library/lms.c

/*
 *  The LMS stateful-hash public-key signature scheme
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  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.
 */

/*
 *  The following sources were referenced in the design of this implementation
 *  of the LMS algorithm:
 *
 *  [1] IETF RFC8554
 *      D. McGrew, M. Curcio, S.Fluhrer
 *      https://datatracker.ietf.org/doc/html/rfc8554
 *
 *  [2] NIST Special Publication 800-208
 *      David A. Cooper et. al.
 *      https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-208.pdf
 */

#include "common.h"

#if defined(MBEDTLS_LMS_C)

#include <string.h>

#include "lmots.h"

#include "psa/crypto.h"

#include "mbedtls/lms.h"
#include "mbedtls/error.h"
#include "mbedtls/platform_util.h"

#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdlib.h>
#include <stdio.h>
#define mbedtls_printf printf
#define mbedtls_calloc calloc
#define mbedtls_free   free
#endif

#define SIG_Q_LEAF_ID_OFFSET     (0)
#define SIG_OTS_SIG_OFFSET       (SIG_Q_LEAF_ID_OFFSET + \
                                  MBEDTLS_LMOTS_Q_LEAF_ID_LEN)
#define SIG_TYPE_OFFSET(otstype) (SIG_OTS_SIG_OFFSET   + \
                                  MBEDTLS_LMOTS_SIG_LEN(otstype))
#define SIG_PATH_OFFSET(otstype) (SIG_TYPE_OFFSET(otstype) + \
                                  MBEDTLS_LMS_TYPE_LEN)

#define PUBLIC_KEY_TYPE_OFFSET      (0)
#define PUBLIC_KEY_OTSTYPE_OFFSET   (PUBLIC_KEY_TYPE_OFFSET + \
                                     MBEDTLS_LMS_TYPE_LEN)
#define PUBLIC_KEY_I_KEY_ID_OFFSET  (PUBLIC_KEY_OTSTYPE_OFFSET  + \
                                     MBEDTLS_LMOTS_TYPE_LEN)
#define PUBLIC_KEY_ROOT_NODE_OFFSET (PUBLIC_KEY_I_KEY_ID_OFFSET + \
                                     MBEDTLS_LMOTS_I_KEY_ID_LEN)


/* Currently only support H=10 */
#define H_TREE_HEIGHT_MAX                  10
#define MERKLE_TREE_NODE_AM_MAX            (1u << (H_TREE_HEIGHT_MAX + 1u))
#define MERKLE_TREE_NODE_AM(type)          (1u << (MBEDTLS_LMS_H_TREE_HEIGHT(type) + 1u))
#define MERKLE_TREE_LEAF_NODE_AM(type)     (1u << MBEDTLS_LMS_H_TREE_HEIGHT(type))
#define MERKLE_TREE_INTERNAL_NODE_AM(type) (1u << MBEDTLS_LMS_H_TREE_HEIGHT(type))

#define D_CONST_LEN           (2)
static const unsigned char D_LEAF_CONSTANT_BYTES[D_CONST_LEN] = {0x82, 0x82};
static const unsigned char D_INTR_CONSTANT_BYTES[D_CONST_LEN] = {0x83, 0x83};


/* Calculate the value of a leaf node of the merkle tree (which is a hash of a
 * public key and some other parameters like the leaf index). This function
 * implements RFC8554 section 5.3, in the case where r >= 2^h.
 *
 *  params              The LMS parameter set, the underlying LMOTS
 *                      parameter set, and I value which describe the key
 *                      being used.
 *
 *  pub_key             The public key of the private whose index
 *                      corresponds to the index of this leaf node. This
 *                      is a hash output.
 *
 *  r_node_idx          The index of this node in the merkle tree. Note
 *                      that the root node of the merkle tree is
 *                      1-indexed.
 *
 *  out                 The output node value, which is a hash output.
 */
static int create_merkle_leaf_value( const mbedtls_lms_parameters_t *params,
                                     unsigned char *pub_key,
                                     unsigned int r_node_idx,
                                     unsigned char *out )
{
    psa_hash_operation_t op;
    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
    size_t output_hash_len;
    unsigned char r_node_idx_bytes[4];

    op = psa_hash_operation_init( );
    status = psa_hash_setup( &op, PSA_ALG_SHA_256 );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, params->I_key_identifier,
                              MBEDTLS_LMOTS_I_KEY_ID_LEN );
    if( status != PSA_SUCCESS )
        goto exit;

    mbedtls_lms_unsigned_int_to_network_bytes( r_node_idx, 4, r_node_idx_bytes );
    status = psa_hash_update( &op, r_node_idx_bytes, 4 );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, D_LEAF_CONSTANT_BYTES, D_CONST_LEN );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, pub_key,
                              MBEDTLS_LMOTS_N_HASH_LEN(params->otstype) );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_finish( &op, out, MBEDTLS_LMS_M_NODE_BYTES(params->type),
                              &output_hash_len );
    if( status != PSA_SUCCESS )
        goto exit;

exit:
    psa_hash_abort( &op );

    return ( mbedtls_lms_error_from_psa( status ) );
}

/* Calculate the value of an internal node of the merkle tree (which is a hash
 * of a public key and some other parameters like the node index). This function
 * implements RFC8554 section 5.3, in the case where r < 2^h.
 *
 *  params              The LMS parameter set, the underlying LMOTS
 *                      parameter set, and I value which describe the key
 *                      being used.
 *
 *  left_node           The value of the child of this node which is on
 *                      the left-hand side. As with all nodes on the
 *                      merkle tree, this is a hash output.
 *
 *  right_node          The value of the child of this node which is on
 *                      the right-hand side. As with all nodes on the
 *                      merkle tree, this is a hash output.
 *
 *  r_node_idx          The index of this node in the merkle tree. Note
 *                      that the root node of the merkle tree is
 *                      1-indexed.
 *
 *  out                 The output node value, which is a hash output.
 */
static int create_merkle_internal_value( const mbedtls_lms_parameters_t *params,
                                         const unsigned char *left_node,
                                         const unsigned char *right_node,
                                         unsigned int r_node_idx,
                                         unsigned char *out )
{
    psa_hash_operation_t op;
    psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
    size_t output_hash_len;
    unsigned char r_node_idx_bytes[4];

    op = psa_hash_operation_init( );
    status = psa_hash_setup( &op, PSA_ALG_SHA_256 );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, params->I_key_identifier,
                              MBEDTLS_LMOTS_I_KEY_ID_LEN );
    if( status != PSA_SUCCESS )
        goto exit;

    mbedtls_lms_unsigned_int_to_network_bytes( r_node_idx, 4, r_node_idx_bytes );
    status = psa_hash_update( &op, r_node_idx_bytes, 4 );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, D_INTR_CONSTANT_BYTES, D_CONST_LEN );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, left_node,
                              MBEDTLS_LMS_M_NODE_BYTES(params->type) );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_update( &op, right_node,
                              MBEDTLS_LMS_M_NODE_BYTES(params->type) );
    if( status != PSA_SUCCESS )
        goto exit;

    status = psa_hash_finish( &op, out, MBEDTLS_LMS_M_NODE_BYTES(params->type),
                           &output_hash_len );
    if( status != PSA_SUCCESS )
        goto exit;

exit:
    psa_hash_abort( &op );

    return( mbedtls_lms_error_from_psa( status ) );
}

void mbedtls_lms_public_init( mbedtls_lms_public_t *ctx )
{
    mbedtls_platform_zeroize( ctx, sizeof( *ctx ) ) ;
}

void mbedtls_lms_public_free( mbedtls_lms_public_t *ctx )
{
    mbedtls_platform_zeroize( ctx, sizeof( *ctx ) );
}

int mbedtls_lms_import_public_key( mbedtls_lms_public_t *ctx,
                               const unsigned char *key, size_t key_size )
{
    mbedtls_lms_algorithm_type_t type;
    mbedtls_lmots_algorithm_type_t otstype;

    if( key_size < MBEDTLS_LMS_PUBLIC_KEY_LEN(ctx->params.type) )
    {
        return( MBEDTLS_ERR_LMS_BUFFER_TOO_SMALL );
    }

    type = mbedtls_lms_network_bytes_to_unsigned_int( MBEDTLS_LMS_TYPE_LEN,
            key + PUBLIC_KEY_TYPE_OFFSET );
    if( type != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }
    ctx->params.type = type;

    otstype = mbedtls_lms_network_bytes_to_unsigned_int( MBEDTLS_LMOTS_TYPE_LEN,
            key + PUBLIC_KEY_OTSTYPE_OFFSET );
    if( otstype != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }
    ctx->params.otstype = otstype;

    memcpy( ctx->params.I_key_identifier,
            key + PUBLIC_KEY_I_KEY_ID_OFFSET,
            MBEDTLS_LMOTS_I_KEY_ID_LEN );
    memcpy( ctx->T_1_pub_key, key + PUBLIC_KEY_ROOT_NODE_OFFSET,
            MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type) );

    ctx->have_public_key = 1;

    return( 0 );
}

int mbedtls_lms_verify( const mbedtls_lms_public_t *ctx,
                        const unsigned char *msg, size_t msg_size,
                        const unsigned char *sig, size_t sig_size )
{
    unsigned int q_leaf_identifier;
    unsigned char Kc_candidate_ots_pub_key[MBEDTLS_LMOTS_N_HASH_LEN_MAX];
    unsigned char Tc_candidate_root_node[MBEDTLS_LMS_M_NODE_BYTES_MAX];
    unsigned int height;
    unsigned int curr_node_id;
    unsigned int parent_node_id;
    const unsigned char* left_node;
    const unsigned char* right_node;
    mbedtls_lmots_parameters_t ots_params;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( ! ctx->have_public_key )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( sig_size != MBEDTLS_LMS_SIG_LEN(ctx->params.type, ctx->params.otstype) )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( ctx->params.type
        != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( ctx->params.otstype
        != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( mbedtls_lms_network_bytes_to_unsigned_int( MBEDTLS_LMOTS_TYPE_LEN,
            sig + SIG_OTS_SIG_OFFSET + MBEDTLS_LMOTS_SIG_TYPE_OFFSET )
        != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_VERIFY_FAILED );
    }

    if( mbedtls_lms_network_bytes_to_unsigned_int( MBEDTLS_LMS_TYPE_LEN,
            sig + SIG_TYPE_OFFSET(ctx->params.otstype))
        != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_VERIFY_FAILED );
    }


    q_leaf_identifier = mbedtls_lms_network_bytes_to_unsigned_int(
            MBEDTLS_LMOTS_Q_LEAF_ID_LEN, sig + SIG_Q_LEAF_ID_OFFSET );

    if( q_leaf_identifier >= MERKLE_TREE_LEAF_NODE_AM(ctx->params.type) )
    {
        return( MBEDTLS_ERR_LMS_VERIFY_FAILED );
    }

    memcpy( ots_params.I_key_identifier,
            ctx->params.I_key_identifier,
            MBEDTLS_LMOTS_I_KEY_ID_LEN );
    mbedtls_lms_unsigned_int_to_network_bytes( q_leaf_identifier,
                                              MBEDTLS_LMOTS_Q_LEAF_ID_LEN,
                                              ots_params.q_leaf_identifier );
    ots_params.type = ctx->params.otstype;

    ret = mbedtls_lmots_calculate_public_key_candidate( &ots_params, msg,
            msg_size, sig + SIG_OTS_SIG_OFFSET,
            MBEDTLS_LMOTS_SIG_LEN(ctx->params.otstype), Kc_candidate_ots_pub_key,
            sizeof( Kc_candidate_ots_pub_key ), NULL );
    if( ret != 0 )
    {
        return( ret );
    }

    create_merkle_leaf_value(
            &ctx->params,
            Kc_candidate_ots_pub_key,
            MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type) + q_leaf_identifier,
            Tc_candidate_root_node );

    curr_node_id = MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type) +
                   q_leaf_identifier;

    for( height = 0; height < MBEDTLS_LMS_H_TREE_HEIGHT(ctx->params.type);
         height++ )
    {
        parent_node_id = curr_node_id / 2;

        /* Left/right node ordering matters for the hash */
        if( curr_node_id & 1 )
        {
            left_node = sig + SIG_PATH_OFFSET(ctx->params.otstype) +
                        height * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type);
            right_node = Tc_candidate_root_node;
        }
        else
        {
            left_node = Tc_candidate_root_node;
            right_node = sig + SIG_PATH_OFFSET(ctx->params.otstype) +
                         height * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type);
        }

        create_merkle_internal_value( &ctx->params, left_node, right_node,
                                      parent_node_id, Tc_candidate_root_node);

        curr_node_id /= 2;
    }

    if( memcmp( Tc_candidate_root_node, ctx->T_1_pub_key,
                MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)) )
    {
        return( MBEDTLS_ERR_LMS_VERIFY_FAILED );
    }

    return( 0 );
}

#if defined(MBEDTLS_LMS_PRIVATE)

/* Calculate a full merkle tree based on a private key. This function
 * implements RFC8554 section 5.3, and is used to generate a public key (as the
 * public key is the root node of the merkle tree).
 *
 *  ctx                 The LMS private context, containing a parameter
 *                      set and private key material consisting of both
 *                      public and private OTS.
 *
 *  tree                The output tree, which is 2^(H + 1) hash outputs.
 *                      In the case of H=10 we have 2048 tree nodes (of
 *                      which 1024 of them are leaf nodes). Note that
 *                      because the merkle tree root is 1-indexed, the 0
 *                      index tree node is never used.
 */
static int calculate_merkle_tree( const mbedtls_lms_private_t *ctx,
                                  unsigned char *tree )
{
    unsigned int priv_key_idx;
    unsigned int r_node_idx;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    /* First create the leaf nodes, in ascending order */
    for( priv_key_idx = 0;
         priv_key_idx < MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type);
         priv_key_idx++ )
    {
        r_node_idx = MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type) + priv_key_idx;

        ret = create_merkle_leaf_value( &ctx->params,
                ctx->ots_public_keys[priv_key_idx].public_key, r_node_idx,
                &tree[r_node_idx * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)] );
        if( ret != 0 )
        {
            return( ret );
        }
    }

    /* Then the internal nodes, in reverse order so that we can guarantee the
     * parent has been created */
    for( r_node_idx = MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type) - 1;
         r_node_idx > 0;
         r_node_idx-- )
    {
        ret = create_merkle_internal_value( &ctx->params,
                &tree[( r_node_idx * 2 ) * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)],
                &tree[( r_node_idx * 2 + 1 ) * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)],
                r_node_idx,
                &tree[r_node_idx * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)] );
        if( ret != 0 )
        {
            return( ret );
        }
    }

    return( 0 );
}

/* Calculate a path from a leaf node of the merkle tree to the root of the tree,
 * and return the full path. This function implements RFC8554 section 5.4.1, as
 * the merkle path is the main component of an LMS signature.
 *
 *  ctx                 The LMS private context, containing a parameter
 *                      set and private key material consisting of both
 *                      public and private OTS.
 *
 *  leaf_node_id        Which leaf node to calculate the path from.
 *
 *  tree                The output path, which is H hash outputs.
 */
static int get_merkle_path( mbedtls_lms_private_t *ctx,
                            unsigned int leaf_node_id,
                            unsigned char *path )
{
    unsigned char tree[MERKLE_TREE_NODE_AM_MAX][MBEDTLS_LMS_M_NODE_BYTES_MAX];
    unsigned int curr_node_id = leaf_node_id;
    unsigned int adjacent_node_id;
    unsigned int height;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    ret = calculate_merkle_tree( ctx, ( unsigned char * )tree );
    if( ret != 0 )
    {
        return( ret );
    }

    for( height = 0; height < MBEDTLS_LMS_H_TREE_HEIGHT(ctx->params.type);
         height++ )
    {
        adjacent_node_id = curr_node_id ^ 1;

        memcpy( &path[height * MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type)],
                &tree[adjacent_node_id],
                MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type) );

        curr_node_id >>=1;
    }

    return( 0 );
}

void mbedtls_lms_private_init( mbedtls_lms_private_t *ctx )
{
    mbedtls_platform_zeroize( ctx, sizeof( *ctx ) ) ;
}

void mbedtls_lms_private_free( mbedtls_lms_private_t *ctx )
{
    unsigned int idx;

    if( ctx->have_private_key )
    {
        for( idx = 0; idx < MERKLE_TREE_LEAF_NODE_AM(ctx->params.type); idx++ )
        {
            mbedtls_lmots_private_free( &ctx->ots_private_keys[idx] );
            mbedtls_lmots_public_free( &ctx->ots_public_keys[idx] );
        }

        if( ctx->ots_private_keys != NULL )
            mbedtls_free( ctx->ots_private_keys );

        if( ctx->ots_public_keys != NULL )
            mbedtls_free( ctx->ots_public_keys );
    }

    mbedtls_platform_zeroize( ctx, sizeof( *ctx ) );
}


int mbedtls_lms_generate_private_key( mbedtls_lms_private_t *ctx,
                                      mbedtls_lms_algorithm_type_t type,
                                      mbedtls_lmots_algorithm_type_t otstype,
                                      int (*f_rng)(void *, unsigned char *, size_t),
                                      void* p_rng, const unsigned char *seed,
                                      size_t seed_size )
{
    unsigned int idx = 0;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( type != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( otstype != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( ctx->have_private_key )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    ctx->params.type = type;
    ctx->params.otstype = otstype;

    f_rng( p_rng,
           ctx->params.I_key_identifier,
           MBEDTLS_LMOTS_I_KEY_ID_LEN );

    ctx->ots_private_keys = mbedtls_calloc( ( size_t )MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
                                            sizeof( *ctx->ots_private_keys ) );
    if( ctx->ots_private_keys == NULL )
    {
        ret = MBEDTLS_ERR_LMS_ALLOC_FAILED;
        goto exit;
    }

    ctx->ots_public_keys = mbedtls_calloc( ( size_t )MERKLE_TREE_LEAF_NODE_AM(ctx->params.type),
                                           sizeof( *ctx->ots_public_keys ) );
    if( ctx->ots_public_keys == NULL )
    {
        /* Free just the ots private keys (since they've been allocated at this
         * point) so that we can pass the context to lms_private_free (which
         * will not try to free the private keys since have_private_key is not
         * set.
         */
        mbedtls_free(ctx->ots_private_keys);
        ctx->ots_private_keys = NULL;
        ret = MBEDTLS_ERR_LMS_ALLOC_FAILED;
        goto exit;
    }

    /* Now that all the allocation has succeeded we set have_private_key, since
     * that causes lms_private_free to free the ots keys.
     */
    ctx->have_private_key = 1;

    for( idx = 0; idx < MERKLE_TREE_LEAF_NODE_AM(ctx->params.type); idx++ )
    {
        mbedtls_lmots_private_init( &ctx->ots_private_keys[idx] );
        mbedtls_lmots_public_init( &ctx->ots_public_keys[idx] );
    }


    for( idx = 0; idx < MERKLE_TREE_LEAF_NODE_AM(ctx->params.type); idx++ )
    {
        ret = mbedtls_lmots_generate_private_key( &ctx->ots_private_keys[idx],
                                                  otstype,
                                                  ctx->params.I_key_identifier,
                                                  idx, seed, seed_size );
        if( ret != 0 )
            goto exit;

        ret = mbedtls_lmots_calculate_public_key( &ctx->ots_public_keys[idx],
                                                  &ctx->ots_private_keys[idx] );
        if( ret != 0 )
            goto exit;
    }

    ctx->q_next_usable_key = 0;

exit:
    if( ret != 0 )
    {
        mbedtls_lms_private_free(ctx);
    }

    return( ret );
}

int mbedtls_lms_calculate_public_key( mbedtls_lms_public_t *ctx,
                                      const mbedtls_lms_private_t *priv_ctx )
{
    unsigned char tree[MERKLE_TREE_NODE_AM_MAX][MBEDTLS_LMS_M_NODE_BYTES_MAX];
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( ! priv_ctx->have_private_key )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( priv_ctx->params.type
        != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( priv_ctx->params.otstype
        != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    memcpy( &ctx->params, &priv_ctx->params,
            sizeof( mbedtls_lmots_parameters_t ) );

    ret = calculate_merkle_tree( priv_ctx, ( unsigned char * )tree );
    if( ret != 0 )
    {
        return( ret );
    }

    /* Root node is always at position 1, due to 1-based indexing */
    memcpy( ctx->T_1_pub_key, &tree[1],
            MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type) );

    ctx->have_public_key = 1;

    return( 0 );
}


int mbedtls_lms_export_public_key( const mbedtls_lms_public_t *ctx,
                                   unsigned char *key,
                                   size_t key_size, size_t *key_len )
{
    if( key_size < MBEDTLS_LMS_PUBLIC_KEY_LEN(ctx->params.type) )
    {
        return( MBEDTLS_ERR_LMS_BUFFER_TOO_SMALL );
    }

    if( ! ctx->have_public_key )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    mbedtls_lms_unsigned_int_to_network_bytes(
            ctx->params.type,
            MBEDTLS_LMS_TYPE_LEN, key + PUBLIC_KEY_TYPE_OFFSET );
    mbedtls_lms_unsigned_int_to_network_bytes( ctx->params.otstype,
                                   MBEDTLS_LMOTS_TYPE_LEN,
                                   key + PUBLIC_KEY_OTSTYPE_OFFSET );
    memcpy( key + PUBLIC_KEY_I_KEY_ID_OFFSET,
            ctx->params.I_key_identifier,
            MBEDTLS_LMOTS_I_KEY_ID_LEN );
    memcpy( key +PUBLIC_KEY_ROOT_NODE_OFFSET,
            ctx->T_1_pub_key,
            MBEDTLS_LMS_M_NODE_BYTES(ctx->params.type) );

    if( key_len != NULL )
    {
        *key_len = MBEDTLS_LMS_PUBLIC_KEY_LEN(ctx->params.type);
    }

    return( 0 );
}


int mbedtls_lms_sign( mbedtls_lms_private_t *ctx,
                      int (*f_rng)(void *, unsigned char *, size_t),
                      void* p_rng, const unsigned char *msg,
                      unsigned int msg_size, unsigned char *sig, size_t sig_size,
                      size_t *sig_len )
{
    uint32_t q_leaf_identifier;
    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;

    if( ! ctx->have_private_key )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( sig_size < MBEDTLS_LMS_SIG_LEN(ctx->params.type, ctx->params.otstype) )
    {
        return( MBEDTLS_ERR_LMS_BUFFER_TOO_SMALL );
    }

    if( ctx->params.type != MBEDTLS_LMS_SHA256_M32_H10 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( ctx->params.otstype
        != MBEDTLS_LMOTS_SHA256_N32_W8 )
    {
        return( MBEDTLS_ERR_LMS_BAD_INPUT_DATA );
    }

    if( ctx->q_next_usable_key >= MERKLE_TREE_LEAF_NODE_AM(ctx->params.type) )
    {
        return( MBEDTLS_ERR_LMS_OUT_OF_PRIVATE_KEYS );
    }


    q_leaf_identifier = ctx->q_next_usable_key;
    /* This new value must _always_ be written back to the disk before the
     * signature is returned.
     */
    ctx->q_next_usable_key += 1;

    ret = mbedtls_lmots_sign( &ctx->ots_private_keys[q_leaf_identifier],
                              f_rng, p_rng, msg, msg_size,
                              sig + SIG_OTS_SIG_OFFSET,
                              MBEDTLS_LMS_SIG_LEN(ctx->params.type, ctx->params.otstype),
                              NULL );
    if( ret != 0 )
    {
        return( ret );
    }

    mbedtls_lms_unsigned_int_to_network_bytes( ctx->params.type,
            MBEDTLS_LMS_TYPE_LEN,
            sig + SIG_TYPE_OFFSET(ctx->params.otstype) );
    mbedtls_lms_unsigned_int_to_network_bytes( q_leaf_identifier,
            MBEDTLS_LMOTS_Q_LEAF_ID_LEN,
            sig + SIG_Q_LEAF_ID_OFFSET );

    ret = get_merkle_path( ctx,
            MERKLE_TREE_INTERNAL_NODE_AM(ctx->params.type) + q_leaf_identifier,
            sig + SIG_PATH_OFFSET(ctx->params.otstype) );
    if( ret != 0 )
    {
        return( ret );
    }

    if( sig_len != NULL )
    {
        *sig_len = MBEDTLS_LMS_SIG_LEN(ctx->params.type, ctx->params.otstype);
    }


    return( 0 );
}

#endif /* defined(MBEDTLS_LMS_PRIVATE) */
#endif /* defined(MBEDTLS_LMS_C) */