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diff --git a/include/olm/crypto.hh b/include/olm/crypto.hh
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+/* 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_CRYPTO_HH_
+#define OLM_CRYPTO_HH_
+
+#include <cstdint>
+#include <cstddef>
+
+namespace olm {
+
+
+struct Curve25519PublicKey {
+    static const int LENGTH = 32;
+    std::uint8_t public_key[32];
+};
+
+
+struct Curve25519KeyPair : public Curve25519PublicKey {
+    static const int LENGTH = 64;
+    std::uint8_t private_key[32];
+};
+
+
+/** Generate a curve25519 key pair from 32 random bytes. */
+void generate_key(
+    std::uint8_t const * random_32_bytes,
+    Curve25519KeyPair & key_pair
+);
+
+
+const std::size_t CURVE25519_SHARED_SECRET_LENGTH = 32;
+
+
+/** Create a shared secret using our private key and their public key.
+ * The output buffer must be at least 32 bytes long. */
+void curve25519_shared_secret(
+    Curve25519KeyPair const & our_key,
+    Curve25519PublicKey const & their_key,
+    std::uint8_t * output
+);
+
+
+/** Signs the message using our private key.
+ * The output buffer must be at least 64 bytes long. */
+void curve25519_sign(
+    Curve25519KeyPair const & our_key,
+    std::uint8_t const * message, std::size_t message_length,
+    std::uint8_t * output
+);
+
+
+/** Verify thei message using their public key.
+ * The signature input buffer must be 64 bytes long.
+ * Returns true if the signature is valid. */
+bool curve25519_verify(
+    Curve25519PublicKey const & their_key,
+    std::uint8_t const * message, std::size_t message_length,
+    std::uint8_t const * signature
+);
+
+
+struct Aes256Key {
+    static const int LENGTH = 32;
+    std::uint8_t key[32];
+};
+
+
+struct Aes256Iv {
+    static const int LENGTH = 16;
+    std::uint8_t iv[16];
+};
+
+
+/** The length of output the aes_encrypt_cbc function will write */
+std::size_t aes_encrypt_cbc_length(
+    std::size_t input_length
+);
+
+
+/** Encrypts the input using AES256 in CBC mode with PKCS#7 padding.
+ * The output buffer must be big enough to hold the output including padding */
+void aes_encrypt_cbc(
+    Aes256Key const & key,
+    Aes256Iv const & iv,
+    std::uint8_t const * input, std::size_t input_length,
+    std::uint8_t * output
+);
+
+
+/** Decrypts the input using AES256 in CBC mode. The output buffer must be at
+ * least the same size as the input buffer. Returns the length of the plaintext
+ * without padding on success or std::size_t(-1) if the padding is invalid.
+ */
+std::size_t aes_decrypt_cbc(
+    Aes256Key const & key,
+    Aes256Iv const & iv,
+    std::uint8_t const * input, std::size_t input_length,
+    std::uint8_t * output
+);
+
+
+/** Computes SHA-256 of the input. The output buffer must be a least 32
+ * bytes long. */
+void sha256(
+    std::uint8_t const * input, std::size_t input_length,
+    std::uint8_t * output
+);
+
+
+const std::size_t HMAC_SHA256_OUTPUT_LENGTH = 32;
+
+
+/** HMAC: Keyed-Hashing for Message Authentication
+ * http://tools.ietf.org/html/rfc2104
+ * Computes HMAC-SHA-256 of the input for the key. The output buffer must
+ * be at least 32 bytes long. */
+void hmac_sha256(
+    std::uint8_t const * key, std::size_t key_length,
+    std::uint8_t const * input, std::size_t input_length,
+    std::uint8_t * output
+);
+
+
+/** HMAC-based Key Derivation Function (HKDF)
+ * https://tools.ietf.org/html/rfc5869
+ * Derives key material from the input bytes. */
+void hkdf_sha256(
+    std::uint8_t const * input, std::size_t input_length,
+    std::uint8_t const * info, std::size_t info_length,
+    std::uint8_t const * salt, std::size_t salt_length,
+    std::uint8_t * output, std::size_t output_length
+);
+
+} // namespace olm
+
+#endif /* OLM_CRYPTO_HH_ */