diff --git a/presence/implementation/np_ldt.h b/presence/implementation/np_ldt.h
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+++ b/presence/implementation/np_ldt.h
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+// Copyright 2022 Google LLC
+//
+// 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
+//
+//     https://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 API for Rust implementation of LDT [1], tailored to Nearby Presence's
+// BLE 4.2 legacy format advertisement parsing usecase.
+//
+// [1] https://eprint.iacr.org/2017/841.pdf
+
+// TODO pluggable memory allocation for embedded
+
+// TODO include guard name based on final file location
+#ifndef NP_LDT_H_
+#define NP_LDT_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <stddef.h>
+#include <stdint.h>
+
+// Individual encrypt/decrypt API, useful when creating advertisements or when
+// decrypting advertisements from a known origin
+
+// Handle for accessing the rust ldt implementation apis
+typedef void* NpLdtHandle;
+
+// Key material from the Nearby Presence credential from which keys will be
+// derived.
+typedef struct {
+  uint8_t bytes[32];
+} NpLdtKeySeed;
+
+typedef struct {
+  uint8_t bytes[32];
+} NpMetadataKeyHmac;
+
+typedef struct {
+  uint8_t bytes[16];
+} NpLdtAes128Key;
+
+typedef struct {
+  uint8_t bytes[16];
+} NpLdtAesBlock;
+
+typedef struct {
+  uint8_t bytes[2];
+} NpLdtSalt;
+
+// Handle for accessing the AES implementation provided by caller
+typedef void* NpLdtAesCipherHandle;
+
+// Create an AES cipher from a key, returning a handle (or 0 on error)
+typedef NpLdtAesCipherHandle (*NpLdtAesCreateCipher)(NpLdtAes128Key key);
+// Release resources for the cipher, returning 0 on success.
+typedef int32_t (*NpLdtAesCloseCipher)(NpLdtAesCipherHandle handle);
+// Encrypt the block in place with the cipher corresponding to the handle
+typedef void (*NpLdtAesEncrypt)(NpLdtAesCipherHandle handle,
+                                NpLdtAesBlock* block);
+// Decrypt the block in place with the cipher corresponding to the handle
+typedef void (*NpLdtAesDecrypt)(NpLdtAesCipherHandle handle,
+                                NpLdtAesBlock* block);
+
+// Functions for creating, destroying, and using AES ciphers.
+//
+// Users on platforms with optimized AES implementations available may use this
+// to have LDT use their preferred AES.
+typedef struct {
+  NpLdtAesCreateCipher create_cipher;
+  NpLdtAesCloseCipher close_cipher;
+  NpLdtAesEncrypt encrypt;
+  NpLdtAesDecrypt decrypt;
+} NpLdtAesConfig;
+
+// Possible result codes returned from the LDT NP API's
+typedef enum {
+  // Call to api was succesful
+  NP_LDT_SUCCESS = 0,
+  // Payload of invalid length was provided must be >= 16 and <=31 bytes
+  NP_LDT_ERROR_INVALID_LENGTH = -1,
+  // The provided metadata hmac did not match the calculated hmac on call to
+  // decrypt and verify
+  NP_LDT_ERROR_MAC_MISMATCH = -2,
+} NP_LDT_RESULT;
+
+// Allocate an LDT-XTS-AES128 cipher using the "swap" mix function.
+//
+// `aes_config` defines the AES impl that will be used.
+// `key_seed` is the key material from the Nearby Presence credential from which
+// the LDT key will be derived.
+//
+// Returns 0 on error, or a non-zero handle on success.
+NpLdtHandle NpLdtCreate(NpLdtAesConfig aes_config, NpLdtKeySeed key_seed,
+                        NpMetadataKeyHmac known_hmac);
+
+// Release resources for an NpLdtHandle allocated by
+// `np_ldt_create_xts_aes_128`.
+//
+// Returns 0 on success or an NP_LDT_RESULT error code on failure
+NP_LDT_RESULT NpLdtClose(NpLdtHandle handle);
+
+// Encrypt a 16-31 byte buffer in-place.
+//
+// `buffer` is a pointer to a 16-31 byte plaintext, with length in `buffer_len`.
+// `salt` is the big-endian  2 byte salt that will be used in the Nearby
+// Presence advertisement, which will be incorporated into the tweaks LDT uses
+// while encrypting.
+//
+// Returns 0 on success, in which case `buffer` will now contain ciphertext.
+// Returns an NP_LDT_RESULT error code on failure
+NP_LDT_RESULT NpLdtEncrypt(NpLdtHandle handle, uint8_t* buffer,
+                           size_t buffer_len, NpLdtSalt salt);
+
+// Decrypt a 16-31 byte buffer in-place.
+//
+// `buffer` is a pointer to a 16-31 byte ciphertext, with length in
+// `buffer_len`.
+// `salt` is the big-endian  2 byte salt found in the Nearby Presence
+// advertisement, which will be incorporated into the tweaks LDT uses while
+// decrypting.
+//
+// Returns 0 on success, in which case `buffer` will now contain plaintext.
+// Returns an NP_LDT_RESULT error code on failure
+NP_LDT_RESULT NpLdtDecryptAndVerify(NpLdtHandle handle, uint8_t* buffer,
+                                    size_t buffer_len, NpLdtSalt salt);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // NP_LDT_H_