diff --git a/hotp/ta/hotp_ta.c b/hotp/ta/hotp_ta.c
new file mode 100644
index 0000000..18b7ec2
--- /dev/null
+++ b/hotp/ta/hotp_ta.c
@@ -0,0 +1,229 @@
+/*
+ * Copyright (c) 2017, Linaro Limited
+ * All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+#include <hotp_ta.h>
+#include <string.h>
+#include <tee_internal_api_extensions.h>
+#include <tee_internal_api.h>
+
+/* The size of a SHA1 hash in bytes. */
+#define SHA1_HASH_SIZE 20
+
+/* GP says that for HMAC SHA-1, max is 512 bits and min 80 bits. */
+#define MAX_KEY_SIZE 64 /* In bytes */
+#define MIN_KEY_SIZE 10 /* In bytes */
+
+/* Dynamic Binary Code 2 Modulo, which is 10^6 according to the spec. */
+#define DBC2_MODULO 1000000
+
+/*
+ * Currently this only supports a single key, in the future this could be
+ * updated to support multiple users, all with different unique keys (stored
+ * using secure storage).
+ */
+static uint8_t K[MAX_KEY_SIZE];
+static uint32_t K_len;
+
+/* The counter as defined by RFC4226. */
+static uint8_t counter[] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
+
+/*
+ *  HMAC a block of memory to produce the authentication tag
+ *  @param key       The secret key
+ *  @param keylen    The length of the secret key (bytes)
+ *  @param in        The data to HMAC
+ *  @param inlen     The length of the data to HMAC (bytes)
+ *  @param out       [out] Destination of the authentication tag
+ *  @param outlen    [in/out] Max size and resulting size of authentication tag
+ */
+static TEE_Result hmac_sha1(const uint8_t *key, const size_t keylen,
+			    const uint8_t *in, const size_t inlen,
+			    uint8_t *out, size_t *outlen)
+{
+	TEE_Attribute attr = { 0 };
+	TEE_ObjectHandle key_handle = TEE_HANDLE_NULL;
+	TEE_OperationHandle op_handle = TEE_HANDLE_NULL;
+	TEE_Result res = TEE_SUCCESS;
+
+	if (keylen < MIN_KEY_SIZE || keylen > MAX_KEY_SIZE)
+		return TEE_ERROR_BAD_PARAMETERS;
+
+	if (!in || !out || !outlen)
+		return TEE_ERROR_BAD_PARAMETERS;
+
+	/*
+	 * 1. Allocate cryptographic (operation) handle for the HMAC operation.
+	 *    Note that the expected size here is in bits (and therefore times
+	 *    8)!
+	 */
+	res = TEE_AllocateOperation(&op_handle, TEE_ALG_HMAC_SHA1, TEE_MODE_MAC,
+				    keylen * 8);
+	if (res != TEE_SUCCESS) {
+		EMSG("0x%08x", res);
+		goto exit;
+	}
+
+	/*
+	 * 2. Allocate a container (key handle) for the HMAC attributes. Note
+	 *    that the expected size here is in bits (and therefore times 8)!
+	 */
+	res = TEE_AllocateTransientObject(TEE_TYPE_HMAC_SHA1, keylen * 8,
+					  &key_handle);
+	if (res != TEE_SUCCESS) {
+		EMSG("0x%08x", res);
+		goto exit;
+	}
+
+	/*
+	 * 3. Initialize the attributes, i.e., point to the actual HMAC key.
+	 *    Here, the expected size is in bytes and not bits as above!
+	 */
+	TEE_InitRefAttribute(&attr, TEE_ATTR_SECRET_VALUE, key, keylen);
+
+	/* 4. Populate/assign the attributes with the key object */
+	res = TEE_PopulateTransientObject(key_handle, &attr, 1);
+	if (res != TEE_SUCCESS) {
+		EMSG("0x%08x", res);
+		goto exit;
+	}
+
+	/* 5. Associate the key (object) with the operation */
+	res = TEE_SetOperationKey(op_handle, key_handle);
+	if (res != TEE_SUCCESS) {
+		EMSG("0x%08x", res);
+		goto exit;
+	}
+
+	/* 6. Do the HMAC operations */
+	TEE_MACInit(op_handle, NULL, 0);
+	TEE_MACUpdate(op_handle, in, inlen);
+	res = TEE_MACComputeFinal(op_handle, NULL, 0, out, outlen);
+exit:
+	if (op_handle != TEE_HANDLE_NULL)
+		TEE_FreeOperation(op_handle);
+
+	/* It is OK to call this when key_handle is TEE_HANDLE_NULL */
+	TEE_FreeTransientObject(key_handle);
+
+	return res;
+}
+
+/*
+ * Truncate function working as described in RFC4226.
+ */
+static void truncate(uint8_t *hmac_result, uint32_t *bin_code)
+{
+	int offset = hmac_result[19] & 0xf;
+
+	*bin_code = (hmac_result[offset] & 0x7f) << 24 |
+		(hmac_result[offset+1] & 0xff) << 16 |
+		(hmac_result[offset+2] & 0xff) <<  8 |
+		(hmac_result[offset+3] & 0xff);
+
+	*bin_code %= DBC2_MODULO;
+}
+
+static TEE_Result register_shared_key(uint32_t param_types, TEE_Param params[4])
+{
+	TEE_Result res = TEE_SUCCESS;
+
+	uint32_t exp_param_types = TEE_PARAM_TYPES(TEE_PARAM_TYPE_MEMREF_INPUT,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE);
+
+	if (param_types != exp_param_types) {
+		EMSG("Expected: 0x%x, got: 0x%x", exp_param_types, param_types);
+		return TEE_ERROR_BAD_PARAMETERS;
+	}
+
+	memset(K, 0, sizeof(K));
+	memcpy(K, params[0].memref.buffer, params[0].memref.size);
+
+	K_len = params[0].memref.size;
+	DMSG("Got shared key %s (%u bytes).", K, params[0].memref.size);
+
+	return res;
+}
+
+static TEE_Result get_hotp(uint32_t param_types, TEE_Param params[4])
+{
+	TEE_Result res = TEE_SUCCESS;
+	uint32_t hotp_val;
+	uint8_t mac[SHA1_HASH_SIZE];
+	size_t mac_len = sizeof(mac);
+	int i;
+
+	uint32_t exp_param_types = TEE_PARAM_TYPES(TEE_PARAM_TYPE_VALUE_OUTPUT,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE);
+
+	if (param_types != exp_param_types) {
+		EMSG("Expected: 0x%x, got: 0x%x", exp_param_types, param_types);
+		return TEE_ERROR_BAD_PARAMETERS;
+	}
+
+	res = hmac_sha1(K, K_len, counter, sizeof(counter), mac, &mac_len);
+
+	/* Increment the counter. */
+	for (i = sizeof(counter) - 1; i >= 0; i--) {
+		if (++counter[i])
+			break;
+	}
+
+	truncate(mac, &hotp_val);
+	DMSG("HOTP is: %d", hotp_val);
+	params[0].value.a = hotp_val;
+
+	return res;
+}
+
+/*******************************************************************************
+ * Mandatory TA functions.
+ ******************************************************************************/
+TEE_Result TA_CreateEntryPoint(void)
+{
+	return TEE_SUCCESS;
+}
+
+void TA_DestroyEntryPoint(void)
+{
+}
+
+TEE_Result TA_OpenSessionEntryPoint(uint32_t param_types,
+				    TEE_Param __unused params[4],
+				    void __unused **sess_ctx)
+{
+	uint32_t exp_param_types = TEE_PARAM_TYPES(TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE,
+						   TEE_PARAM_TYPE_NONE);
+	if (param_types != exp_param_types)
+		return TEE_ERROR_BAD_PARAMETERS;
+
+	return TEE_SUCCESS;
+}
+
+void TA_CloseSessionEntryPoint(void __unused *sess_ctx)
+{
+}
+
+TEE_Result TA_InvokeCommandEntryPoint(void __unused *sess_ctx,
+				      uint32_t cmd_id,
+				      uint32_t param_types, TEE_Param params[4])
+{
+	switch (cmd_id) {
+	case TA_HOTP_CMD_REGISTER_SHARED_KEY:
+		return register_shared_key(param_types, params);
+
+	case TA_HOTP_CMD_GET_HOTP:
+		return get_hotp(param_types, params);
+
+	default:
+		return TEE_ERROR_BAD_PARAMETERS;
+	}
+}