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diff --git a/include/olm/crypto.h b/include/olm/crypto.h
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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.
+ */
+
+/* C-compatible crpyto utility functions. At some point all of crypto.hh will
+ * move here.
+ */
+
+#ifndef OLM_CRYPTO_H_
+#define OLM_CRYPTO_H_
+
+#include <stdint.h>
+#include <stdlib.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** 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
+
+struct _olm_aes256_key {
+    uint8_t key[AES256_KEY_LENGTH];
+};
+
+struct _olm_aes256_iv {
+    uint8_t iv[AES256_IV_LENGTH];
+};
+
+
+struct _olm_curve25519_public_key {
+    uint8_t public_key[CURVE25519_KEY_LENGTH];
+};
+
+struct _olm_curve25519_private_key {
+    uint8_t private_key[CURVE25519_KEY_LENGTH];
+};
+
+struct _olm_curve25519_key_pair {
+    struct _olm_curve25519_public_key public_key;
+    struct _olm_curve25519_private_key private_key;
+};
+
+struct _olm_ed25519_public_key {
+    uint8_t public_key[ED25519_PUBLIC_KEY_LENGTH];
+};
+
+struct _olm_ed25519_private_key {
+    uint8_t private_key[ED25519_PRIVATE_KEY_LENGTH];
+};
+
+struct _olm_ed25519_key_pair {
+    struct _olm_ed25519_public_key public_key;
+    struct _olm_ed25519_private_key private_key;
+};
+
+
+/** The length of output the aes_encrypt_cbc function will write */
+size_t _olm_crypto_aes_encrypt_cbc_length(
+    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 _olm_crypto_aes_encrypt_cbc(
+    const struct _olm_aes256_key *key,
+    const struct _olm_aes256_iv *iv,
+    const uint8_t *input, size_t input_length,
+    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.
+ */
+size_t _olm_crypto_aes_decrypt_cbc(
+    const struct _olm_aes256_key *key,
+    const struct _olm_aes256_iv *iv,
+    uint8_t const * input, size_t input_length,
+    uint8_t * output
+);
+
+
+/** Computes SHA-256 of the input. The output buffer must be a least
+ * SHA256_OUTPUT_LENGTH (32) bytes long. */
+void _olm_crypto_sha256(
+    uint8_t const * input, size_t input_length,
+    uint8_t * output
+);
+
+/** 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 SHA256_OUTPUT_LENGTH (32) bytes long. */
+void _olm_crypto_hmac_sha256(
+    uint8_t const * key, size_t key_length,
+    uint8_t const * input, size_t input_length,
+    uint8_t * output
+);
+
+
+/** HMAC-based Key Derivation Function (HKDF)
+ * https://tools.ietf.org/html/rfc5869
+ * Derives key material from the input bytes. */
+void _olm_crypto_hkdf_sha256(
+    uint8_t const * input, size_t input_length,
+    uint8_t const * info, size_t info_length,
+    uint8_t const * salt, size_t salt_length,
+    uint8_t * output, size_t output_length
+);
+
+
+/** Generate a curve25519 key pair
+ * random_32_bytes should be CURVE25519_RANDOM_LENGTH (32) bytes long.
+ */
+void _olm_crypto_curve25519_generate_key(
+    uint8_t const * random_32_bytes,
+    struct _olm_curve25519_key_pair *output
+);
+
+
+/** Create a shared secret using our private key and their public key.
+ * The output buffer must be at least CURVE25519_SHARED_SECRET_LENGTH (32) bytes long.
+ */
+void _olm_crypto_curve25519_shared_secret(
+    const struct _olm_curve25519_key_pair *our_key,
+    const struct _olm_curve25519_public_key *their_key,
+    uint8_t * output
+);
+
+/** Generate an ed25519 key pair
+ * random_32_bytes should be ED25519_RANDOM_LENGTH (32) bytes long.
+ */
+void _olm_crypto_ed25519_generate_key(
+    uint8_t const * random_bytes,
+    struct _olm_ed25519_key_pair *output
+);
+
+/** Signs the message using our private key.
+ *
+ * The output buffer must be at least ED25519_SIGNATURE_LENGTH (64) bytes
+ * long. */
+void _olm_crypto_ed25519_sign(
+    const struct _olm_ed25519_key_pair *our_key,
+    const uint8_t * message, size_t message_length,
+    uint8_t * output
+);
+
+/** Verify an ed25519 signature
+ * The signature input buffer must be ED25519_SIGNATURE_LENGTH (64) bytes long.
+ * Returns non-zero if the signature is valid. */
+int _olm_crypto_ed25519_verify(
+    const struct _olm_ed25519_public_key *their_key,
+    const uint8_t * message, size_t message_length,
+    const uint8_t * signature
+);
+
+
+
+#ifdef __cplusplus
+} // extern "C"
+#endif
+
+#endif /* OLM_CRYPTO_H_ */