diff --git a/src/aead/chacha20_poly1305/mod.rs b/src/aead/chacha20_poly1305/mod.rs
index b2506d46a..c45de4b0a 100644
--- a/src/aead/chacha20_poly1305/mod.rs
+++ b/src/aead/chacha20_poly1305/mod.rs
@@ -73,8 +73,7 @@ pub(super) fn seal_fallback(
     in_out: &mut [u8],
     cpu_features: cpu::Features,
 ) -> Result<Tag, InputTooLongError> {
-    check_input_lengths(aad, in_out)?;
-    let (counter, poly1305_key) = begin(chacha20_key, nonce);
+    let (counter, poly1305_key) = begin(chacha20_key, nonce, aad, in_out)?;
     let mut auth = poly1305::Context::from_key(poly1305_key, cpu_features);
 
     poly1305_update_padded_16(&mut auth, aad.as_ref());
@@ -108,8 +107,7 @@ pub(super) fn open_fallback(
     in_out: Overlapping<'_>,
     cpu_features: cpu::Features,
 ) -> Result<Tag, InputTooLongError> {
-    check_input_lengths(aad, in_out.input())?;
-    let (counter, poly1305_key) = begin(chacha20_key, nonce);
+    let (counter, poly1305_key) = begin(chacha20_key, nonce, aad, in_out.input())?;
     let mut auth = poly1305::Context::from_key(poly1305_key, cpu_features);
 
     poly1305_update_padded_16(&mut auth, aad.as_ref());
@@ -134,11 +132,18 @@ fn check_input_lengths(aad: Aad<&[u8]>, input: &[u8]) -> Result<(), InputTooLong
 }
 
 // Also used by chacha20_poly1305_openssh.
-pub(super) fn begin(key: &chacha::Key, nonce: Nonce) -> (Counter, poly1305::Key) {
+pub(super) fn begin(
+    key: &chacha::Key,
+    nonce: Nonce,
+    aad: Aad<&[u8]>,
+    input: &[u8],
+) -> Result<(Counter, poly1305::Key), InputTooLongError> {
+    check_input_lengths(aad, input)?;
+
     let mut key_bytes = [0u8; poly1305::KEY_LEN];
     let counter = key.encrypt_single_block_with_ctr_0(nonce, &mut key_bytes);
     let poly1305_key = poly1305::Key::new(key_bytes);
-    (counter, poly1305_key)
+    Ok((counter, poly1305_key))
 }
 
 fn finish(mut auth: poly1305::Context, aad_len: usize, in_out_len: usize) -> Tag {
diff --git a/src/aead/chacha20_poly1305_openssh.rs b/src/aead/chacha20_poly1305_openssh.rs
index f1993b053..cc6a19237 100644
--- a/src/aead/chacha20_poly1305_openssh.rs
+++ b/src/aead/chacha20_poly1305_openssh.rs
@@ -31,9 +31,13 @@
 
 use super::{
     chacha::{self, *},
-    chacha20_poly1305, cpu, poly1305, Nonce, Tag,
+    chacha20_poly1305, cpu, poly1305, Aad, Nonce, Tag,
+};
+use crate::{
+    constant_time,
+    error::{self, InputTooLongError},
+    polyfill::slice,
 };
-use crate::{constant_time, error, polyfill::slice};
 
 /// A key for sealing packets.
 pub struct SealingKey {
@@ -59,6 +63,10 @@ impl SealingKey {
     /// # Panics
     ///
     /// Panics if `plaintext_in_ciphertext_out.len() < PACKET_LENGTH_LEN`.
+    ///
+    /// Panics if `plaintext_in_ciphertext_out` is longer than the maximum
+    /// input size for ChaCha20-Poly1305. Note that this limit is much,
+    /// much larger than SSH's 256KB maximum record size.
     pub fn seal_in_place(
         &self,
         sequence_number: u32,
@@ -70,8 +78,15 @@ impl SealingKey {
             slice::split_first_chunk_mut(plaintext_in_ciphertext_out).unwrap();
 
         let cpu_features = cpu::features();
-        let (counter, poly_key) =
-            chacha20_poly1305::begin(&self.key.k_2, make_nonce(sequence_number));
+        // XXX/TODO(SemVer): Refactor API to return an error.
+        let (counter, poly_key) = chacha20_poly1305::begin(
+            &self.key.k_2,
+            make_nonce(sequence_number),
+            Aad::from(len_in_out),
+            data_and_padding_in_out,
+        )
+        .map_err(error::erase::<InputTooLongError>)
+        .unwrap();
 
         let _: Counter = self
             .key
@@ -131,13 +146,17 @@ impl OpeningKey {
         ciphertext_in_plaintext_out: &'a mut [u8],
         tag: &[u8; TAG_LEN],
     ) -> Result<&'a [u8], error::Unspecified> {
-        if ciphertext_in_plaintext_out.len() < PACKET_LENGTH_LEN {
-            return Err(error::Unspecified);
-        }
+        let (packet_length, after_packet_length): (&mut [u8; PACKET_LENGTH_LEN], _) =
+            slice::split_first_chunk_mut(ciphertext_in_plaintext_out).ok_or(error::Unspecified)?;
 
         let cpu = cpu::features();
-        let (counter, poly_key) =
-            chacha20_poly1305::begin(&self.key.k_2, make_nonce(sequence_number));
+        let (counter, poly_key) = chacha20_poly1305::begin(
+            &self.key.k_2,
+            make_nonce(sequence_number),
+            Aad::from(packet_length),
+            after_packet_length,
+        )
+        .map_err(error::erase::<InputTooLongError>)?;
 
         // We must verify the tag before decrypting so that
         // `ciphertext_in_plaintext_out` is unmodified if verification fails.