Create new constants for key lengths, etc

We were using olm::KEY_LENGTH for everything under the sun which happened to be
32 bytes long, and making a bunch of assumptions in the process. Create a bunch
of new constants (as C #defines rather than C++ consts so that I can use them
in another forthcoming refactor).

11 files changed, 103 insertions, 68 deletions

diff --git a/include/olm/crypto.h b/include/olm/crypto.h
index 31b9b60..325080e 100644
--- a/include/olm/crypto.h
+++ b/include/olm/crypto.h
@@ -27,7 +27,36 @@
 extern "C" {
 #endif
 
-const size_t SHA256_OUTPUT_LENGTH = 32;
+/** length of a sha256 hash */
+#define SHA256_OUTPUT_LENGTH 32
+
+/** length of a public or private Curve25519 key */
+#define CURVE25519_KEY_LENGTH 32
+
+/** length of the shared secret created by a Curve25519 ECDH operation */
+#define CURVE25519_SHARED_SECRET_LENGTH 32
+
+/** amount of random data required to create a Curve25519 keypair */
+#define CURVE25519_RANDOM_LENGTH CURVE25519_KEY_LENGTH
+
+/** length of a public Ed25519 key */
+#define ED25519_PUBLIC_KEY_LENGTH 32
+
+/** length of a private Ed25519 key */
+#define ED25519_PRIVATE_KEY_LENGTH 64
+
+/** amount of random data required to create a Ed25519 keypair */
+#define ED25519_RANDOM_LENGTH 32
+
+/** length of an Ed25519 signature */
+#define ED25519_SIGNATURE_LENGTH 64
+
+/** length of an aes256 key */
+#define AES256_KEY_LENGTH 32
+
+/** length of an aes256 initialisation vector */
+#define AES256_IV_LENGTH 16
+
 
 /** Computes SHA-256 of the input. The output buffer must be a least 32
  * bytes long. */
diff --git a/include/olm/crypto.hh b/include/olm/crypto.hh
index 484dc83..13fd7e9 100644
--- a/include/olm/crypto.hh
+++ b/include/olm/crypto.hh
@@ -25,23 +25,18 @@
 
 namespace olm {
 
-static const std::size_t ED25519_PRIVATE_KEY_LENGTH = 64;
-static const std::size_t KEY_LENGTH = 32;
-static const std::size_t SIGNATURE_LENGTH = 64;
-static const std::size_t IV_LENGTH = 16;
-
 struct Curve25519PublicKey {
-    std::uint8_t public_key[KEY_LENGTH];
+    std::uint8_t public_key[CURVE25519_KEY_LENGTH];
 };
 
 
 struct Curve25519KeyPair : public Curve25519PublicKey {
-    std::uint8_t private_key[KEY_LENGTH];
+    std::uint8_t private_key[CURVE25519_KEY_LENGTH];
 };
 
 
 struct Ed25519PublicKey {
-    std::uint8_t public_key[KEY_LENGTH];
+    std::uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH];
 };
 
 
@@ -93,12 +88,12 @@ bool ed25519_verify(
 
 
 struct Aes256Key {
-    std::uint8_t key[KEY_LENGTH];
+    std::uint8_t key[AES256_KEY_LENGTH];
 };
 
 
 struct Aes256Iv {
-    std::uint8_t iv[IV_LENGTH];
+    std::uint8_t iv[AES256_IV_LENGTH];
 };
 
 
diff --git a/include/olm/ratchet.hh b/include/olm/ratchet.hh
index 13f7097..e91d634 100644
--- a/include/olm/ratchet.hh
+++ b/include/olm/ratchet.hh
@@ -21,8 +21,13 @@ struct _olm_cipher;
 
 namespace olm {
 
-typedef std::uint8_t SharedKey[olm::KEY_LENGTH];
+/** length of a shared key: the root key R(i), chain key C(i,j), and message key
+ * M(i,j)). They are all only used to stuff into HMACs, so could be any length
+ * for that. The chain key and message key are both derived from SHA256
+ * operations, so their length is determined by that. */
+const std::size_t OLM_SHARED_KEY_LENGTH = SHA256_OUTPUT_LENGTH;
 
+typedef std::uint8_t SharedKey[OLM_SHARED_KEY_LENGTH];
 
 struct ChainKey {
     std::uint32_t index;
diff --git a/src/account.cpp b/src/account.cpp
index ec763f8..9512068 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -49,7 +49,7 @@ std::size_t olm::Account::remove_key(
 }
 
 std::size_t olm::Account::new_account_random_length() {
-    return 2 * olm::KEY_LENGTH;
+    return ED25519_RANDOM_LENGTH + CURVE25519_RANDOM_LENGTH;
 }
 
 std::size_t olm::Account::new_account(
@@ -61,7 +61,7 @@ std::size_t olm::Account::new_account(
     }
 
     olm::ed25519_generate_key(random, identity_keys.ed25519_key);
-    random += KEY_LENGTH;
+    random += ED25519_RANDOM_LENGTH;
     olm::curve25519_generate_key(random, identity_keys.curve25519_key);
 
     return 0;
@@ -137,7 +137,7 @@ std::size_t olm::Account::get_identity_json(
 
 std::size_t olm::Account::signature_length(
 ) {
-    return olm::SIGNATURE_LENGTH;
+    return ED25519_SIGNATURE_LENGTH;
 }
 
 
@@ -238,7 +238,7 @@ std::size_t olm::Account::max_number_of_one_time_keys(
 std::size_t olm::Account::generate_one_time_keys_random_length(
     std::size_t number_of_keys
 ) {
-    return olm::KEY_LENGTH * number_of_keys;
+    return CURVE25519_RANDOM_LENGTH * number_of_keys;
 }
 
 std::size_t olm::Account::generate_one_time_keys(
@@ -254,7 +254,7 @@ std::size_t olm::Account::generate_one_time_keys(
         key.id = ++next_one_time_key_id;
         key.published = false;
         olm::curve25519_generate_key(random, key.key);
-        random += olm::KEY_LENGTH;
+        random += CURVE25519_RANDOM_LENGTH;
     }
     return number_of_keys;
 }
diff --git a/src/cipher.cpp b/src/cipher.cpp
index 8c3de92..8e3d7a5 100644
--- a/src/cipher.cpp
+++ b/src/cipher.cpp
@@ -17,11 +17,13 @@
 #include "olm/memory.hh"
 #include <cstring>
 
+const std::size_t HMAC_KEY_LENGTH = 32;
+
 namespace {
 
 struct DerivedKeys {
     olm::Aes256Key aes_key;
-    std::uint8_t mac_key[olm::KEY_LENGTH];
+    std::uint8_t mac_key[HMAC_KEY_LENGTH];
     olm::Aes256Iv aes_iv;
 };
 
@@ -31,7 +33,9 @@ static void derive_keys(
     std::uint8_t const * key, std::size_t key_length,
     DerivedKeys & keys
 ) {
-    std::uint8_t derived_secrets[2 * olm::KEY_LENGTH + olm::IV_LENGTH];
+    std::uint8_t derived_secrets[
+        AES256_KEY_LENGTH + HMAC_KEY_LENGTH + AES256_IV_LENGTH
+    ];
     _olm_crypto_hkdf_sha256(
         key, key_length,
         nullptr, 0,
@@ -81,7 +85,7 @@ size_t aes_sha_256_cipher_encrypt(
     );
 
     _olm_crypto_hmac_sha256(
-        keys.mac_key, olm::KEY_LENGTH, output, output_length - MAC_LENGTH, mac
+        keys.mac_key, HMAC_KEY_LENGTH, output, output_length - MAC_LENGTH, mac
     );
 
     std::memcpy(output + output_length - MAC_LENGTH, mac, MAC_LENGTH);
@@ -113,7 +117,7 @@ size_t aes_sha_256_cipher_decrypt(
     derive_keys(c->kdf_info, c->kdf_info_length, key, key_length, keys);
 
     _olm_crypto_hmac_sha256(
-        keys.mac_key, olm::KEY_LENGTH, input, input_length - MAC_LENGTH, mac
+        keys.mac_key, HMAC_KEY_LENGTH, input, input_length - MAC_LENGTH, mac
     );
 
     std::uint8_t const * input_mac = input + input_length - MAC_LENGTH;
diff --git a/src/crypto.cpp b/src/crypto.cpp
index 83493be..0b08c54 100644
--- a/src/crypto.cpp
+++ b/src/crypto.cpp
@@ -31,7 +31,7 @@ namespace {
 
 static const std::uint8_t CURVE25519_BASEPOINT[32] = {9};
 static const std::size_t AES_KEY_SCHEDULE_LENGTH = 60;
-static const std::size_t AES_KEY_BITS = 8 * olm::KEY_LENGTH;
+static const std::size_t AES_KEY_BITS = 8 * AES256_KEY_LENGTH;
 static const std::size_t AES_BLOCK_LENGTH = 16;
 static const std::size_t SHA256_BLOCK_LENGTH = 64;
 static const std::uint8_t HKDF_DEFAULT_SALT[32] = {};
@@ -104,7 +104,7 @@ void olm::curve25519_generate_key(
     std::uint8_t const * random_32_bytes,
     olm::Curve25519KeyPair & key_pair
 ) {
-    std::memcpy(key_pair.private_key, random_32_bytes, KEY_LENGTH);
+    std::memcpy(key_pair.private_key, random_32_bytes, CURVE25519_KEY_LENGTH);
     ::curve25519_donna(
         key_pair.public_key, key_pair.private_key, CURVE25519_BASEPOINT
     );
diff --git a/src/olm.cpp b/src/olm.cpp
index 682a84c..10e00fa 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -436,8 +436,8 @@ size_t olm_create_outbound_session(
     std::size_t id_key_length = their_identity_key_length;
     std::size_t ot_key_length = their_one_time_key_length;
 
-    if (olm::decode_base64_length(id_key_length) != olm::KEY_LENGTH
-            || olm::decode_base64_length(ot_key_length) != olm::KEY_LENGTH
+    if (olm::decode_base64_length(id_key_length) != CURVE25519_KEY_LENGTH
+            || olm::decode_base64_length(ot_key_length) != CURVE25519_KEY_LENGTH
     ) {
         from_c(session)->last_error = OlmErrorCode::OLM_INVALID_BASE64;
         return std::size_t(-1);
@@ -483,7 +483,7 @@ size_t olm_create_inbound_session_from(
     std::uint8_t const * id_key = from_c(their_identity_key);
     std::size_t id_key_length = their_identity_key_length;
 
-    if (olm::decode_base64_length(id_key_length) != olm::KEY_LENGTH) {
+    if (olm::decode_base64_length(id_key_length) != CURVE25519_KEY_LENGTH) {
         from_c(session)->last_error = OlmErrorCode::OLM_INVALID_BASE64;
         return std::size_t(-1);
     }
@@ -554,7 +554,7 @@ size_t olm_matches_inbound_session_from(
     std::uint8_t const * id_key = from_c(their_identity_key);
     std::size_t id_key_length = their_identity_key_length;
 
-    if (olm::decode_base64_length(id_key_length) != olm::KEY_LENGTH) {
+    if (olm::decode_base64_length(id_key_length) != CURVE25519_KEY_LENGTH) {
         from_c(session)->last_error = OlmErrorCode::OLM_INVALID_BASE64;
         return std::size_t(-1);
     }
@@ -710,7 +710,7 @@ size_t olm_ed25519_verify(
     void const * message, size_t message_length,
     void * signature, size_t signature_length
 ) {
-    if (olm::decode_base64_length(key_length) != olm::KEY_LENGTH) {
+    if (olm::decode_base64_length(key_length) != CURVE25519_KEY_LENGTH) {
         from_c(utility)->last_error = OlmErrorCode::OLM_INVALID_BASE64;
         return std::size_t(-1);
     }
diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index aab5fbb..57cb385 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -49,7 +49,7 @@ static void create_chain_key(
 ) {
     olm::SharedKey secret;
     olm::curve25519_shared_secret(our_key, their_key, secret);
-    std::uint8_t derived_secrets[2 * olm::KEY_LENGTH];
+    std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
     _olm_crypto_hkdf_sha256(
         secret, sizeof(secret),
         root_key, sizeof(root_key),
@@ -191,7 +191,7 @@ void olm::Ratchet::initialise_as_bob(
     std::uint8_t const * shared_secret, std::size_t shared_secret_length,
     olm::Curve25519PublicKey const & their_ratchet_key
 ) {
-    std::uint8_t derived_secrets[2 * olm::KEY_LENGTH];
+    std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
     _olm_crypto_hkdf_sha256(
         shared_secret, shared_secret_length,
         nullptr, 0,
@@ -212,7 +212,7 @@ void olm::Ratchet::initialise_as_alice(
     std::uint8_t const * shared_secret, std::size_t shared_secret_length,
     olm::Curve25519KeyPair const & our_ratchet_key
 ) {
-    std::uint8_t derived_secrets[2 * olm::KEY_LENGTH];
+    std::uint8_t derived_secrets[2 * olm::OLM_SHARED_KEY_LENGTH];
     _olm_crypto_hkdf_sha256(
         shared_secret, shared_secret_length,
         nullptr, 0,
@@ -234,7 +234,7 @@ namespace olm {
 static std::size_t pickle_length(
     const olm::SharedKey & value
 ) {
-    return olm::KEY_LENGTH;
+    return olm::OLM_SHARED_KEY_LENGTH;
 }
 
 
@@ -242,7 +242,7 @@ static std::uint8_t * pickle(
     std::uint8_t * pos,
     const olm::SharedKey & value
 ) {
-    return olm::pickle_bytes(pos, value, olm::KEY_LENGTH);
+    return olm::pickle_bytes(pos, value, olm::OLM_SHARED_KEY_LENGTH);
 }
 
 
@@ -250,7 +250,7 @@ static std::uint8_t const * unpickle(
     std::uint8_t const * pos, std::uint8_t const * end,
     olm::SharedKey & value
 ) {
-    return olm::unpickle_bytes(pos, end, value, olm::KEY_LENGTH);
+    return olm::unpickle_bytes(pos, end, value, olm::OLM_SHARED_KEY_LENGTH);
 }
 
 
@@ -359,7 +359,7 @@ std::size_t olm::pickle_length(
     olm::Ratchet const & value
 ) {
     std::size_t length = 0;
-    length += olm::KEY_LENGTH;
+    length += olm::OLM_SHARED_KEY_LENGTH;
     length += olm::pickle_length(value.sender_chain);
     length += olm::pickle_length(value.receiver_chains);
     length += olm::pickle_length(value.skipped_message_keys);
@@ -409,13 +409,13 @@ std::size_t olm::Ratchet::encrypt_output_length(
         plaintext_length
     );
     return olm::encode_message_length(
-        counter, olm::KEY_LENGTH, padded, ratchet_cipher->ops->mac_length(ratchet_cipher)
+        counter, CURVE25519_KEY_LENGTH, padded, ratchet_cipher->ops->mac_length(ratchet_cipher)
     );
 }
 
 
 std::size_t olm::Ratchet::encrypt_random_length() {
-    return sender_chain.empty() ? olm::KEY_LENGTH : 0;
+    return sender_chain.empty() ? CURVE25519_RANDOM_LENGTH : 0;
 }
 
 
@@ -461,7 +461,8 @@ std::size_t olm::Ratchet::encrypt(
     olm::MessageWriter writer;
 
     olm::encode_message(
-        writer, PROTOCOL_VERSION, counter, olm::KEY_LENGTH, ciphertext_length,
+        writer, PROTOCOL_VERSION, counter, CURVE25519_KEY_LENGTH,
+        ciphertext_length,
         output
     );
 
@@ -529,7 +530,7 @@ std::size_t olm::Ratchet::decrypt(
         return std::size_t(-1);
     }
 
-    if (reader.ratchet_key_length != olm::KEY_LENGTH) {
+    if (reader.ratchet_key_length != CURVE25519_KEY_LENGTH) {
         last_error = OlmErrorCode::OLM_BAD_MESSAGE_FORMAT;
         return std::size_t(-1);
     }
@@ -539,7 +540,7 @@ std::size_t olm::Ratchet::decrypt(
     for (olm::ReceiverChain & receiver_chain : receiver_chains) {
         if (0 == std::memcmp(
                 receiver_chain.ratchet_key.public_key, reader.ratchet_key,
-                olm::KEY_LENGTH
+                CURVE25519_KEY_LENGTH
         )) {
             chain = &receiver_chain;
             break;
@@ -559,7 +560,7 @@ std::size_t olm::Ratchet::decrypt(
             if (reader.counter == skipped.message_key.index
                     && 0 == std::memcmp(
                         skipped.ratchet_key.public_key, reader.ratchet_key,
-                        olm::KEY_LENGTH
+                        CURVE25519_KEY_LENGTH
                     )
             ) {
                 /* Found the key for this message. Check the MAC. */
diff --git a/src/session.cpp b/src/session.cpp
index 6ca7f36..7bde5d1 100644
--- a/src/session.cpp
+++ b/src/session.cpp
@@ -49,7 +49,7 @@ olm::Session::Session(
 
 
 std::size_t olm::Session::new_outbound_session_random_length() {
-    return olm::KEY_LENGTH * 2;
+    return CURVE25519_RANDOM_LENGTH * 2;
 }
 
 
@@ -68,7 +68,7 @@ std::size_t olm::Session::new_outbound_session(
     olm::curve25519_generate_key(random, base_key);
 
     olm::Curve25519KeyPair ratchet_key;
-    olm::curve25519_generate_key(random + olm::KEY_LENGTH, ratchet_key);
+    olm::curve25519_generate_key(random + CURVE25519_RANDOM_LENGTH, ratchet_key);
 
     olm::Curve25519KeyPair const & alice_identity_key_pair = (
         local_account.identity_keys.curve25519_key
@@ -79,13 +79,13 @@ std::size_t olm::Session::new_outbound_session(
     alice_base_key = base_key;
     bob_one_time_key = one_time_key;
 
-    std::uint8_t secret[3 * olm::KEY_LENGTH];
+    // Calculate the shared secret S via triple DH
+    std::uint8_t secret[3 * CURVE25519_SHARED_SECRET_LENGTH];
     std::uint8_t * pos = secret;
-
     olm::curve25519_shared_secret(alice_identity_key_pair, one_time_key, pos);
-    pos += olm::KEY_LENGTH;
+    pos += CURVE25519_SHARED_SECRET_LENGTH;
     olm::curve25519_shared_secret(base_key, identity_key, pos);
-    pos += olm::KEY_LENGTH;
+    pos += CURVE25519_SHARED_SECRET_LENGTH;
     olm::curve25519_shared_secret(base_key, one_time_key, pos);
 
     ratchet.initialise_as_alice(secret, sizeof(secret), ratchet_key);
@@ -105,13 +105,13 @@ static bool check_message_fields(
     bool ok = true;
     ok = ok && (have_their_identity_key || reader.identity_key);
     if (reader.identity_key) {
-        ok = ok && reader.identity_key_length == olm::KEY_LENGTH;
+        ok = ok && reader.identity_key_length == CURVE25519_KEY_LENGTH;
     }
     ok = ok && reader.message;
     ok = ok && reader.base_key;
-    ok = ok && reader.base_key_length == olm::KEY_LENGTH;
+    ok = ok && reader.base_key_length == CURVE25519_KEY_LENGTH;
     ok = ok && reader.one_time_key;
-    ok = ok && reader.one_time_key_length == olm::KEY_LENGTH;
+    ok = ok && reader.one_time_key_length == CURVE25519_KEY_LENGTH;
     return ok;
 }
 
@@ -133,7 +133,7 @@ std::size_t olm::Session::new_inbound_session(
 
     if (reader.identity_key && their_identity_key) {
         bool same = 0 == std::memcmp(
-            their_identity_key->public_key, reader.identity_key, olm::KEY_LENGTH
+            their_identity_key->public_key, reader.identity_key, CURVE25519_KEY_LENGTH
         );
         if (!same) {
             last_error = OlmErrorCode::OLM_BAD_MESSAGE_KEY_ID;
@@ -152,7 +152,7 @@ std::size_t olm::Session::new_inbound_session(
     );
 
     if (!message_reader.ratchet_key
-            || message_reader.ratchet_key_length != olm::KEY_LENGTH) {
+            || message_reader.ratchet_key_length != CURVE25519_KEY_LENGTH) {
         last_error = OlmErrorCode::OLM_BAD_MESSAGE_FORMAT;
         return std::size_t(-1);
     }
@@ -174,12 +174,13 @@ std::size_t olm::Session::new_inbound_session(
     );
     olm::Curve25519KeyPair const & bob_one_time_key = our_one_time_key->key;
 
-    std::uint8_t secret[olm::KEY_LENGTH * 3];
+    // Calculate the shared secret S via triple DH
+    std::uint8_t secret[CURVE25519_SHARED_SECRET_LENGTH * 3];
     std::uint8_t * pos = secret;
     olm::curve25519_shared_secret(bob_one_time_key, alice_identity_key, pos);
-    pos += olm::KEY_LENGTH;
+    pos += CURVE25519_SHARED_SECRET_LENGTH;
     olm::curve25519_shared_secret(bob_identity_key, alice_base_key, pos);
-    pos += olm::KEY_LENGTH;
+    pos += CURVE25519_SHARED_SECRET_LENGTH;
     olm::curve25519_shared_secret(bob_one_time_key, alice_base_key, pos);
 
     ratchet.initialise_as_bob(secret, sizeof(secret), ratchet_key);
@@ -202,7 +203,7 @@ std::size_t olm::Session::session_id(
         last_error = OlmErrorCode::OLM_OUTPUT_BUFFER_TOO_SMALL;
         return std::size_t(-1);
     }
-    std::uint8_t tmp[olm::KEY_LENGTH * 3];
+    std::uint8_t tmp[CURVE25519_KEY_LENGTH * 3];
     std::uint8_t * pos = tmp;
     pos = olm::store_array(pos, alice_identity_key.public_key);
     pos = olm::store_array(pos, alice_base_key.public_key);
@@ -226,20 +227,20 @@ bool olm::Session::matches_inbound_session(
     bool same = true;
     if (reader.identity_key) {
         same = same && 0 == std::memcmp(
-            reader.identity_key, alice_identity_key.public_key, olm::KEY_LENGTH
+            reader.identity_key, alice_identity_key.public_key, CURVE25519_KEY_LENGTH
         );
     }
     if (their_identity_key) {
         same = same && 0 == std::memcmp(
             their_identity_key->public_key, alice_identity_key.public_key,
-            olm::KEY_LENGTH
+            CURVE25519_KEY_LENGTH
         );
     }
     same = same && 0 == std::memcmp(
-        reader.base_key, alice_base_key.public_key, olm::KEY_LENGTH
+        reader.base_key, alice_base_key.public_key, CURVE25519_KEY_LENGTH
     );
     same = same && 0 == std::memcmp(
-        reader.one_time_key, bob_one_time_key.public_key, olm::KEY_LENGTH
+        reader.one_time_key, bob_one_time_key.public_key, CURVE25519_KEY_LENGTH
     );
     return same;
 }
@@ -266,9 +267,9 @@ std::size_t olm::Session::encrypt_message_length(
     }
 
     return encode_one_time_key_message_length(
-        olm::KEY_LENGTH,
-        olm::KEY_LENGTH,
-        olm::KEY_LENGTH,
+        CURVE25519_KEY_LENGTH,
+        CURVE25519_KEY_LENGTH,
+        CURVE25519_KEY_LENGTH,
         message_length
     );
 }
@@ -300,9 +301,9 @@ std::size_t olm::Session::encrypt(
         encode_one_time_key_message(
             writer,
             PROTOCOL_VERSION,
-            olm::KEY_LENGTH,
-            olm::KEY_LENGTH,
-            olm::KEY_LENGTH,
+            CURVE25519_KEY_LENGTH,
+            CURVE25519_KEY_LENGTH,
+            CURVE25519_KEY_LENGTH,
             message_body_length,
             message
         );
diff --git a/src/utility.cpp b/src/utility.cpp
index 67029c9..e33351c 100644
--- a/src/utility.cpp
+++ b/src/utility.cpp
@@ -45,7 +45,7 @@ size_t olm::Utility::ed25519_verify(
     std::uint8_t const * message, std::size_t message_length,
     std::uint8_t const * signature, std::size_t signature_length
 ) {
-    if (signature_length < olm::SIGNATURE_LENGTH) {
+    if (signature_length < ED25519_SIGNATURE_LENGTH) {
         last_error = OlmErrorCode::OLM_BAD_MESSAGE_MAC;
         return std::size_t(-1);
     }
diff --git a/tests/test_crypto.cpp b/tests/test_crypto.cpp
index 175f66f..56abdcd 100644
--- a/tests/test_crypto.cpp
+++ b/tests/test_crypto.cpp
@@ -70,7 +70,7 @@ olm::curve25519_generate_key(bob_private, bob_pair);
 assert_equals(bob_private, bob_pair.private_key, 32);
 assert_equals(bob_public, bob_pair.public_key, 32);
 
-std::uint8_t actual_agreement[olm::KEY_LENGTH] = {};
+std::uint8_t actual_agreement[CURVE25519_SHARED_SECRET_LENGTH] = {};
 
 olm::curve25519_shared_secret(alice_pair, bob_pair, actual_agreement);