTEXT.as_ptr(),
-                    PLAINTEXT.len(),
-                    key.as_ptr(),
-                    sample[std::mem::size_of::<u32>()..].as_ptr(),
-                    counter,
-                );
+                unsafe {
+                    CRYPTO_chacha_20(
+                        new_mask.as_mut_ptr(),
+                        PLAINTEXT.as_ptr(),
+                        PLAINTEXT.len(),
+                        key.as_ptr(),
+                        sample[std::mem::size_of::<u32>()..].as_ptr(),
+                        counter,
+                    );
+                };
+
+                Ok(new_mask)
             },
         }
-
-        Ok(new_mask)
     }
 }
 
diff --git a/quiche/src/crypto/mod.rs b/quiche/src/crypto/mod.rs
index 367215ae60..cbe56714e1 100644
--- a/quiche/src/crypto/mod.rs
+++ b/quiche/src/crypto/mod.rs
@@ -35,6 +35,9 @@ use crate::packet;
 // All the AEAD algorithms we support use 96-bit nonces.
 pub const MAX_NONCE_LEN: usize = 12;
 
+// Length of header protection mask.
+pub const HP_MASK_LEN: usize = 5;
+
 #[repr(C)]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Level {
@@ -120,6 +123,8 @@ struct EVP_MD {
     _unused: c_void,
 }
 
+type HeaderProtectionMask = [u8; HP_MASK_LEN];
+
 pub struct Open {
     alg: Algorithm,
 
diff --git a/quiche/src/crypto/openssl_quictls.rs b/quiche/src/crypto/openssl_quictls.rs
index 2888ecbbc6..c4bc5cb9c3 100644
--- a/quiche/src/crypto/openssl_quictls.rs
+++ b/quiche/src/crypto/openssl_quictls.rs
@@ -316,10 +316,10 @@ impl HeaderProtectionKey {
         })
     }
 
-    pub fn new_mask(&self, sample: &[u8]) -> Result<[u8; 5]> {
+    pub fn new_mask(&self, sample: &[u8]) -> Result<HeaderProtectionMask> {
         const PLAINTEXT: &[u8; 5] = &[0_u8; 5];
 
-        let mut new_mask = [0_u8; 5];
+        let mut new_mask = HeaderProtectionMask::default();
 
         // Set IV (i.e. the sample).
         let rc = unsafe {