diff --git a/include/olm/account.hh b/include/olm/account.hh
index 826b26d..99754ea 100644
--- a/include/olm/account.hh
+++ b/include/olm/account.hh
@@ -49,7 +49,7 @@ struct Account {
     OlmErrorCode last_error;
 
     /** Number of random bytes needed to create a new account */
-    std::size_t new_account_random_length();
+    std::size_t new_account_random_length() const;
 
     /** Create a new account. Returns std::size_t(-1) on error. If the number of
      * random bytes is too small then last_error will be NOT_ENOUGH_RANDOM */
@@ -58,7 +58,7 @@ struct Account {
     );
 
     /** Number of bytes needed to output the identity keys for this account */
-    std::size_t get_identity_json_length();
+    std::size_t get_identity_json_length() const;
 
     /** Output the identity keys for this account as JSON in the following
      * format:
@@ -77,7 +77,7 @@ struct Account {
     /**
      * The length of an ed25519 signature in bytes.
      */
-    std::size_t signature_length();
+    std::size_t signature_length() const;
 
     /**
      * Signs a message with the ed25519 key for this account.
@@ -88,7 +88,7 @@ struct Account {
     );
 
     /** Number of bytes needed to output the one time keys for this account */
-    std::size_t get_one_time_keys_json_length();
+    std::size_t get_one_time_keys_json_length() const;
 
     /** Output the one time keys that haven't been published yet as JSON:
      *
@@ -111,13 +111,13 @@ struct Account {
     std::size_t mark_keys_as_published();
 
     /** The largest number of one time keys this account can store. */
-    std::size_t max_number_of_one_time_keys();
+    std::size_t max_number_of_one_time_keys() const;
 
     /** The number of random bytes needed to generate a given number of new one
      * time keys. */
     std::size_t generate_one_time_keys_random_length(
         std::size_t number_of_keys
-    );
+    ) const;
 
     /** Generates a number of new one time keys. If the total number of keys
      * stored by this account exceeds max_number_of_one_time_keys() then the
@@ -129,7 +129,7 @@ struct Account {
     );
 
     /** The number of random bytes needed to generate a fallback key. */
-    std::size_t generate_fallback_key_random_length();
+    std::size_t generate_fallback_key_random_length() const;
 
     /** Generates a new fallback key. Returns std::size_t(-1) on error. If the
      * number of random bytes is too small then last_error will be
@@ -139,7 +139,7 @@ struct Account {
     );
 
     /** Number of bytes needed to output the one time keys for this account */
-    std::size_t get_fallback_key_json_length();
+    std::size_t get_fallback_key_json_length() const;
 
     /** Output the fallback key as JSON:
      *
diff --git a/include/olm/olm.h b/include/olm/olm.h
index 0d3ea94..e5aff67 100644
--- a/include/olm/olm.h
+++ b/include/olm/olm.h
@@ -119,12 +119,12 @@ size_t olm_clear_utility(
 
 /** Returns the number of bytes needed to store an account */
 size_t olm_pickle_account_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Returns the number of bytes needed to store a session */
 size_t olm_pickle_session_length(
-    OlmSession * session
+    const OlmSession * session
 );
 
 /** Stores an account as a base64 string. Encrypts the account using the
@@ -175,7 +175,7 @@ size_t olm_unpickle_session(
 
 /** The number of random bytes needed to create an account.*/
 size_t olm_create_account_random_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Creates a new account. Returns olm_error() on failure. If there weren't
@@ -188,7 +188,7 @@ size_t olm_create_account(
 
 /** The size of the output buffer needed to hold the identity keys */
 size_t olm_account_identity_keys_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Writes the public parts of the identity keys for the account into the
@@ -203,7 +203,7 @@ size_t olm_account_identity_keys(
 
 /** The length of an ed25519 signature encoded as base64. */
 size_t olm_account_signature_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Signs a message with the ed25519 key for this account. Returns olm_error()
@@ -217,7 +217,7 @@ size_t olm_account_sign(
 
 /** The size of the output buffer needed to hold the one time keys */
 size_t olm_account_one_time_keys_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Writes the public parts of the unpublished one time keys for the account
@@ -250,13 +250,13 @@ size_t olm_account_mark_keys_as_published(
 
 /** The largest number of one time keys this account can store. */
 size_t olm_account_max_number_of_one_time_keys(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** The number of random bytes needed to generate a given number of new one
  * time keys. */
 size_t olm_account_generate_one_time_keys_random_length(
-    OlmAccount * account,
+    const OlmAccount * account,
     size_t number_of_keys
 );
 
@@ -272,7 +272,7 @@ size_t olm_account_generate_one_time_keys(
 
 /** The number of random bytes needed to generate a fallback key. */
 size_t olm_account_generate_fallback_key_random_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 /** Generates a new fallback key. Only one previous fallback key is
@@ -286,7 +286,7 @@ size_t olm_account_generate_fallback_key(
 /** The number of bytes needed to hold the fallback key as returned by
  * olm_account_fallback_key. */
 size_t olm_account_fallback_key_length(
-    OlmAccount * account
+    const OlmAccount * account
 );
 
 size_t olm_account_fallback_key(
@@ -297,7 +297,7 @@ size_t olm_account_fallback_key(
 
 /** The number of random bytes needed to create an outbound session */
 size_t olm_create_outbound_session_random_length(
-    OlmSession * session
+    const OlmSession * session
 );
 
 /** Creates a new out-bound session for sending messages to a given identity_key
@@ -339,7 +339,7 @@ size_t olm_create_inbound_session_from(
 
 /** The length of the buffer needed to return the id for this session. */
 size_t olm_session_id_length(
-    OlmSession * session
+    const OlmSession * session
 );
 
 /** An identifier for this session. Will be the same for both ends of the
@@ -351,7 +351,7 @@ size_t olm_session_id(
 );
 
 int olm_session_has_received_message(
-    OlmSession *session
+    const OlmSession *session
 );
 
 /**
@@ -402,18 +402,18 @@ size_t olm_remove_one_time_keys(
  * Returns OLM_MESSAGE_TYPE_MESSAGE if the message will be a normal message.
  * Returns olm_error on failure. */
 size_t olm_encrypt_message_type(
-    OlmSession * session
+    const OlmSession * session
 );
 
 /** The number of random bytes needed to encrypt the next message. */
 size_t olm_encrypt_random_length(
-    OlmSession * session
+    const OlmSession * session
 );
 
 /** The size of the next message in bytes for the given number of plain-text
  * bytes. */
 size_t olm_encrypt_message_length(
-    OlmSession * session,
+    const OlmSession * session,
     size_t plaintext_length
 );
 
@@ -464,7 +464,7 @@ size_t olm_decrypt(
 
 /** The length of the buffer needed to hold the SHA-256 hash. */
 size_t olm_sha256_length(
-   OlmUtility * utility
+   const OlmUtility * utility
 );
 
 /** Calculates the SHA-256 hash of the input and encodes it as base64. If the
diff --git a/include/olm/pk.h b/include/olm/pk.h
index ddeedca..a3157a4 100644
--- a/include/olm/pk.h
+++ b/include/olm/pk.h
@@ -62,13 +62,13 @@ size_t olm_pk_encryption_set_recipient_key(
 /** Get the length of the ciphertext that will correspond to a plaintext of the
  * given length. */
 size_t olm_pk_ciphertext_length(
-    OlmPkEncryption *encryption,
+    const OlmPkEncryption *encryption,
     size_t plaintext_length
 );
 
 /** Get the length of the message authentication code. */
 size_t olm_pk_mac_length(
-    OlmPkEncryption *encryption
+    const OlmPkEncryption *encryption
 );
 
 /** Get the length of a public or ephemeral key */
@@ -76,7 +76,7 @@ size_t olm_pk_key_length(void);
 
 /** The number of random bytes needed to encrypt a message. */
 size_t olm_pk_encrypt_random_length(
-    OlmPkEncryption *encryption
+    const OlmPkEncryption *encryption
 );
 
 /** Encrypt a plaintext for the recipient set using
@@ -160,7 +160,7 @@ size_t olm_pk_generate_key(
 
 /** Returns the number of bytes needed to store a decryption object. */
 size_t olm_pickle_pk_decryption_length(
-    OlmPkDecryption * decryption
+    const OlmPkDecryption * decryption
 );
 
 /** Stores decryption object as a base64 string. Encrypts the object using the
@@ -191,7 +191,7 @@ size_t olm_unpickle_pk_decryption(
 /** Get the length of the plaintext that will correspond to a ciphertext of the
  * given length. */
 size_t olm_pk_max_plaintext_length(
-    OlmPkDecryption * decryption,
+    const OlmPkDecryption * decryption,
     size_t ciphertext_length
 );
 
diff --git a/include/olm/ratchet.hh b/include/olm/ratchet.hh
index 4925e37..050ddd8 100644
--- a/include/olm/ratchet.hh
+++ b/include/olm/ratchet.hh
@@ -124,12 +124,12 @@ struct Ratchet {
      * a given message length. */
     std::size_t encrypt_output_length(
         std::size_t plaintext_length
-    );
+    ) const;
 
     /** The number of bytes of random data the encrypt method will need to
      * encrypt a message. This will be 32 bytes if the session needs to
      * generate a new ephemeral key, or will be 0 bytes otherwise.*/
-    std::size_t encrypt_random_length();
+    std::size_t encrypt_random_length() const;
 
     /** Encrypt some plain-text. Returns the length of the encrypted message
      * or std::size_t(-1) on failure. On failure last_error will be set with
diff --git a/include/olm/sas.h b/include/olm/sas.h
index c455435..e9acd62 100644
--- a/include/olm/sas.h
+++ b/include/olm/sas.h
@@ -61,7 +61,7 @@ size_t olm_clear_sas(
 
 /** The number of random bytes needed to create an SAS object. */
 size_t olm_create_sas_random_length(
-    OlmSAS * sas
+    const OlmSAS * sas
 );
 
 /** Creates a new SAS object.
@@ -81,7 +81,7 @@ size_t olm_create_sas(
 );
 
 /** The size of a public key in bytes. */
-size_t olm_sas_pubkey_length(OlmSAS * sas);
+size_t olm_sas_pubkey_length(const OlmSAS * sas);
 
 /** Get the public key for the SAS object.
  *
@@ -119,7 +119,7 @@ size_t olm_sas_set_their_key(
  *
  */
 int olm_sas_is_their_key_set(
-    OlmSAS *sas
+    const OlmSAS *sas
 );
 
 /** Generate bytes to use for the short authentication string.
@@ -144,7 +144,7 @@ size_t olm_sas_generate_bytes(
 /** The size of the message authentication code generated by
  * olm_sas_calculate_mac()`. */
 size_t olm_sas_mac_length(
-    OlmSAS *sas
+    const OlmSAS *sas
 );
 
 /** Generate a message authentication code (MAC) based on the shared secret.
diff --git a/include/olm/session.hh b/include/olm/session.hh
index ce05fc8..5859632 100644
--- a/include/olm/session.hh
+++ b/include/olm/session.hh
@@ -41,7 +41,7 @@ struct Session {
 
     /** The number of random bytes that are needed to create a new outbound
      * session. This will be 64 bytes since two ephemeral keys are needed. */
-    std::size_t new_outbound_session_random_length();
+    std::size_t new_outbound_session_random_length() const;
 
     /** Start a new outbound session. Returns std::size_t(-1) on failure. On
      * failure last_error will be set with an error code. The last_error will be
@@ -64,7 +64,7 @@ struct Session {
     );
 
     /** The number of bytes written by session_id() */
-    std::size_t session_id_length();
+    std::size_t session_id_length() const;
 
     /** An identifier for this session. Generated by hashing the public keys
      * used to create the session. Returns the length of the session id on
@@ -84,21 +84,21 @@ struct Session {
     bool matches_inbound_session(
         _olm_curve25519_public_key const * their_identity_key,
         std::uint8_t const * pre_key_message, std::size_t message_length
-    );
+    ) const;
 
     /** Whether the next message will be a pre-key message or a normal message.
      * An outbound session will send pre-key messages until it receives a
      * message with a ratchet key. */
-    MessageType encrypt_message_type();
+    MessageType encrypt_message_type() const;
 
     std::size_t encrypt_message_length(
         std::size_t plaintext_length
-    );
+    ) const;
 
     /** The number of bytes of random data the encrypt method will need to
      * encrypt a message. This will be 32 bytes if the session needs to
      * generate a new ephemeral key, or will be 0 bytes otherwise. */
-    std::size_t encrypt_random_length();
+    std::size_t encrypt_random_length() const;
 
     /** Encrypt some plain-text. Returns the length of the encrypted message
       * or std::size_t(-1) on failure. On failure last_error will be set with
diff --git a/include/olm/utility.hh b/include/olm/utility.hh
index d650abc..3e5f5b7 100644
--- a/include/olm/utility.hh
+++ b/include/olm/utility.hh
@@ -32,7 +32,7 @@ struct Utility {
     OlmErrorCode last_error;
 
     /** The length of a SHA-256 hash in bytes. */
-    std::size_t sha256_length();
+    std::size_t sha256_length() const;
 
     /** Compute a SHA-256 hash. Returns the length of the SHA-256 hash in bytes
      * on success. Returns std::size_t(-1) on failure. On failure last_error
diff --git a/libolm.version b/libolm.version
index f93b6f2..343b0cf 100644
--- a/libolm.version
+++ b/libolm.version
@@ -1,4 +1,4 @@
 {
-  global: olm*; _olm*; megolm*; _ZN3olm*;
+  global: olm*; _olm*; megolm*; _ZN3olm*; _ZNK3olm*;
   local: *;
 };
diff --git a/src/account.cpp b/src/account.cpp
index d7a3e4c..7b81dc7 100644
--- a/src/account.cpp
+++ b/src/account.cpp
@@ -84,7 +84,7 @@ std::size_t olm::Account::remove_key(
     return std::size_t(-1);
 }
 
-std::size_t olm::Account::new_account_random_length() {
+std::size_t olm::Account::new_account_random_length() const {
     return ED25519_RANDOM_LENGTH + CURVE25519_RANDOM_LENGTH;
 }
 
@@ -120,7 +120,7 @@ static std::uint8_t * write_string(
 }
 
 
-std::size_t olm::Account::get_identity_json_length() {
+std::size_t olm::Account::get_identity_json_length() const {
     std::size_t length = 0;
     length += 1; /* { */
     length += sizeof(KEY_JSON_CURVE25519) - 1;
@@ -172,7 +172,7 @@ std::size_t olm::Account::get_identity_json(
 
 
 std::size_t olm::Account::signature_length(
-) {
+) const {
     return ED25519_SIGNATURE_LENGTH;
 }
 
@@ -193,7 +193,7 @@ std::size_t olm::Account::sign(
 
 
 std::size_t olm::Account::get_one_time_keys_json_length(
-) {
+) const {
     std::size_t length = 0;
     bool is_empty = true;
     for (auto const & key : one_time_keys) {
@@ -267,13 +267,13 @@ std::size_t olm::Account::mark_keys_as_published(
 
 
 std::size_t olm::Account::max_number_of_one_time_keys(
-) {
+) const {
     return olm::MAX_ONE_TIME_KEYS;
 }
 
 std::size_t olm::Account::generate_one_time_keys_random_length(
     std::size_t number_of_keys
-) {
+) const {
     return CURVE25519_RANDOM_LENGTH * number_of_keys;
 }
 
@@ -295,7 +295,7 @@ std::size_t olm::Account::generate_one_time_keys(
     return number_of_keys;
 }
 
-std::size_t olm::Account::generate_fallback_key_random_length() {
+std::size_t olm::Account::generate_fallback_key_random_length() const {
     return CURVE25519_RANDOM_LENGTH;
 }
 
@@ -315,9 +315,9 @@ std::size_t olm::Account::generate_fallback_key(
 
 
 std::size_t olm::Account::get_fallback_key_json_length(
-) {
+) const {
     std::size_t length = 4 + sizeof(KEY_JSON_CURVE25519) - 1; /* {"curve25519":{}} */
-    OneTimeKey & key = current_fallback_key;
+    const OneTimeKey & key = current_fallback_key;
     if (key.published) {
         length += 1; /* " */
         length += olm::encode_base64_length(_olm_pickle_uint32_length(key.id));
diff --git a/src/olm.cpp b/src/olm.cpp
index b8896c7..5024874 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -231,14 +231,14 @@ size_t olm_clear_utility(
 
 
 size_t olm_pickle_account_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return _olm_enc_output_length(pickle_length(*from_c(account)));
 }
 
 
 size_t olm_pickle_session_length(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return _olm_enc_output_length(pickle_length(*from_c(session)));
 }
@@ -334,7 +334,7 @@ size_t olm_unpickle_session(
 
 
 size_t olm_create_account_random_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->new_account_random_length();
 }
@@ -351,7 +351,7 @@ size_t olm_create_account(
 
 
 size_t olm_account_identity_keys_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->get_identity_json_length();
 }
@@ -368,7 +368,7 @@ size_t olm_account_identity_keys(
 
 
 size_t olm_account_signature_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return b64_output_length(from_c(account)->signature_length());
 }
@@ -394,7 +394,7 @@ size_t olm_account_sign(
 
 
 size_t olm_account_one_time_keys_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->get_one_time_keys_json_length();
 }
@@ -418,14 +418,14 @@ size_t olm_account_mark_keys_as_published(
 
 
 size_t olm_account_max_number_of_one_time_keys(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->max_number_of_one_time_keys();
 }
 
 
 size_t olm_account_generate_one_time_keys_random_length(
-    OlmAccount * account,
+    const OlmAccount * account,
     size_t number_of_keys
 ) {
     return from_c(account)->generate_one_time_keys_random_length(number_of_keys);
@@ -447,7 +447,7 @@ size_t olm_account_generate_one_time_keys(
 
 
 size_t olm_account_generate_fallback_key_random_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->generate_fallback_key_random_length();
 }
@@ -466,7 +466,7 @@ size_t olm_account_generate_fallback_key(
 
 
 size_t olm_account_fallback_key_length(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     return from_c(account)->get_fallback_key_json_length();
 }
@@ -483,7 +483,7 @@ size_t olm_account_fallback_key(
 
 
 size_t olm_create_outbound_session_random_length(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return from_c(session)->new_outbound_session_random_length();
 }
@@ -569,7 +569,7 @@ size_t olm_create_inbound_session_from(
 
 
 size_t olm_session_id_length(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return b64_output_length(from_c(session)->session_id_length());
 }
@@ -595,7 +595,7 @@ size_t olm_session_id(
 
 
 int olm_session_has_received_message(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return from_c(session)->received_message;
 }
@@ -666,21 +666,21 @@ size_t olm_remove_one_time_keys(
 
 
 size_t olm_encrypt_message_type(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return size_t(from_c(session)->encrypt_message_type());
 }
 
 
 size_t olm_encrypt_random_length(
-    OlmSession * session
+    const OlmSession * session
 ) {
     return from_c(session)->encrypt_random_length();
 }
 
 
 size_t olm_encrypt_message_length(
-    OlmSession * session,
+    const OlmSession * session,
     size_t plaintext_length
 ) {
     return b64_output_length(
@@ -753,7 +753,7 @@ size_t olm_decrypt(
 
 
 size_t olm_sha256_length(
-   OlmUtility * utility
+   const OlmUtility * utility
 ) {
     return b64_output_length(from_c(utility)->sha256_length());
 }
diff --git a/src/pk.cpp b/src/pk.cpp
index 99882db..70d64a5 100644
--- a/src/pk.cpp
+++ b/src/pk.cpp
@@ -90,7 +90,7 @@ size_t olm_pk_encryption_set_recipient_key (
 }
 
 size_t olm_pk_ciphertext_length(
-    OlmPkEncryption *encryption,
+    const OlmPkEncryption *encryption,
     size_t plaintext_length
 ) {
     return olm::encode_base64_length(
@@ -99,13 +99,13 @@ size_t olm_pk_ciphertext_length(
 }
 
 size_t olm_pk_mac_length(
-    OlmPkEncryption *encryption
+    const OlmPkEncryption *encryption
 ) {
     return olm::encode_base64_length(_olm_cipher_aes_sha_256_ops.mac_length(olm_pk_cipher));
 }
 
 size_t olm_pk_encrypt_random_length(
-    OlmPkEncryption *encryption
+    const OlmPkEncryption *encryption
 ) {
     return CURVE25519_KEY_LENGTH;
 }
@@ -291,7 +291,7 @@ namespace {
 }
 
 size_t olm_pickle_pk_decryption_length(
-    OlmPkDecryption * decryption
+    const OlmPkDecryption * decryption
 ) {
     return _olm_enc_output_length(pickle_length(*decryption));
 }
@@ -355,7 +355,7 @@ size_t olm_unpickle_pk_decryption(
 }
 
 size_t olm_pk_max_plaintext_length(
-    OlmPkDecryption * decryption,
+    const OlmPkDecryption * decryption,
     size_t ciphertext_length
 ) {
     return _olm_cipher_aes_sha_256_ops.decrypt_max_plaintext_length(
diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index 026a105..360a20b 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -399,7 +399,7 @@ std::uint8_t const * olm::unpickle(
 
 std::size_t olm::Ratchet::encrypt_output_length(
     std::size_t plaintext_length
-) {
+) const {
     std::size_t counter = 0;
     if (!sender_chain.empty()) {
         counter = sender_chain[0].chain_key.index;
@@ -414,7 +414,7 @@ std::size_t olm::Ratchet::encrypt_output_length(
 }
 
 
-std::size_t olm::Ratchet::encrypt_random_length() {
+std::size_t olm::Ratchet::encrypt_random_length() const {
     return sender_chain.empty() ? CURVE25519_RANDOM_LENGTH : 0;
 }
 
diff --git a/src/sas.c b/src/sas.c
index 765a4a6..d9cec7e 100644
--- a/src/sas.c
+++ b/src/sas.c
@@ -56,7 +56,7 @@ size_t olm_clear_sas(
     return sizeof(OlmSAS);
 }
 
-size_t olm_create_sas_random_length(OlmSAS * sas) {
+size_t olm_create_sas_random_length(const OlmSAS * sas) {
     return CURVE25519_KEY_LENGTH;
 }
 
@@ -73,7 +73,7 @@ size_t olm_create_sas(
     return 0;
 }
 
-size_t olm_sas_pubkey_length(OlmSAS * sas) {
+size_t olm_sas_pubkey_length(const OlmSAS * sas) {
     return _olm_encode_base64_length(CURVE25519_KEY_LENGTH);
 }
 
@@ -114,7 +114,7 @@ size_t olm_sas_set_their_key(
 }
 
 int olm_sas_is_their_key_set(
-    OlmSAS *sas
+    const OlmSAS *sas
 ) {
     return sas->their_key_set;
 }
@@ -138,7 +138,7 @@ size_t olm_sas_generate_bytes(
 }
 
 size_t olm_sas_mac_length(
-    OlmSAS *sas
+    const OlmSAS *sas
 ) {
     return _olm_encode_base64_length(SHA256_OUTPUT_LENGTH);
 }
diff --git a/src/session.cpp b/src/session.cpp
index b619e56..bd622fc 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() {
+std::size_t olm::Session::new_outbound_session_random_length() const {
     return CURVE25519_RANDOM_LENGTH * 2;
 }
 
@@ -193,7 +193,7 @@ std::size_t olm::Session::new_inbound_session(
 }
 
 
-std::size_t olm::Session::session_id_length() {
+std::size_t olm::Session::session_id_length() const {
     return SHA256_OUTPUT_LENGTH;
 }
 
@@ -218,7 +218,7 @@ std::size_t olm::Session::session_id(
 bool olm::Session::matches_inbound_session(
     _olm_curve25519_public_key const * their_identity_key,
     std::uint8_t const * one_time_key_message, std::size_t message_length
-) {
+) const {
     olm::PreKeyMessageReader reader;
     decode_one_time_key_message(reader, one_time_key_message, message_length);
 
@@ -248,7 +248,7 @@ bool olm::Session::matches_inbound_session(
 }
 
 
-olm::MessageType olm::Session::encrypt_message_type() {
+olm::MessageType olm::Session::encrypt_message_type() const {
     if (received_message) {
         return olm::MessageType::MESSAGE;
     } else {
@@ -259,7 +259,7 @@ olm::MessageType olm::Session::encrypt_message_type() {
 
 std::size_t olm::Session::encrypt_message_length(
     std::size_t plaintext_length
-) {
+) const {
     std::size_t message_length = ratchet.encrypt_output_length(
         plaintext_length
     );
@@ -277,7 +277,7 @@ std::size_t olm::Session::encrypt_message_length(
 }
 
 
-std::size_t olm::Session::encrypt_random_length() {
+std::size_t olm::Session::encrypt_random_length() const {
     return ratchet.encrypt_random_length();
 }
 
diff --git a/src/utility.cpp b/src/utility.cpp
index e9688de..b6bb56e 100644
--- a/src/utility.cpp
+++ b/src/utility.cpp
@@ -22,7 +22,7 @@ olm::Utility::Utility(
 }
 
 
-size_t olm::Utility::sha256_length() {
+size_t olm::Utility::sha256_length() const {
     return SHA256_OUTPUT_LENGTH;
 }