diff --git a/shell_wrapper/kahe.rs b/shell_wrapper/kahe.rs
index 92a5f3b..11b40f5 100644
--- a/shell_wrapper/kahe.rs
+++ b/shell_wrapper/kahe.rs
@@ -18,7 +18,7 @@
 use shell_types::{Moduli, RnsContextRef, RnsPolynomial, RnsPolynomialVec};
 use single_thread_hkdf::{SeedWrapper, SingleThreadHkdfWrapper};
 use status::rust_status_from_cpp;
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 use std::marker::PhantomData;
 use std::mem::MaybeUninit;
 
@@ -197,7 +197,7 @@ pub use ffi::BigIntVectorWrapper;
 /// Returns the resulting ciphertexts.
 pub fn encrypt(
     input_vectors: &HashMap<&str, &[u64]>,
-    packed_vector_configs: &HashMap<String, PackedVectorConfig>,
+    packed_vector_configs: &BTreeMap<String, PackedVectorConfig>,
     secret_key: &RnsPolynomial,
     params: &KahePublicParametersWrapper,
     prng: &mut SingleThreadHkdfWrapper,
@@ -246,7 +246,7 @@ pub fn decrypt(
     ciphertext: &RnsPolynomialVec,
     secret_key: &RnsPolynomial,
     params: &KahePublicParametersWrapper,
-    packed_vector_configs: &HashMap<String, PackedVectorConfig>,
+    packed_vector_configs: &BTreeMap<String, PackedVectorConfig>,
 ) -> Result<HashMap<String, Vec<u64>>, status::StatusError> {
     let mut packed_values = MaybeUninit::<BigIntVectorWrapper>::zeroed();
     // SAFETY: No lifetime constraints (`packed_values` does not keep any reference to the inputs).
diff --git a/shell_wrapper/kahe_test.rs b/shell_wrapper/kahe_test.rs
index d9db5ee..a6eb31a 100644
--- a/shell_wrapper/kahe_test.rs
+++ b/shell_wrapper/kahe_test.rs
@@ -21,7 +21,7 @@ use kahe::{create_public_parameters, decrypt, encrypt, generate_secret_key, Pack
 use rand::Rng;
 use status::StatusErrorCode;
 use status_matchers_rs::status_is;
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 // RNS configuration. LOG_T is the bit length of the KAHE plaintext modulus.
 const LOG_T: u64 = 11;
@@ -45,7 +45,7 @@ fn encrypt_decrypt() -> Result<()> {
     // Encrypt small vector. `ciphertext` is a wrapper around a C++ pointer.
     let input_values = vec![1, 2, 3];
     let plaintext = HashMap::from([(DEFAULT_ID, input_values.as_slice())]);
-    let packed_vector_configs = HashMap::from([(
+    let packed_vector_configs = BTreeMap::from([(
         DEFAULT_ID.to_string(),
         PackedVectorConfig { base: 10, dimension: 2, num_packed_coeffs: 2, length: 3 },
     )]);
@@ -83,7 +83,7 @@ fn encrypt_decrypt_padding() -> Result<()> {
     // Encrypt the vector. Pass a longer length than what we need.
     let padded_length = (num_packed_coeffs * packing_dimension) as usize;
     let plaintext = HashMap::from([(DEFAULT_ID, input_values.as_slice())]);
-    let packed_vector_configs = HashMap::from([(
+    let packed_vector_configs = BTreeMap::from([(
         DEFAULT_ID.to_string(),
         PackedVectorConfig {
             base: input_domain as u64,
@@ -133,7 +133,7 @@ fn encrypt_decrypt_long() -> Result<()> {
     let input_values: Vec<u64> =
         (0..num_input_values).map(|_| rand::thread_rng().gen_range(0..input_domain)).collect();
     let plaintext = HashMap::from([(DEFAULT_ID, input_values.as_slice())]);
-    let packed_vector_configs = HashMap::from([(
+    let packed_vector_configs = BTreeMap::from([(
         DEFAULT_ID.to_string(),
         PackedVectorConfig {
             base: input_domain as u64,
@@ -184,7 +184,7 @@ fn encrypt_decrypt_two_vectors() -> Result<()> {
     // The number of packed coefficients for both vectors.
     let num_packed_coeffs = [5, 5];
 
-    let packed_vector_configs = HashMap::from([
+    let packed_vector_configs = BTreeMap::from([
         (
             ID0.to_string(),
             PackedVectorConfig {
diff --git a/willow/src/shell/kahe.rs b/willow/src/shell/kahe.rs
index 5103f0d..2551307 100644
--- a/willow/src/shell/kahe.rs
+++ b/willow/src/shell/kahe.rs
@@ -26,7 +26,7 @@ use shell_types::{
     RnsPolynomialVec,
 };
 use single_thread_hkdf::SingleThreadHkdfPrng;
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 /// Number of bits supported by the C++ big integer type used for KAHE
 /// plaintext.
@@ -38,7 +38,7 @@ pub struct ShellKaheConfig {
     pub moduli: Vec<u64>,
     pub log_t: usize,
     pub num_public_polynomials: usize,
-    pub packed_vector_configs: HashMap<String, PackedVectorConfig>,
+    pub packed_vector_configs: BTreeMap<String, PackedVectorConfig>,
 }
 
 /// Base type holding public KAHE configuration and C++ parameters.
@@ -376,7 +376,7 @@ mod test {
     use proto_serialization_traits::{FromProto, ToProto};
     use shell_testing_parameters::{make_kahe_config_for, set_kahe_num_public_polynomials};
     use single_thread_hkdf::SingleThreadHkdfPrng;
-    use std::collections::HashMap;
+    use std::collections::{BTreeMap, HashMap};
     use testing_utils::generate_random_unsigned_vector;
 
     /// Standard deviation of the discrete Gaussian distribution used for
@@ -400,7 +400,7 @@ mod test {
     #[gtest]
     fn test_encrypt_decrypt_short() -> googletest::Result<()> {
         let plaintext_modulus_bits = 39;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             DEFAULT_ID.to_string(),
             PackedVectorConfig { base: 10, dimension: 2, num_packed_coeffs: 5, length: 10 },
         )]);
@@ -420,7 +420,7 @@ mod test {
     #[gtest]
     fn test_encrypt_decrypt_short_padding() -> googletest::Result<()> {
         let plaintext_modulus_bits = 39;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             DEFAULT_ID.to_string(),
             PackedVectorConfig { base: 10, dimension: 2, num_packed_coeffs: 5, length: 8 },
         )]);
@@ -440,7 +440,7 @@ mod test {
     #[gtest]
     fn test_encrypt_decrypt_with_serialized_key() -> googletest::Result<()> {
         let plaintext_modulus_bits = 39;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             DEFAULT_ID.to_string(),
             PackedVectorConfig { base: 10, dimension: 2, num_packed_coeffs: 5, length: 10 },
         )]);
@@ -467,7 +467,7 @@ mod test {
     fn test_encrypt_decrypt_long() -> googletest::Result<()> {
         let plaintext_modulus_bits = 17;
         let input_domain = 5;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             DEFAULT_ID.to_string(),
             PackedVectorConfig {
                 base: input_domain,
@@ -507,7 +507,7 @@ mod test {
         let plaintext_modulus_bits = 93;
         let input_domain = 10;
         let num_messages = 50;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             DEFAULT_ID.to_string(),
             PackedVectorConfig {
                 base: input_domain * 2,
@@ -553,7 +553,7 @@ mod test {
     #[gtest]
     fn read_write_secret_key() -> googletest::Result<()> {
         let plaintext_modulus_bits = 17;
-        let packed_vector_configs = HashMap::from([]);
+        let packed_vector_configs = BTreeMap::from([]);
         let kahe_config = make_kahe_config_for(plaintext_modulus_bits, packed_vector_configs)?;
 
         let kahe = ShellKahe::new(kahe_config, CONTEXT_STRING)?;
@@ -595,7 +595,7 @@ mod test {
     #[gtest]
     fn test_encrypt_decrypt_serialized_proto() -> googletest::Result<()> {
         let plaintext_modulus_bits = 39;
-        let packed_vector_configs = HashMap::from([(
+        let packed_vector_configs = BTreeMap::from([(
             String::from(DEFAULT_ID),
             PackedVectorConfig { base: 10, dimension: 2, num_packed_coeffs: 5, length: 10 },
         )]);
@@ -624,7 +624,7 @@ mod test {
     fn test_key_serialization_is_homomorphic() -> googletest::Result<()> {
         // Set up a ShellKahe instance.
         let plaintext_modulus_bits = 39;
-        let packed_vector_configs = HashMap::from([]);
+        let packed_vector_configs = BTreeMap::from([]);
         let kahe_config = make_kahe_config_for(plaintext_modulus_bits, packed_vector_configs)?;
         let kahe = ShellKahe::new(kahe_config, CONTEXT_STRING)?;
 
diff --git a/willow/src/shell/parameters_generation.rs b/willow/src/shell/parameters_generation.rs
index 8d5f348..99d897e 100644
--- a/willow/src/shell/parameters_generation.rs
+++ b/willow/src/shell/parameters_generation.rs
@@ -14,7 +14,7 @@
 
 use aggregation_config::AggregationConfig;
 use kahe::PackedVectorConfig;
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 
 /// Generating KAHE and AHE parameters given the Willow protocol configuration.
 
@@ -34,7 +34,7 @@ pub fn divide_and_roundup(x: usize, y: usize) -> usize {
 pub fn generate_packing_config(
     plaintext_bits: usize,
     agg_config: &AggregationConfig,
-) -> Result<HashMap<String, PackedVectorConfig>, status::StatusError> {
+) -> Result<BTreeMap<String, PackedVectorConfig>, status::StatusError> {
     if plaintext_bits == 0 {
         return Err(status::invalid_argument("`plaintext_bits` must be positive."));
     }
@@ -47,7 +47,7 @@ pub fn generate_packing_config(
     if agg_config.max_number_of_clients <= 0 {
         return Err(status::invalid_argument("`max_number_of_clients` must be positive."));
     }
-    let mut packing_configs = HashMap::<String, PackedVectorConfig>::new();
+    let mut packing_configs = BTreeMap::<String, PackedVectorConfig>::new();
     for (id, (length, bound)) in agg_config.vector_lengths_and_bounds.iter() {
         if *length <= 0 {
             return Err(status::invalid_argument(format!(
@@ -210,12 +210,7 @@ mod test {
         );
         expect_eq!(
             packed_vector_configs.get("large").unwrap(),
-            &PackedVectorConfig {
-                base: 1 << 24,
-                dimension: 1,
-                num_packed_coeffs: 32,
-                length: 32
-            }
+            &PackedVectorConfig { base: 1 << 24, dimension: 1, num_packed_coeffs: 32, length: 32 }
         );
         expect_eq!(
             packed_vector_configs.get("long").unwrap(),
diff --git a/willow/src/shell/parameters_utils.rs b/willow/src/shell/parameters_utils.rs
index d472a1d..cf3bd11 100644
--- a/willow/src/shell/parameters_utils.rs
+++ b/willow/src/shell/parameters_utils.rs
@@ -19,7 +19,7 @@ use shell_parameters_rust_proto::{
     PackedVectorConfigProto, PackedVectorConfigProtoView, ShellKaheConfigProto,
     ShellKaheConfigProtoView,
 };
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 
 /// This file contains some utility functions for working with Willow parameters:
 /// - Conversions between Rust structs and their corresponding protos.
@@ -81,7 +81,7 @@ pub fn kahe_config_from_proto(
                     Err(status::invalid_argument("invalid id in `packed_vectors`."))
                 }
             })
-            .collect::<Result<HashMap<String, PackedVectorConfig>, _>>()?,
+            .collect::<Result<BTreeMap<String, PackedVectorConfig>, _>>()?,
     })
 }
 
@@ -110,7 +110,7 @@ mod test {
             moduli: vec![65537u64, 12289u64],
             log_t: 5usize,
             num_public_polynomials: 2usize,
-            packed_vector_configs: HashMap::from([
+            packed_vector_configs: BTreeMap::from([
                 (
                     String::from("vector0"),
                     PackedVectorConfig {
diff --git a/willow/src/testing_utils/shell_testing_parameters.rs b/willow/src/testing_utils/shell_testing_parameters.rs
index 2feb9f6..6d699ed 100644
--- a/willow/src/testing_utils/shell_testing_parameters.rs
+++ b/willow/src/testing_utils/shell_testing_parameters.rs
@@ -17,13 +17,13 @@ use ahe_shell::ShellAheConfig;
 use kahe::PackedVectorConfig;
 use kahe_shell::ShellKaheConfig;
 use shell_parameters_generation::{divide_and_roundup, generate_packing_config};
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 
 /// Creates an KAHE configuration with the given plaintext modulus bits, by
 /// looking up some pre-generated configurations.
 pub fn make_kahe_config_for(
     plaintext_modulus_bits: usize,
-    packed_vector_configs: HashMap<String, PackedVectorConfig>,
+    packed_vector_configs: BTreeMap<String, PackedVectorConfig>,
 ) -> Result<ShellKaheConfig, status::StatusError> {
     // Configurations below come from:
     // google3/experimental/users/baiyuli/async_rlwe_secagg/parameters.cc,