Convert cipher.hh to plain C

8 files changed, 263 insertions, 210 deletions

diff --git a/include/olm/cipher.h b/include/olm/cipher.h
new file mode 100644
index 0000000..0d6fd5b
--- /dev/null
+++ b/include/olm/cipher.h
@@ -0,0 +1,134 @@
+/* Copyright 2015 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef OLM_CIPHER_H_
+#define OLM_CIPHER_H_
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct olm_cipher;
+
+struct cipher_ops {
+    /**
+     * Returns the length of the message authentication code that will be
+     * appended to the output.
+     */
+    size_t (*mac_length)(const struct olm_cipher *cipher);
+
+    /**
+     * Returns the length of cipher-text for a given length of plain-text.
+     */
+    size_t (*encrypt_ciphertext_length)(const struct olm_cipher *cipher,
+                                        size_t plaintext_length);
+
+    /*
+     * Encrypts the plain-text into the output buffer and authenticates the
+     * contents of the output buffer covering both cipher-text and any other
+     * associated data in the output buffer.
+     *
+     *  |---------------------------------------output_length-->|
+     *  output  |--ciphertext_length-->|       |---mac_length-->|
+     *          ciphertext
+     *
+     * The plain-text pointers and cipher-text pointers may be the same.
+     *
+     * Returns size_t(-1) if the length of the cipher-text or the output
+     * buffer is too small. Otherwise returns the length of the output buffer.
+     */
+    size_t (*encrypt)(
+        const struct olm_cipher *cipher,
+        uint8_t const * key, size_t key_length,
+        uint8_t const * plaintext, size_t plaintext_length,
+        uint8_t * ciphertext, size_t ciphertext_length,
+        uint8_t * output, size_t output_length
+    );
+
+    /**
+     * Returns the maximum length of plain-text that a given length of
+     * cipher-text can contain.
+     */
+    size_t (*decrypt_max_plaintext_length)(
+        const struct olm_cipher *cipher,
+        size_t ciphertext_length
+    );
+
+    /**
+     * Authenticates the input and decrypts the cipher-text into the plain-text
+     * buffer.
+     *
+     *  |----------------------------------------input_length-->|
+     *  input   |--ciphertext_length-->|       |---mac_length-->|
+     *          ciphertext
+     *
+     * The plain-text pointers and cipher-text pointers may be the same.
+     *
+     *  Returns size_t(-1) if the length of the plain-text buffer is too
+     *  small or if the authentication check fails. Otherwise returns the length
+     *  of the plain text.
+     */
+    size_t (*decrypt)(
+        const struct olm_cipher *cipher,
+        uint8_t const * key, size_t key_length,
+        uint8_t const * input, size_t input_length,
+        uint8_t const * ciphertext, size_t ciphertext_length,
+        uint8_t * plaintext, size_t max_plaintext_length
+    );
+
+    /** destroy any private data associated with this cipher */
+    void (*destruct)(struct olm_cipher *cipher);
+};
+
+struct olm_cipher {
+    const struct cipher_ops *ops;
+    /* cipher-specific fields follow */
+};
+
+struct olm_cipher_aes_sha_256 {
+    struct olm_cipher base_cipher;
+
+    uint8_t const * kdf_info;
+    size_t kdf_info_length;
+};
+
+
+/**
+ * initialises a cipher type which uses AES256 for encryption and SHA256 for
+ * authentication.
+ *
+ * cipher: structure to be initialised
+ *
+ * kdf_info: context string for the HKDF used for deriving the AES256 key, HMAC
+ * key, and AES IV, from the key material passed to encrypt/decrypt. Note that
+ * this is NOT copied so must have a lifetime at least as long as the cipher
+ * instance.
+ *
+ * kdf_info_length: length of context string kdf_info
+ */
+struct olm_cipher *olm_cipher_aes_sha_256_init(
+    struct olm_cipher_aes_sha_256 *cipher,
+    uint8_t const * kdf_info,
+    size_t kdf_info_length);
+
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* OLM_CIPHER_H_ */
diff --git a/include/olm/cipher.hh b/include/olm/cipher.hh
deleted file mode 100644
index c561972..0000000
--- a/include/olm/cipher.hh
+++ /dev/null
@@ -1,132 +0,0 @@
-/* Copyright 2015 OpenMarket Ltd
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#ifndef OLM_CIPHER_HH_
-#define OLM_CIPHER_HH_
-
-#include <cstdint>
-#include <cstddef>
-
-namespace olm {
-
-class Cipher {
-public:
-    virtual ~Cipher();
-
-    /**
-     * Returns the length of the message authentication code that will be
-     * appended to the output.
-     */
-    virtual std::size_t mac_length() const = 0;
-
-    /**
-     * Returns the length of cipher-text for a given length of plain-text.
-     */
-    virtual std::size_t encrypt_ciphertext_length(
-        std::size_t plaintext_length
-    ) const = 0;
-
-    /*
-     * Encrypts the plain-text into the output buffer and authenticates the
-     * contents of the output buffer covering both cipher-text and any other
-     * associated data in the output buffer.
-     *
-     *  |---------------------------------------output_length-->|
-     *  output  |--ciphertext_length-->|       |---mac_length-->|
-     *          ciphertext
-     *
-     * The plain-text pointers and cipher-text pointers may be the same.
-     *
-     * Returns std::size_t(-1) if the length of the cipher-text or the output
-     * buffer is too small. Otherwise returns the length of the output buffer.
-     */
-    virtual std::size_t encrypt(
-        std::uint8_t const * key, std::size_t key_length,
-        std::uint8_t const * plaintext, std::size_t plaintext_length,
-        std::uint8_t * ciphertext, std::size_t ciphertext_length,
-        std::uint8_t * output, std::size_t output_length
-    ) const = 0;
-
-    /**
-     * Returns the maximum length of plain-text that a given length of
-     * cipher-text can contain.
-     */
-    virtual std::size_t decrypt_max_plaintext_length(
-        std::size_t ciphertext_length
-    ) const = 0;
-
-    /**
-     * Authenticates the input and decrypts the cipher-text into the plain-text
-     * buffer.
-     *
-     *  |----------------------------------------input_length-->|
-     *  input   |--ciphertext_length-->|       |---mac_length-->|
-     *          ciphertext
-     *
-     * The plain-text pointers and cipher-text pointers may be the same.
-     *
-     *  Returns std::size_t(-1) if the length of the plain-text buffer is too
-     *  small or if the authentication check fails. Otherwise returns the length
-     *  of the plain text.
-     */
-    virtual std::size_t decrypt(
-       std::uint8_t const * key, std::size_t key_length,
-       std::uint8_t const * input, std::size_t input_length,
-       std::uint8_t const * ciphertext, std::size_t ciphertext_length,
-       std::uint8_t * plaintext, std::size_t max_plaintext_length
-    ) const = 0;
-};
-
-
-class CipherAesSha256 : public Cipher {
-public:
-    CipherAesSha256(
-        std::uint8_t const * kdf_info, std::size_t kdf_info_length
-    );
-
-    virtual std::size_t mac_length() const;
-
-    virtual std::size_t encrypt_ciphertext_length(
-        std::size_t plaintext_length
-    ) const;
-
-    virtual std::size_t encrypt(
-        std::uint8_t const * key, std::size_t key_length,
-        std::uint8_t const * plaintext, std::size_t plaintext_length,
-        std::uint8_t * ciphertext, std::size_t ciphertext_length,
-        std::uint8_t * output, std::size_t output_length
-    ) const;
-
-    virtual std::size_t decrypt_max_plaintext_length(
-        std::size_t ciphertext_length
-    ) const;
-
-    virtual std::size_t decrypt(
-        std::uint8_t const * key, std::size_t key_length,
-        std::uint8_t const * input, std::size_t input_length,
-        std::uint8_t const * ciphertext, std::size_t ciphertext_length,
-        std::uint8_t * plaintext, std::size_t max_plaintext_length
-    ) const;
-
-private:
-    std::uint8_t const * kdf_info;
-    std::size_t kdf_info_length;
-};
-
-
-} // namespace
-
-
-#endif /* OLM_CIPHER_HH_ */
diff --git a/include/olm/ratchet.hh b/include/olm/ratchet.hh
index b2787c7..e1d462d 100644
--- a/include/olm/ratchet.hh
+++ b/include/olm/ratchet.hh
@@ -17,9 +17,9 @@
 #include "olm/list.hh"
 #include "olm/error.h"
 
-namespace olm {
+struct olm_cipher;
 
-class Cipher;
+namespace olm {
 
 typedef std::uint8_t SharedKey[olm::KEY_LENGTH];
 
@@ -69,14 +69,14 @@ struct Ratchet {
 
     Ratchet(
         KdfInfo const & kdf_info,
-        Cipher const & ratchet_cipher
+        olm_cipher const *ratchet_cipher
     );
 
     /** A some strings identifying the application to feed into the KDF. */
     KdfInfo const & kdf_info;
 
     /** The AEAD cipher to use for encrypting messages. */
-    Cipher const & ratchet_cipher;
+    olm_cipher const *ratchet_cipher;
 
     /** The last error that happened encrypting or decrypting a message. */
     OlmErrorCode last_error;
diff --git a/src/cipher.cpp b/src/cipher.cpp
index a550312..8c56efa 100644
--- a/src/cipher.cpp
+++ b/src/cipher.cpp
@@ -12,15 +12,11 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-#include "olm/cipher.hh"
+#include "olm/cipher.h"
 #include "olm/crypto.hh"
 #include "olm/memory.hh"
 #include <cstring>
 
-olm::Cipher::~Cipher() {
-
-}
-
 namespace {
 
 struct DerivedKeys {
@@ -51,41 +47,34 @@ static void derive_keys(
 
 static const std::size_t MAC_LENGTH = 8;
 
-} // namespace
-
-
-olm::CipherAesSha256::CipherAesSha256(
-    std::uint8_t const * kdf_info, std::size_t kdf_info_length
-) : kdf_info(kdf_info), kdf_info_length(kdf_info_length) {
-
-}
-
-
-std::size_t olm::CipherAesSha256::mac_length() const {
+size_t aes_sha_256_cipher_mac_length(const struct olm_cipher *cipher) {
     return MAC_LENGTH;
 }
 
-
-std::size_t olm::CipherAesSha256::encrypt_ciphertext_length(
-    std::size_t plaintext_length
-) const {
+size_t aes_sha_256_cipher_encrypt_ciphertext_length(
+        const struct olm_cipher *cipher, size_t plaintext_length
+) {
     return olm::aes_encrypt_cbc_length(plaintext_length);
 }
 
+size_t aes_sha_256_cipher_encrypt(
+    const struct olm_cipher *cipher,
+    uint8_t const * key, size_t key_length,
+    uint8_t const * plaintext, size_t plaintext_length,
+    uint8_t * ciphertext, size_t ciphertext_length,
+    uint8_t * output, size_t output_length
+) {
+    auto *c = reinterpret_cast<const olm_cipher_aes_sha_256 *>(cipher);
 
-std::size_t olm::CipherAesSha256::encrypt(
-    std::uint8_t const * key, std::size_t key_length,
-    std::uint8_t const * plaintext, std::size_t plaintext_length,
-    std::uint8_t * ciphertext, std::size_t ciphertext_length,
-    std::uint8_t * output, std::size_t output_length
-) const {
-    if (encrypt_ciphertext_length(plaintext_length) < ciphertext_length) {
+    if (aes_sha_256_cipher_encrypt_ciphertext_length(cipher, plaintext_length)
+            < ciphertext_length) {
         return std::size_t(-1);
     }
+
     struct DerivedKeys keys;
     std::uint8_t mac[SHA256_OUTPUT_LENGTH];
 
-    derive_keys(kdf_info, kdf_info_length, key, key_length, keys);
+    derive_keys(c->kdf_info, c->kdf_info_length, key, key_length, keys);
 
     olm::aes_encrypt_cbc(
         keys.aes_key, keys.aes_iv, plaintext, plaintext_length, ciphertext
@@ -102,22 +91,26 @@ std::size_t olm::CipherAesSha256::encrypt(
 }
 
 
-std::size_t olm::CipherAesSha256::decrypt_max_plaintext_length(
-    std::size_t ciphertext_length
-) const {
+size_t aes_sha_256_cipher_decrypt_max_plaintext_length(
+    const struct olm_cipher *cipher,
+    size_t ciphertext_length
+) {
     return ciphertext_length;
 }
 
-std::size_t olm::CipherAesSha256::decrypt(
-     std::uint8_t const * key, std::size_t key_length,
-     std::uint8_t const * input, std::size_t input_length,
-     std::uint8_t const * ciphertext, std::size_t ciphertext_length,
-     std::uint8_t * plaintext, std::size_t max_plaintext_length
-) const {
+size_t aes_sha_256_cipher_decrypt(
+    const struct olm_cipher *cipher,
+    uint8_t const * key, size_t key_length,
+    uint8_t const * input, size_t input_length,
+    uint8_t const * ciphertext, size_t ciphertext_length,
+    uint8_t * plaintext, size_t max_plaintext_length
+) {
+    auto *c = reinterpret_cast<const olm_cipher_aes_sha_256 *>(cipher);
+
     DerivedKeys keys;
     std::uint8_t mac[SHA256_OUTPUT_LENGTH];
 
-    derive_keys(kdf_info, kdf_info_length, key, key_length, keys);
+    derive_keys(c->kdf_info, c->kdf_info_length, key, key_length, keys);
 
     crypto_hmac_sha256(
         keys.mac_key, olm::KEY_LENGTH, input, input_length - MAC_LENGTH, mac
@@ -136,3 +129,30 @@ std::size_t olm::CipherAesSha256::decrypt(
     olm::unset(keys);
     return plaintext_length;
 }
+
+
+void aes_sha_256_cipher_destruct(struct olm_cipher *cipher) {
+}
+
+
+const cipher_ops aes_sha_256_cipher_ops = {
+  aes_sha_256_cipher_mac_length,
+  aes_sha_256_cipher_encrypt_ciphertext_length,
+  aes_sha_256_cipher_encrypt,
+  aes_sha_256_cipher_decrypt_max_plaintext_length,
+  aes_sha_256_cipher_decrypt,
+  aes_sha_256_cipher_destruct
+};
+
+} // namespace
+
+
+olm_cipher *olm_cipher_aes_sha_256_init(struct olm_cipher_aes_sha_256 *cipher,
+                                        uint8_t const * kdf_info,
+                                        size_t kdf_info_length)
+{
+    cipher->base_cipher.ops = &aes_sha_256_cipher_ops;
+    cipher->kdf_info = kdf_info;
+    cipher->kdf_info_length = kdf_info_length;
+    return &(cipher->base_cipher);
+}
diff --git a/src/olm.cpp b/src/olm.cpp
index 56bb11f..9d84758 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -15,9 +15,9 @@
 #include "olm/olm.h"
 #include "olm/session.hh"
 #include "olm/account.hh"
+#include "olm/cipher.h"
 #include "olm/utility.hh"
 #include "olm/base64.hh"
-#include "olm/cipher.hh"
 #include "olm/memory.hh"
 
 #include <new>
@@ -59,15 +59,24 @@ static std::uint8_t const * from_c(void const * bytes) {
 
 static const std::uint8_t CIPHER_KDF_INFO[] = "Pickle";
 
-static const olm::CipherAesSha256 PICKLE_CIPHER(
-    CIPHER_KDF_INFO, sizeof(CIPHER_KDF_INFO) -1
-);
+const olm_cipher *get_pickle_cipher() {
+    static olm_cipher *cipher = NULL;
+    static olm_cipher_aes_sha_256 PICKLE_CIPHER;
+    if (!cipher) {
+        cipher = olm_cipher_aes_sha_256_init(
+            &PICKLE_CIPHER,
+            CIPHER_KDF_INFO, sizeof(CIPHER_KDF_INFO) - 1
+        );
+    }
+    return cipher;
+}
 
 std::size_t enc_output_length(
     size_t raw_length
 ) {
-    std::size_t length = PICKLE_CIPHER.encrypt_ciphertext_length(raw_length);
-    length += PICKLE_CIPHER.mac_length();
+    auto *cipher = get_pickle_cipher();
+    std::size_t length = cipher->ops->encrypt_ciphertext_length(cipher, raw_length);
+    length += cipher->ops->mac_length(cipher);
     return olm::encode_base64_length(length);
 }
 
@@ -76,8 +85,9 @@ std::uint8_t * enc_output_pos(
     std::uint8_t * output,
     size_t raw_length
 ) {
-    std::size_t length = PICKLE_CIPHER.encrypt_ciphertext_length(raw_length);
-    length += PICKLE_CIPHER.mac_length();
+    auto *cipher = get_pickle_cipher();
+    std::size_t length = cipher->ops->encrypt_ciphertext_length(cipher, raw_length);
+    length += cipher->ops->mac_length(cipher);
     return output + olm::encode_base64_length(length) - length;
 }
 
@@ -85,13 +95,15 @@ std::size_t enc_output(
     std::uint8_t const * key, std::size_t key_length,
     std::uint8_t * output, size_t raw_length
 ) {
-    std::size_t ciphertext_length = PICKLE_CIPHER.encrypt_ciphertext_length(
-        raw_length
+    auto *cipher = get_pickle_cipher();
+    std::size_t ciphertext_length = cipher->ops->encrypt_ciphertext_length(
+        cipher, raw_length
     );
-    std::size_t length = ciphertext_length + PICKLE_CIPHER.mac_length();
+    std::size_t length = ciphertext_length + cipher->ops->mac_length(cipher);
     std::size_t base64_length = olm::encode_base64_length(length);
     std::uint8_t * raw_output = output + base64_length - length;
-    PICKLE_CIPHER.encrypt(
+    cipher->ops->encrypt(
+        cipher,
         key, key_length,
         raw_output, raw_length,
         raw_output, ciphertext_length,
@@ -112,8 +124,10 @@ std::size_t enc_input(
         return std::size_t(-1);
     }
     olm::decode_base64(input, b64_length, input);
-    std::size_t raw_length = enc_length - PICKLE_CIPHER.mac_length();
-    std::size_t result = PICKLE_CIPHER.decrypt(
+    auto *cipher = get_pickle_cipher();
+    std::size_t raw_length = enc_length - cipher->ops->mac_length(cipher);
+    std::size_t result = cipher->ops->decrypt(
+        cipher,
         key, key_length,
         input, enc_length,
         input, raw_length,
diff --git a/src/ratchet.cpp b/src/ratchet.cpp
index 56ea106..de46be4 100644
--- a/src/ratchet.cpp
+++ b/src/ratchet.cpp
@@ -15,7 +15,7 @@
 #include "olm/ratchet.hh"
 #include "olm/message.hh"
 #include "olm/memory.hh"
-#include "olm/cipher.hh"
+#include "olm/cipher.h"
 #include "olm/pickle.hh"
 
 #include <cstring>
@@ -94,12 +94,13 @@ static void create_message_keys(
 
 
 static std::size_t verify_mac_and_decrypt(
-    olm::Cipher const & cipher,
+    olm_cipher const *cipher,
     olm::MessageKey const & message_key,
     olm::MessageReader const & reader,
     std::uint8_t * plaintext, std::size_t max_plaintext_length
 ) {
-    return cipher.decrypt(
+    return cipher->ops->decrypt(
+        cipher,
         message_key.key, sizeof(message_key.key),
         reader.input, reader.input_length,
         reader.ciphertext, reader.ciphertext_length,
@@ -183,7 +184,7 @@ static std::size_t verify_mac_and_decrypt_for_new_chain(
 
 olm::Ratchet::Ratchet(
     olm::KdfInfo const & kdf_info,
-    Cipher const & ratchet_cipher
+    olm_cipher const * ratchet_cipher
 ) : kdf_info(kdf_info),
     ratchet_cipher(ratchet_cipher),
     last_error(OlmErrorCode::OLM_SUCCESS) {
@@ -405,11 +406,12 @@ std::size_t olm::Ratchet::encrypt_output_length(
     if (!sender_chain.empty()) {
         counter = sender_chain[0].chain_key.index;
     }
-    std::size_t padded = ratchet_cipher.encrypt_ciphertext_length(
+    std::size_t padded = ratchet_cipher->ops->encrypt_ciphertext_length(
+        ratchet_cipher,
         plaintext_length
     );
     return olm::encode_message_length(
-        counter, olm::KEY_LENGTH, padded, ratchet_cipher.mac_length()
+        counter, olm::KEY_LENGTH, padded, ratchet_cipher->ops->mac_length(ratchet_cipher)
     );
 }
 
@@ -452,7 +454,8 @@ std::size_t olm::Ratchet::encrypt(
     create_message_keys(chain_index, sender_chain[0].chain_key, kdf_info, keys);
     advance_chain_key(chain_index, sender_chain[0].chain_key, sender_chain[0].chain_key);
 
-    std::size_t ciphertext_length = ratchet_cipher.encrypt_ciphertext_length(
+    std::size_t ciphertext_length = ratchet_cipher->ops->encrypt_ciphertext_length(
+        ratchet_cipher,
         plaintext_length
     );
     std::uint32_t counter = keys.index;
@@ -467,7 +470,8 @@ std::size_t olm::Ratchet::encrypt(
 
     olm::store_array(writer.ratchet_key, ratchet_key.public_key);
 
-    ratchet_cipher.encrypt(
+    ratchet_cipher->ops->encrypt(
+        ratchet_cipher,
         keys.key, sizeof(keys.key),
         plaintext, plaintext_length,
         writer.ciphertext, ciphertext_length,
@@ -484,7 +488,8 @@ std::size_t olm::Ratchet::decrypt_max_plaintext_length(
 ) {
     olm::MessageReader reader;
     olm::decode_message(
-        reader, input, input_length, ratchet_cipher.mac_length()
+        reader, input, input_length,
+        ratchet_cipher->ops->mac_length(ratchet_cipher)
     );
 
     if (!reader.ciphertext) {
@@ -492,7 +497,8 @@ std::size_t olm::Ratchet::decrypt_max_plaintext_length(
         return std::size_t(-1);
     }
 
-    return ratchet_cipher.decrypt_max_plaintext_length(reader.ciphertext_length);
+    return ratchet_cipher->ops->decrypt_max_plaintext_length(
+        ratchet_cipher, reader.ciphertext_length);
 }
 
 
@@ -502,7 +508,8 @@ std::size_t olm::Ratchet::decrypt(
 ) {
     olm::MessageReader reader;
     olm::decode_message(
-        reader, input, input_length, ratchet_cipher.mac_length()
+        reader, input, input_length,
+        ratchet_cipher->ops->mac_length(ratchet_cipher)
     );
 
     if (reader.version != PROTOCOL_VERSION) {
@@ -515,7 +522,8 @@ std::size_t olm::Ratchet::decrypt(
         return std::size_t(-1);
     }
 
-    std::size_t max_length = ratchet_cipher.decrypt_max_plaintext_length(
+    std::size_t max_length = ratchet_cipher->ops->decrypt_max_plaintext_length(
+        ratchet_cipher,
         reader.ciphertext_length
     );
 
diff --git a/src/session.cpp b/src/session.cpp
index c0b6cf4..0d9b58a 100644
--- a/src/session.cpp
+++ b/src/session.cpp
@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 #include "olm/session.hh"
-#include "olm/cipher.hh"
+#include "olm/cipher.h"
 #include "olm/crypto.hh"
 #include "olm/account.hh"
 #include "olm/memory.hh"
@@ -30,19 +30,27 @@ static const std::uint8_t ROOT_KDF_INFO[] = "OLM_ROOT";
 static const std::uint8_t RATCHET_KDF_INFO[] = "OLM_RATCHET";
 static const std::uint8_t CIPHER_KDF_INFO[] = "OLM_KEYS";
 
-static const olm::CipherAesSha256 OLM_CIPHER(
-    CIPHER_KDF_INFO, sizeof(CIPHER_KDF_INFO) -1
-);
-
 static const olm::KdfInfo OLM_KDF_INFO = {
     ROOT_KDF_INFO, sizeof(ROOT_KDF_INFO) - 1,
     RATCHET_KDF_INFO, sizeof(RATCHET_KDF_INFO) - 1
 };
 
+const olm_cipher *get_cipher() {
+    static olm_cipher *cipher;
+    static olm_cipher_aes_sha_256 OLM_CIPHER;
+    if (!cipher) {
+        cipher = olm_cipher_aes_sha_256_init(
+            &OLM_CIPHER,
+            CIPHER_KDF_INFO, sizeof(CIPHER_KDF_INFO) - 1
+        );
+    }
+    return cipher;
+}
+
 } // namespace
 
 olm::Session::Session(
-) : ratchet(OLM_KDF_INFO, OLM_CIPHER),
+) : ratchet(OLM_KDF_INFO, get_cipher()),
     last_error(OlmErrorCode::OLM_SUCCESS),
     received_message(false) {
 
@@ -149,7 +157,7 @@ std::size_t olm::Session::new_inbound_session(
     olm::MessageReader message_reader;
     decode_message(
         message_reader, reader.message, reader.message_length,
-        ratchet.ratchet_cipher.mac_length()
+        ratchet.ratchet_cipher->ops->mac_length(ratchet.ratchet_cipher)
     );
 
     if (!message_reader.ratchet_key
diff --git a/tests/test_ratchet.cpp b/tests/test_ratchet.cpp
index 8f89048..2c8bc1b 100644
--- a/tests/test_ratchet.cpp
+++ b/tests/test_ratchet.cpp
@@ -13,7 +13,7 @@
  * limitations under the License.
  */
 #include "olm/ratchet.hh"
-#include "olm/cipher.hh"
+#include "olm/cipher.h"
 #include "unittest.hh"
 
 
@@ -28,8 +28,9 @@ olm::KdfInfo kdf_info = {
     ratchet_info, sizeof(ratchet_info) - 1
 };
 
-olm::CipherAesSha256 cipher(
-    message_info, sizeof(message_info) - 1
+olm_cipher_aes_sha_256 cipher0;
+olm_cipher *cipher = olm_cipher_aes_sha_256_init(
+    &cipher0, message_info, sizeof(message_info) - 1
 );
 
 std::uint8_t random_bytes[] = "0123456789ABDEF0123456789ABCDEF";