diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0bd5ec8..b377737 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -92,6 +92,7 @@ install(FILES
     ${CMAKE_CURRENT_SOURCE_DIR}/include/olm/inbound_group_session.h
     ${CMAKE_CURRENT_SOURCE_DIR}/include/olm/pk.h
     ${CMAKE_CURRENT_SOURCE_DIR}/include/olm/sas.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/include/olm/error.h
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/olm)
 
 if (UNIX AND NOT APPLE)
diff --git a/include/olm/inbound_group_session.h b/include/olm/inbound_group_session.h
index 9b2f9df..af86b71 100644
--- a/include/olm/inbound_group_session.h
+++ b/include/olm/inbound_group_session.h
@@ -18,6 +18,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "olm/error.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -43,6 +45,13 @@ const char *olm_inbound_group_session_last_error(
     const OlmInboundGroupSession *session
 );
 
+/**
+ * An error code describing the most recent error to happen to a group
+ * session */
+enum OlmErrorCode olm_inbound_group_session_last_error_code(
+    const OlmInboundGroupSession *session
+);
+
 /** Clears the memory used to back this group session */
 size_t olm_clear_inbound_group_session(
     OlmInboundGroupSession *session
diff --git a/include/olm/olm.h b/include/olm/olm.h
index 777007b..0d3ea94 100644
--- a/include/olm/olm.h
+++ b/include/olm/olm.h
@@ -19,6 +19,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "olm/error.h"
 #include "olm/inbound_group_session.h"
 #include "olm/outbound_group_session.h"
 
@@ -71,19 +72,34 @@ size_t olm_error(void);
 /** A null terminated string describing the most recent error to happen to an
  * account */
 const char * olm_account_last_error(
-    OlmAccount * account
+    const OlmAccount * account
+);
+
+/** An error code describing the most recent error to happen to an account */
+enum OlmErrorCode olm_account_last_error_code(
+    const OlmAccount * account
 );
 
 /** A null terminated string describing the most recent error to happen to a
  * session */
 const char * olm_session_last_error(
-    OlmSession * session
+    const OlmSession * session
+);
+
+/** An error code describing the most recent error to happen to a session */
+enum OlmErrorCode olm_session_last_error_code(
+    const OlmSession * session
 );
 
 /** A null terminated string describing the most recent error to happen to a
  * utility */
 const char * olm_utility_last_error(
-    OlmUtility * utility
+    const OlmUtility * utility
+);
+
+/** An error code describing the most recent error to happen to a utility */
+enum OlmErrorCode olm_utility_last_error_code(
+    const OlmUtility * utility
 );
 
 /** Clears the memory used to back this account */
diff --git a/include/olm/outbound_group_session.h b/include/olm/outbound_group_session.h
index eafaf80..fa227b6 100644
--- a/include/olm/outbound_group_session.h
+++ b/include/olm/outbound_group_session.h
@@ -18,6 +18,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "olm/error.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -43,6 +45,13 @@ const char *olm_outbound_group_session_last_error(
     const OlmOutboundGroupSession *session
 );
 
+/**
+ * An error code describing the most recent error to happen to a group
+ * session */
+enum OlmErrorCode olm_outbound_group_session_last_error_code(
+    const OlmOutboundGroupSession *session
+);
+
 /** Clears the memory used to back this group session */
 size_t olm_clear_outbound_group_session(
     OlmOutboundGroupSession *session
diff --git a/include/olm/pk.h b/include/olm/pk.h
index d6476f2..ddeedca 100644
--- a/include/olm/pk.h
+++ b/include/olm/pk.h
@@ -19,6 +19,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "olm/error.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -37,7 +39,13 @@ OlmPkEncryption *olm_pk_encryption(
 /** A null terminated string describing the most recent error to happen to an
  * encryption object */
 const char * olm_pk_encryption_last_error(
-    OlmPkEncryption * encryption
+    const OlmPkEncryption * encryption
+);
+
+/** An error code describing the most recent error to happen to an encryption
+ * object */
+enum OlmErrorCode olm_pk_encryption_last_error_code(
+    const OlmPkEncryption * encryption
 );
 
 /** Clears the memory used to back this encryption object */
@@ -104,7 +112,13 @@ OlmPkDecryption *olm_pk_decryption(
 /** A null terminated string describing the most recent error to happen to a
  * decription object */
 const char * olm_pk_decryption_last_error(
-    OlmPkDecryption * decryption
+    const OlmPkDecryption * decryption
+);
+
+/** An error code describing the most recent error to happen to a decription
+ * object */
+enum OlmErrorCode olm_pk_decryption_last_error_code(
+    const OlmPkDecryption * decryption
 );
 
 /** Clears the memory used to back this decryption object */
@@ -221,7 +235,13 @@ OlmPkSigning *olm_pk_signing(
 /** A null terminated string describing the most recent error to happen to a
  * signing object */
 const char * olm_pk_signing_last_error(
-    OlmPkSigning * sign
+    const OlmPkSigning * sign
+);
+
+/** A null terminated string describing the most recent error to happen to a
+ * signing object */
+enum OlmErrorCode olm_pk_signing_last_error_code(
+    const OlmPkSigning * sign
 );
 
 /** Clears the memory used to back this signing object */
diff --git a/include/olm/sas.h b/include/olm/sas.h
index effa2cf..c455435 100644
--- a/include/olm/sas.h
+++ b/include/olm/sas.h
@@ -19,6 +19,8 @@
 
 #include <stddef.h>
 
+#include "olm/error.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -34,7 +36,13 @@ typedef struct OlmSAS OlmSAS;
 /** A null terminated string describing the most recent error to happen to an
  * SAS object. */
 const char * olm_sas_last_error(
-    OlmSAS * sas
+    const OlmSAS * sas
+);
+
+/** An error code describing the most recent error to happen to an SAS
+ * object. */
+enum OlmErrorCode olm_sas_last_error_code(
+    const OlmSAS * sas
 );
 
 /** The size of an SAS object in bytes. */
diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index 7e96294..4d8e770 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -76,6 +76,12 @@ const char *olm_inbound_group_session_last_error(
     return _olm_error_to_string(session->last_error);
 }
 
+enum OlmErrorCode olm_inbound_group_session_last_error_code(
+    const OlmInboundGroupSession *session
+) {
+    return session->last_error;
+}
+
 size_t olm_clear_inbound_group_session(
     OlmInboundGroupSession *session
 ) {
diff --git a/src/olm.cpp b/src/olm.cpp
index 50742cc..b8896c7 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -42,14 +42,26 @@ static olm::Account * from_c(OlmAccount * account) {
     return reinterpret_cast<olm::Account *>(account);
 }
 
+static const olm::Account * from_c(const OlmAccount * account) {
+    return reinterpret_cast<const olm::Account *>(account);
+}
+
 static olm::Session * from_c(OlmSession * session) {
     return reinterpret_cast<olm::Session *>(session);
 }
 
+static const olm::Session * from_c(const OlmSession * session) {
+    return reinterpret_cast<const olm::Session *>(session);
+}
+
 static olm::Utility * from_c(OlmUtility * utility) {
     return reinterpret_cast<olm::Utility *>(utility);
 }
 
+static const olm::Utility * from_c(const OlmUtility * utility) {
+    return reinterpret_cast<const olm::Utility *>(utility);
+}
+
 static std::uint8_t * from_c(void * bytes) {
     return reinterpret_cast<std::uint8_t *>(bytes);
 }
@@ -110,27 +122,44 @@ size_t olm_error(void) {
 
 
 const char * olm_account_last_error(
-    OlmAccount * account
+    const OlmAccount * account
 ) {
     auto error = from_c(account)->last_error;
     return _olm_error_to_string(error);
 }
 
+enum OlmErrorCode olm_account_last_error_code(
+    const OlmAccount * account
+) {
+    return from_c(account)->last_error;
+}
 
 const char * olm_session_last_error(
-    OlmSession * session
+    const OlmSession * session
 ) {
     auto error = from_c(session)->last_error;
     return _olm_error_to_string(error);
 }
 
+enum OlmErrorCode olm_session_last_error_code(
+    const OlmSession * session
+) {
+    return from_c(session)->last_error;
+}
+
 const char * olm_utility_last_error(
-    OlmUtility * utility
+    const OlmUtility * utility
 ) {
     auto error = from_c(utility)->last_error;
     return _olm_error_to_string(error);
 }
 
+enum OlmErrorCode olm_utility_last_error_code(
+    const OlmUtility * utility
+) {
+    return from_c(utility)->last_error;
+}
+
 size_t olm_account_size(void) {
     return sizeof(olm::Account);
 }
diff --git a/src/outbound_group_session.c b/src/outbound_group_session.c
index 74ce68d..6acad5a 100644
--- a/src/outbound_group_session.c
+++ b/src/outbound_group_session.c
@@ -61,6 +61,12 @@ const char *olm_outbound_group_session_last_error(
     return _olm_error_to_string(session->last_error);
 }
 
+enum OlmErrorCode olm_outbound_group_session_last_error_code(
+    const OlmOutboundGroupSession *session
+) {
+    return session->last_error;
+}
+
 size_t olm_clear_outbound_group_session(
     OlmOutboundGroupSession *session
 ) {
diff --git a/src/pk.cpp b/src/pk.cpp
index 05d5db1..99882db 100644
--- a/src/pk.cpp
+++ b/src/pk.cpp
@@ -37,12 +37,18 @@ struct OlmPkEncryption {
 };
 
 const char * olm_pk_encryption_last_error(
-    OlmPkEncryption * encryption
+    const OlmPkEncryption * encryption
 ) {
     auto error = encryption->last_error;
     return _olm_error_to_string(error);
 }
 
+OlmErrorCode olm_pk_encryption_last_error_code(
+    const OlmPkEncryption * encryption
+) {
+    return encryption->last_error;
+}
+
 size_t olm_pk_encryption_size(void) {
     return sizeof(OlmPkEncryption);
 }
@@ -162,12 +168,18 @@ struct OlmPkDecryption {
 };
 
 const char * olm_pk_decryption_last_error(
-    OlmPkDecryption * decryption
+    const OlmPkDecryption * decryption
 ) {
     auto error = decryption->last_error;
     return _olm_error_to_string(error);
 }
 
+OlmErrorCode olm_pk_decryption_last_error_code(
+    const OlmPkDecryption * decryption
+) {
+    return decryption->last_error;
+}
+
 size_t olm_pk_decryption_size(void) {
     return sizeof(OlmPkDecryption);
 }
@@ -446,11 +458,15 @@ OlmPkSigning *olm_pk_signing(void * memory) {
     return new(memory) OlmPkSigning;
 }
 
-const char * olm_pk_signing_last_error(OlmPkSigning * sign) {
+const char * olm_pk_signing_last_error(const OlmPkSigning * sign) {
     auto error = sign->last_error;
     return _olm_error_to_string(error);
 }
 
+OlmErrorCode olm_pk_signing_last_error_code(const OlmPkSigning * sign) {
+    return sign->last_error;
+}
+
 size_t olm_clear_pk_signing(OlmPkSigning *sign) {
     /* Clear the memory backing the signing */
     olm::unset(sign, sizeof(OlmPkSigning));
diff --git a/src/sas.c b/src/sas.c
index 9a84233..765a4a6 100644
--- a/src/sas.c
+++ b/src/sas.c
@@ -27,11 +27,17 @@ struct OlmSAS {
 };
 
 const char * olm_sas_last_error(
-    OlmSAS * sas
+    const OlmSAS * sas
 ) {
     return _olm_error_to_string(sas->last_error);
 }
 
+enum OlmErrorCode olm_sas_last_error_code(
+    const OlmSAS * sas
+) {
+    return sas->last_error;
+}
+
 size_t olm_sas_size(void) {
     return sizeof(OlmSAS);
 }