programs/psa/key_ladder_demo.c

/**
 * PSA API key derivation demonstration
 *
 * This program calculates a key ladder: a chain of secret material, each
 * derived from the previous one in a deterministic way based on a label.
 * Two keys are identical if and only if they are derived from the same key
 * using the same label.
 *
 * The initial key is called the master key. The master key is normally
 * randomly generated, but it could itself be derived from another key.
 *
 * This program derives a series of keys called intermediate keys.
 * The first intermediate key is derived from the master key using the
 * first label passed on the command line. Each subsequent intermediate
 * key is derived from the previous one using the next label passed
 * on the command line.
 *
 * This program has four modes of operation:
 *
 * - "generate": generate a random master key.
 * - "wrap": derive a wrapping key from the last intermediate key,
 *           and use that key to encrypt-and-authenticate some data.
 * - "unwrap": derive a wrapping key from the last intermediate key,
 *             and use that key to decrypt-and-authenticate some
 *             ciphertext created by wrap mode.
 * - "save": save the last intermediate key so that it can be reused as
 *           the master key in another run of the program.
 *
 * See the usage() output for the command line usage. See the file
 * `key_ladder_demo.sh` for an example run.
 */

/*
 *  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.
 */

/* First include Mbed TLS headers to get the Mbed TLS configuration and
 * platform definitions that we'll use in this program. Also include
 * standard C headers for functions we'll use here. */
#include "mbedtls/build_info.h"

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#include "mbedtls/platform.h" // for mbedtls_setbuf
#include "mbedtls/platform_util.h" // for mbedtls_platform_zeroize

#include <psa/crypto.h>

/* If the build options we need are not enabled, compile a placeholder. */
#if !defined(PSA_WANT_ALG_SHA_256) || !defined(MBEDTLS_MD_C) ||      \
    !defined(MBEDTLS_AES_C) || !defined(MBEDTLS_CCM_C) ||        \
    !defined(MBEDTLS_PSA_CRYPTO_C) || !defined(MBEDTLS_FS_IO) || \
    defined(MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER)
int main(void)
{
    printf("PSA_WANT_ALG_SHA_256 and/or MBEDTLS_MD_C and/or "
           "MBEDTLS_AES_C and/or MBEDTLS_CCM_C and/or "
           "MBEDTLS_PSA_CRYPTO_C and/or MBEDTLS_FS_IO "
           "not defined and/or MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER "
           "defined.\n");
    return 0;
}
#else

/* The real program starts here. */

/* Run a system function and bail out if it fails. */
#define SYS_CHECK(expr)                                       \
    do                                                          \
    {                                                           \
        if (!(expr))                                        \
        {                                                       \
            perror( #expr);                                    \
            status = DEMO_ERROR;                                \
            goto exit;                                          \
        }                                                       \
    }                                                           \
    while (0)

/* Run a PSA function and bail out if it fails. */
#define PSA_CHECK(expr)                                       \
    do                                                          \
    {                                                           \
        status = (expr);                                      \
        if (status != PSA_SUCCESS)                             \
        {                                                       \
            printf("Error %d at line %d: %s\n",                \
                   (int) status,                               \
                   __LINE__,                                   \
                   #expr);                                    \
            goto exit;                                          \
        }                                                       \
    }                                                           \
    while (0)

/* To report operational errors in this program, use an error code that is
 * different from every PSA error code. */
#define DEMO_ERROR 120

/* The maximum supported key ladder depth. */
#define MAX_LADDER_DEPTH 10

/* Salt to use when deriving an intermediate key. */
#define DERIVE_KEY_SALT ((uint8_t *) "key_ladder_demo.derive")
#define DERIVE_KEY_SALT_LENGTH (strlen((const char *) DERIVE_KEY_SALT))

/* Salt to use when deriving a wrapping key. */
#define WRAPPING_KEY_SALT ((uint8_t *) "key_ladder_demo.wrap")
#define WRAPPING_KEY_SALT_LENGTH (strlen((const char *) WRAPPING_KEY_SALT))

/* Size of the key derivation keys (applies both to the master key and
 * to intermediate keys). */
#define KEY_SIZE_BYTES 40

/* Algorithm for key derivation. */
#define KDF_ALG PSA_ALG_HKDF(PSA_ALG_SHA_256)

/* Type and size of the key used to wrap data. */
#define WRAPPING_KEY_TYPE PSA_KEY_TYPE_AES
#define WRAPPING_KEY_BITS 128

/* Cipher mode used to wrap data. */
#define WRAPPING_ALG PSA_ALG_CCM

/* Nonce size used to wrap data. */
#define WRAPPING_IV_SIZE 13

/* Header used in files containing wrapped data. We'll save this header
 * directly without worrying about data representation issues such as
 * integer sizes and endianness, because the data is meant to be read
 * back by the same program on the same machine. */
#define WRAPPED_DATA_MAGIC "key_ladder_demo" // including trailing null byte
#define WRAPPED_DATA_MAGIC_LENGTH (sizeof(WRAPPED_DATA_MAGIC))
typedef struct {
    char magic[WRAPPED_DATA_MAGIC_LENGTH];
    size_t ad_size; /* Size of the additional data, which is this header. */
    size_t payload_size; /* Size of the encrypted data. */
    /* Store the IV inside the additional data. It's convenient. */
    uint8_t iv[WRAPPING_IV_SIZE];
} wrapped_data_header_t;

/* The modes that this program can operate in (see usage). */
enum program_mode {
    MODE_GENERATE,
    MODE_SAVE,
    MODE_UNWRAP,
    MODE_WRAP
};

/* Save a key to a file. In the real world, you may want to export a derived
 * key sometimes, to share it with another party. */
static psa_status_t save_key(psa_key_id_t key,
                             const char *output_file_name)
{
    psa_status_t status = PSA_SUCCESS;
    uint8_t key_data[KEY_SIZE_BYTES];
    size_t key_size;
    FILE *key_file = NULL;

    PSA_CHECK(psa_export_key(key,
                             key_data, sizeof(key_data),
                             &key_size));
    SYS_CHECK((key_file = fopen(output_file_name, "wb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(key_file, NULL);
    SYS_CHECK(fwrite(key_data, 1, key_size, key_file) == key_size);
    SYS_CHECK(fclose(key_file) == 0);
    key_file = NULL;

exit:
    if (key_file != NULL) {
        fclose(key_file);
    }
    return status;
}

/* Generate a master key for use in this demo.
 *
 * Normally a master key would be non-exportable. For the purpose of this
 * demo, we want to save it to a file, to avoid relying on the keystore
 * capability of the PSA crypto library. */
static psa_status_t generate(const char *key_file_name)
{
    psa_status_t status = PSA_SUCCESS;
    psa_key_id_t key = 0;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;

    psa_set_key_usage_flags(&attributes,
                            PSA_KEY_USAGE_DERIVE | PSA_KEY_USAGE_EXPORT);
    psa_set_key_algorithm(&attributes, KDF_ALG);
    psa_set_key_type(&attributes, PSA_KEY_TYPE_DERIVE);
    psa_set_key_bits(&attributes, PSA_BYTES_TO_BITS(KEY_SIZE_BYTES));

    PSA_CHECK(psa_generate_key(&attributes, &key));

    PSA_CHECK(save_key(key, key_file_name));

exit:
    (void) psa_destroy_key(key);
    return status;
}

/* Load the master key from a file.
 *
 * In the real world, this master key would be stored in an internal memory
 * and the storage would be managed by the keystore capability of the PSA
 * crypto library. */
static psa_status_t import_key_from_file(psa_key_usage_t usage,
                                         psa_algorithm_t alg,
                                         const char *key_file_name,
                                         psa_key_id_t *master_key)
{
    psa_status_t status = PSA_SUCCESS;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
    uint8_t key_data[KEY_SIZE_BYTES];
    size_t key_size;
    FILE *key_file = NULL;
    unsigned char extra_byte;

    SYS_CHECK((key_file = fopen(key_file_name, "rb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(key_file, NULL);
    SYS_CHECK((key_size = fread(key_data, 1, sizeof(key_data),
                                key_file)) != 0);
    if (fread(&extra_byte, 1, 1, key_file) != 0) {
        printf("Key file too large (max: %u).\n",
               (unsigned) sizeof(key_data));
        status = DEMO_ERROR;
        goto exit;
    }
    SYS_CHECK(fclose(key_file) == 0);
    key_file = NULL;

    psa_set_key_usage_flags(&attributes, usage);
    psa_set_key_algorithm(&attributes, alg);
    psa_set_key_type(&attributes, PSA_KEY_TYPE_DERIVE);
    PSA_CHECK(psa_import_key(&attributes, key_data, key_size, master_key));
exit:
    if (key_file != NULL) {
        fclose(key_file);
    }
    mbedtls_platform_zeroize(key_data, sizeof(key_data));
    if (status != PSA_SUCCESS) {
        /* If the key creation hasn't happened yet or has failed,
         * *master_key is null. psa_destroy_key( 0 ) is
         * guaranteed to do nothing and return PSA_SUCCESS. */
        (void) psa_destroy_key(*master_key);
        *master_key = 0;
    }
    return status;
}

/* Derive the intermediate keys, using the list of labels provided on
 * the command line. On input, *key is the master key identifier.
 * This function destroys the master key. On successful output, *key
 * is the identifier of the final derived key.
 */
static psa_status_t derive_key_ladder(const char *ladder[],
                                      size_t ladder_depth,
                                      psa_key_id_t *key)
{
    psa_status_t status = PSA_SUCCESS;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
    psa_key_derivation_operation_t operation = PSA_KEY_DERIVATION_OPERATION_INIT;
    size_t i;

    psa_set_key_usage_flags(&attributes,
                            PSA_KEY_USAGE_DERIVE | PSA_KEY_USAGE_EXPORT);
    psa_set_key_algorithm(&attributes, KDF_ALG);
    psa_set_key_type(&attributes, PSA_KEY_TYPE_DERIVE);
    psa_set_key_bits(&attributes, PSA_BYTES_TO_BITS(KEY_SIZE_BYTES));

    /* For each label in turn, ... */
    for (i = 0; i < ladder_depth; i++) {
        /* Start deriving material from the master key (if i=0) or from
         * the current intermediate key (if i>0). */
        PSA_CHECK(psa_key_derivation_setup(&operation, KDF_ALG));
        PSA_CHECK(psa_key_derivation_input_bytes(
                      &operation, PSA_KEY_DERIVATION_INPUT_SALT,
                      DERIVE_KEY_SALT, DERIVE_KEY_SALT_LENGTH));
        PSA_CHECK(psa_key_derivation_input_key(
                      &operation, PSA_KEY_DERIVATION_INPUT_SECRET,
                      *key));
        PSA_CHECK(psa_key_derivation_input_bytes(
                      &operation, PSA_KEY_DERIVATION_INPUT_INFO,
                      (uint8_t *) ladder[i], strlen(ladder[i])));
        /* When the parent key is not the master key, destroy it,
         * since it is no longer needed. */
        PSA_CHECK(psa_destroy_key(*key));
        *key = 0;
        /* Derive the next intermediate key from the parent key. */
        PSA_CHECK(psa_key_derivation_output_key(&attributes, &operation,
                                                key));
        PSA_CHECK(psa_key_derivation_abort(&operation));
    }

exit:
    psa_key_derivation_abort(&operation);
    if (status != PSA_SUCCESS) {
        psa_destroy_key(*key);
        *key = 0;
    }
    return status;
}

/* Derive a wrapping key from the last intermediate key. */
static psa_status_t derive_wrapping_key(psa_key_usage_t usage,
                                        psa_key_id_t derived_key,
                                        psa_key_id_t *wrapping_key)
{
    psa_status_t status = PSA_SUCCESS;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
    psa_key_derivation_operation_t operation = PSA_KEY_DERIVATION_OPERATION_INIT;

    *wrapping_key = 0;

    /* Set up a key derivation operation from the key derived from
     * the master key. */
    PSA_CHECK(psa_key_derivation_setup(&operation, KDF_ALG));
    PSA_CHECK(psa_key_derivation_input_bytes(
                  &operation, PSA_KEY_DERIVATION_INPUT_SALT,
                  WRAPPING_KEY_SALT, WRAPPING_KEY_SALT_LENGTH));
    PSA_CHECK(psa_key_derivation_input_key(
                  &operation, PSA_KEY_DERIVATION_INPUT_SECRET,
                  derived_key));
    PSA_CHECK(psa_key_derivation_input_bytes(
                  &operation, PSA_KEY_DERIVATION_INPUT_INFO,
                  NULL, 0));

    /* Create the wrapping key. */
    psa_set_key_usage_flags(&attributes, usage);
    psa_set_key_algorithm(&attributes, WRAPPING_ALG);
    psa_set_key_type(&attributes, PSA_KEY_TYPE_AES);
    psa_set_key_bits(&attributes, WRAPPING_KEY_BITS);
    PSA_CHECK(psa_key_derivation_output_key(&attributes, &operation,
                                            wrapping_key));

exit:
    psa_key_derivation_abort(&operation);
    return status;
}

static psa_status_t wrap_data(const char *input_file_name,
                              const char *output_file_name,
                              psa_key_id_t wrapping_key)
{
    psa_status_t status;
    FILE *input_file = NULL;
    FILE *output_file = NULL;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
    psa_key_type_t key_type;
    long input_position;
    size_t input_size;
    size_t buffer_size = 0;
    unsigned char *buffer = NULL;
    size_t ciphertext_size;
    wrapped_data_header_t header;

    /* Find the size of the data to wrap. */
    SYS_CHECK((input_file = fopen(input_file_name, "rb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(input_file, NULL);
    SYS_CHECK(fseek(input_file, 0, SEEK_END) == 0);
    SYS_CHECK((input_position = ftell(input_file)) != -1);
#if LONG_MAX > SIZE_MAX
    if (input_position > SIZE_MAX) {
        printf("Input file too large.\n");
        status = DEMO_ERROR;
        goto exit;
    }
#endif
    input_size = input_position;
    PSA_CHECK(psa_get_key_attributes(wrapping_key, &attributes));
    key_type = psa_get_key_type(&attributes);
    buffer_size =
        PSA_AEAD_ENCRYPT_OUTPUT_SIZE(key_type, WRAPPING_ALG, input_size);
    /* Check for integer overflow. */
    if (buffer_size < input_size) {
        printf("Input file too large.\n");
        status = DEMO_ERROR;
        goto exit;
    }

    /* Load the data to wrap. */
    SYS_CHECK(fseek(input_file, 0, SEEK_SET) == 0);
    SYS_CHECK((buffer = calloc(1, buffer_size)) != NULL);
    SYS_CHECK(fread(buffer, 1, input_size, input_file) == input_size);
    SYS_CHECK(fclose(input_file) == 0);
    input_file = NULL;

    /* Construct a header. */
    memcpy(&header.magic, WRAPPED_DATA_MAGIC, WRAPPED_DATA_MAGIC_LENGTH);
    header.ad_size = sizeof(header);
    header.payload_size = input_size;

    /* Wrap the data. */
    PSA_CHECK(psa_generate_random(header.iv, WRAPPING_IV_SIZE));
    PSA_CHECK(psa_aead_encrypt(wrapping_key, WRAPPING_ALG,
                               header.iv, WRAPPING_IV_SIZE,
                               (uint8_t *) &header, sizeof(header),
                               buffer, input_size,
                               buffer, buffer_size,
                               &ciphertext_size));

    /* Write the output. */
    SYS_CHECK((output_file = fopen(output_file_name, "wb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(output_file, NULL);
    SYS_CHECK(fwrite(&header, 1, sizeof(header),
                     output_file) == sizeof(header));
    SYS_CHECK(fwrite(buffer, 1, ciphertext_size,
                     output_file) == ciphertext_size);
    SYS_CHECK(fclose(output_file) == 0);
    output_file = NULL;

exit:
    if (input_file != NULL) {
        fclose(input_file);
    }
    if (output_file != NULL) {
        fclose(output_file);
    }
    if (buffer != NULL) {
        mbedtls_platform_zeroize(buffer, buffer_size);
    }
    free(buffer);
    return status;
}

static psa_status_t unwrap_data(const char *input_file_name,
                                const char *output_file_name,
                                psa_key_id_t wrapping_key)
{
    psa_status_t status;
    FILE *input_file = NULL;
    FILE *output_file = NULL;
    psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
    psa_key_type_t key_type;
    unsigned char *buffer = NULL;
    size_t ciphertext_size = 0;
    size_t plaintext_size;
    wrapped_data_header_t header;
    unsigned char extra_byte;

    /* Load and validate the header. */
    SYS_CHECK((input_file = fopen(input_file_name, "rb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(input_file, NULL);
    SYS_CHECK(fread(&header, 1, sizeof(header),
                    input_file) == sizeof(header));
    if (memcmp(&header.magic, WRAPPED_DATA_MAGIC,
               WRAPPED_DATA_MAGIC_LENGTH) != 0) {
        printf("The input does not start with a valid magic header.\n");
        status = DEMO_ERROR;
        goto exit;
    }
    if (header.ad_size != sizeof(header)) {
        printf("The header size is not correct.\n");
        status = DEMO_ERROR;
        goto exit;
    }
    PSA_CHECK(psa_get_key_attributes(wrapping_key, &attributes));
    key_type = psa_get_key_type(&attributes);
    ciphertext_size =
        PSA_AEAD_ENCRYPT_OUTPUT_SIZE(key_type, WRAPPING_ALG, header.payload_size);
    /* Check for integer overflow. */
    if (ciphertext_size < header.payload_size) {
        printf("Input file too large.\n");
        status = DEMO_ERROR;
        goto exit;
    }

    /* Load the payload data. */
    SYS_CHECK((buffer = calloc(1, ciphertext_size)) != NULL);
    SYS_CHECK(fread(buffer, 1, ciphertext_size,
                    input_file) == ciphertext_size);
    if (fread(&extra_byte, 1, 1, input_file) != 0) {
        printf("Extra garbage after ciphertext\n");
        status = DEMO_ERROR;
        goto exit;
    }
    SYS_CHECK(fclose(input_file) == 0);
    input_file = NULL;

    /* Unwrap the data. */
    PSA_CHECK(psa_aead_decrypt(wrapping_key, WRAPPING_ALG,
                               header.iv, WRAPPING_IV_SIZE,
                               (uint8_t *) &header, sizeof(header),
                               buffer, ciphertext_size,
                               buffer, ciphertext_size,
                               &plaintext_size));
    if (plaintext_size != header.payload_size) {
        printf("Incorrect payload size in the header.\n");
        status = DEMO_ERROR;
        goto exit;
    }

    /* Write the output. */
    SYS_CHECK((output_file = fopen(output_file_name, "wb")) != NULL);
    /* Ensure no stdio buffering of secrets, as such buffers cannot be wiped. */
    mbedtls_setbuf(output_file, NULL);
    SYS_CHECK(fwrite(buffer, 1, plaintext_size,
                     output_file) == plaintext_size);
    SYS_CHECK(fclose(output_file) == 0);
    output_file = NULL;

exit:
    if (input_file != NULL) {
        fclose(input_file);
    }
    if (output_file != NULL) {
        fclose(output_file);
    }
    if (buffer != NULL) {
        mbedtls_platform_zeroize(buffer, ciphertext_size);
    }
    free(buffer);
    return status;
}

static psa_status_t run(enum program_mode mode,
                        const char *key_file_name,
                        const char *ladder[], size_t ladder_depth,
                        const char *input_file_name,
                        const char *output_file_name)
{
    psa_status_t status = PSA_SUCCESS;
    psa_key_id_t derivation_key = 0;
    psa_key_id_t wrapping_key = 0;

    /* Initialize the PSA crypto library. */
    PSA_CHECK(psa_crypto_init());

    /* Generate mode is unlike the others. Generate the master key and exit. */
    if (mode == MODE_GENERATE) {
        return generate(key_file_name);
    }

    /* Read the master key. */
    PSA_CHECK(import_key_from_file(PSA_KEY_USAGE_DERIVE | PSA_KEY_USAGE_EXPORT,
                                   KDF_ALG,
                                   key_file_name,
                                   &derivation_key));

    /* Calculate the derived key for this session. */
    PSA_CHECK(derive_key_ladder(ladder, ladder_depth,
                                &derivation_key));

    switch (mode) {
        case MODE_SAVE:
            PSA_CHECK(save_key(derivation_key, output_file_name));
            break;
        case MODE_UNWRAP:
            PSA_CHECK(derive_wrapping_key(PSA_KEY_USAGE_DECRYPT,
                                          derivation_key,
                                          &wrapping_key));
            PSA_CHECK(unwrap_data(input_file_name, output_file_name,
                                  wrapping_key));
            break;
        case MODE_WRAP:
            PSA_CHECK(derive_wrapping_key(PSA_KEY_USAGE_ENCRYPT,
                                          derivation_key,
                                          &wrapping_key));
            PSA_CHECK(wrap_data(input_file_name, output_file_name,
                                wrapping_key));
            break;
        default:
            /* Unreachable but some compilers don't realize it. */
            break;
    }

exit:
    /* Destroy any remaining key. Deinitializing the crypto library would do
     * this anyway since they are volatile keys, but explicitly destroying
     * keys makes the code easier to reuse. */
    (void) psa_destroy_key(derivation_key);
    (void) psa_destroy_key(wrapping_key);
    /* Deinitialize the PSA crypto library. */
    mbedtls_psa_crypto_free();
    return status;
}

static void usage(void)
{
    printf("Usage: key_ladder_demo MODE [OPTION=VALUE]...\n");
    printf("Demonstrate the usage of a key derivation ladder.\n");
    printf("\n");
    printf("Modes:\n");
    printf("  generate  Generate the master key\n");
    printf("  save      Save the derived key\n");
    printf("  unwrap    Unwrap (decrypt) input with the derived key\n");
    printf("  wrap      Wrap (encrypt) input with the derived key\n");
    printf("\n");
    printf("Options:\n");
    printf("  input=FILENAME    Input file (required for wrap/unwrap)\n");
    printf("  master=FILENAME   File containing the master key (default: master.key)\n");
    printf("  output=FILENAME   Output file (required for save/wrap/unwrap)\n");
    printf("  label=TEXT        Label for the key derivation.\n");
    printf("                    This may be repeated multiple times.\n");
    printf("                    To get the same key, you must use the same master key\n");
    printf("                    and the same sequence of labels.\n");
}

int main(int argc, char *argv[])
{
    const char *key_file_name = "master.key";
    const char *input_file_name = NULL;
    const char *output_file_name = NULL;
    const char *ladder[MAX_LADDER_DEPTH];
    size_t ladder_depth = 0;
    int i;
    enum program_mode mode;
    psa_status_t status;

    if (argc <= 1 ||
        strcmp(argv[1], "help") == 0 ||
        strcmp(argv[1], "-help") == 0 ||
        strcmp(argv[1], "--help") == 0) {
        usage();
        return EXIT_SUCCESS;
    }

    for (i = 2; i < argc; i++) {
        char *q = strchr(argv[i], '=');
        if (q == NULL) {
            printf("Missing argument to option %s\n", argv[i]);
            goto usage_failure;
        }
        *q = 0;
        ++q;
        if (strcmp(argv[i], "input") == 0) {
            input_file_name = q;
        } else if (strcmp(argv[i], "label") == 0) {
            if (ladder_depth == MAX_LADDER_DEPTH) {
                printf("Maximum ladder depth %u exceeded.\n",
                       (unsigned) MAX_LADDER_DEPTH);
                return EXIT_FAILURE;
            }
            ladder[ladder_depth] = q;
            ++ladder_depth;
        } else if (strcmp(argv[i], "master") == 0) {
            key_file_name = q;
        } else if (strcmp(argv[i], "output") == 0) {
            output_file_name = q;
        } else {
            printf("Unknown option: %s\n", argv[i]);
            goto usage_failure;
        }
    }

    if (strcmp(argv[1], "generate") == 0) {
        mode = MODE_GENERATE;
    } else if (strcmp(argv[1], "save") == 0) {
        mode = MODE_SAVE;
    } else if (strcmp(argv[1], "unwrap") == 0) {
        mode = MODE_UNWRAP;
    } else if (strcmp(argv[1], "wrap") == 0) {
        mode = MODE_WRAP;
    } else {
        printf("Unknown action: %s\n", argv[1]);
        goto usage_failure;
    }

    if (input_file_name == NULL &&
        (mode == MODE_WRAP || mode == MODE_UNWRAP)) {
        printf("Required argument missing: input\n");
        return DEMO_ERROR;
    }
    if (output_file_name == NULL &&
        (mode == MODE_SAVE || mode == MODE_WRAP || mode == MODE_UNWRAP)) {
        printf("Required argument missing: output\n");
        return DEMO_ERROR;
    }

    status = run(mode, key_file_name,
                 ladder, ladder_depth,
                 input_file_name, output_file_name);
    return status == PSA_SUCCESS ?
           EXIT_SUCCESS :
           EXIT_FAILURE;

usage_failure:
    usage();
    return EXIT_FAILURE;
}
#endif /* PSA_WANT_ALG_SHA_256 && MBEDTLS_MD_C &&
          MBEDTLS_AES_C && MBEDTLS_CCM_C &&
          MBEDTLS_PSA_CRYPTO_C && MBEDTLS_FS_IO */