fe-auth-scram.c

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/*-------------------------------------------------------------------------
 *
 * fe-auth-scram.c
 *     The front-end (client) implementation of SCRAM authentication.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/interfaces/libpq/fe-auth-scram.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres_fe.h"

#include "common/base64.h"
#include "common/hmac.h"
#include "common/saslprep.h"
#include "common/scram-common.h"
#include "fe-auth.h"


/*
 * Status of exchange messages used for SCRAM authentication via the
 * SASL protocol.
 */
typedef enum
{
    FE_SCRAM_INIT,
    FE_SCRAM_NONCE_SENT,
    FE_SCRAM_PROOF_SENT,
    FE_SCRAM_FINISHED
} fe_scram_state_enum;

typedef struct
{
    fe_scram_state_enum state;

    /* These are supplied by the user */
    PGconn     *conn;
    char       *password;
    char       *sasl_mechanism;

    /* We construct these */
    uint8       SaltedPassword[SCRAM_KEY_LEN];
    char       *client_nonce;
    char       *client_first_message_bare;
    char       *client_final_message_without_proof;

    /* These come from the server-first message */
    char       *server_first_message;
    char       *salt;
    int         saltlen;
    int         iterations;
    char       *nonce;

    /* These come from the server-final message */
    char       *server_final_message;
    char        ServerSignature[SCRAM_KEY_LEN];
} fe_scram_state;

static bool read_server_first_message(fe_scram_state *state, char *input);
static bool read_server_final_message(fe_scram_state *state, char *input);
static char *build_client_first_message(fe_scram_state *state);
static char *build_client_final_message(fe_scram_state *state);
static bool verify_server_signature(fe_scram_state *state, bool *match);
static bool calculate_client_proof(fe_scram_state *state,
                                   const char *client_final_message_without_proof,
                                   uint8 *result);

/*
 * Initialize SCRAM exchange status.
 */
void *
pg_fe_scram_init(PGconn *conn,
                 const char *password,
                 const char *sasl_mechanism)
{
    fe_scram_state *state;
    char       *prep_password;
    pg_saslprep_rc rc;

    Assert(sasl_mechanism != NULL);

    state = (fe_scram_state *) malloc(sizeof(fe_scram_state));
    if (!state)
        return NULL;
    memset(state, 0, sizeof(fe_scram_state));
    state->conn = conn;
    state->state = FE_SCRAM_INIT;
    state->sasl_mechanism = strdup(sasl_mechanism);

    if (!state->sasl_mechanism)
    {
        free(state);
        return NULL;
    }

    /* Normalize the password with SASLprep, if possible */
    rc = pg_saslprep(password, &prep_password);
    if (rc == SASLPREP_OOM)
    {
        free(state->sasl_mechanism);
        free(state);
        return NULL;
    }
    if (rc != SASLPREP_SUCCESS)
    {
        prep_password = strdup(password);
        if (!prep_password)
        {
            free(state->sasl_mechanism);
            free(state);
            return NULL;
        }
    }
    state->password = prep_password;

    return state;
}

/*
 * Return true if channel binding was employed and the SCRAM exchange
 * completed. This should be used after a successful exchange to determine
 * whether the server authenticated itself to the client.
 *
 * Note that the caller must also ensure that the exchange was actually
 * successful.
 */
bool
pg_fe_scram_channel_bound(void *opaq)
{
    fe_scram_state *state = (fe_scram_state *) opaq;

    /* no SCRAM exchange done */
    if (state == NULL)
        return false;

    /* SCRAM exchange not completed */
    if (state->state != FE_SCRAM_FINISHED)
        return false;

    /* channel binding mechanism not used */
    if (strcmp(state->sasl_mechanism, SCRAM_SHA_256_PLUS_NAME) != 0)
        return false;

    /* all clear! */
    return true;
}

/*
 * Free SCRAM exchange status
 */
void
pg_fe_scram_free(void *opaq)
{
    fe_scram_state *state = (fe_scram_state *) opaq;

    if (state->password)
        free(state->password);
    if (state->sasl_mechanism)
        free(state->sasl_mechanism);

    /* client messages */
    if (state->client_nonce)
        free(state->client_nonce);
    if (state->client_first_message_bare)
        free(state->client_first_message_bare);
    if (state->client_final_message_without_proof)
        free(state->client_final_message_without_proof);

    /* first message from server */
    if (state->server_first_message)
        free(state->server_first_message);
    if (state->salt)
        free(state->salt);
    if (state->nonce)
        free(state->nonce);

    /* final message from server */
    if (state->server_final_message)
        free(state->server_final_message);

    free(state);
}

/*
 * Exchange a SCRAM message with backend.
 */
void
pg_fe_scram_exchange(void *opaq, char *input, int inputlen,
                     char **output, int *outputlen,
                     bool *done, bool *success)
{
    fe_scram_state *state = (fe_scram_state *) opaq;
    PGconn     *conn = state->conn;

    *done = false;
    *success = false;
    *output = NULL;
    *outputlen = 0;

    /*
     * Check that the input length agrees with the string length of the input.
     * We can ignore inputlen after this.
     */
    if (state->state != FE_SCRAM_INIT)
    {
        if (inputlen == 0)
        {
            appendPQExpBufferStr(&conn->errorMessage,
                                 libpq_gettext("malformed SCRAM message (empty message)\n"));
            goto error;
        }
        if (inputlen != strlen(input))
        {
            appendPQExpBufferStr(&conn->errorMessage,
                                 libpq_gettext("malformed SCRAM message (length mismatch)\n"));
            goto error;
        }
    }

    switch (state->state)
    {
        case FE_SCRAM_INIT:
            /* Begin the SCRAM handshake, by sending client nonce */
            *output = build_client_first_message(state);
            if (*output == NULL)
                goto error;

            *outputlen = strlen(*output);
            *done = false;
            state->state = FE_SCRAM_NONCE_SENT;
            break;

        case FE_SCRAM_NONCE_SENT:
            /* Receive salt and server nonce, send response. */
            if (!read_server_first_message(state, input))
                goto error;

            *output = build_client_final_message(state);
            if (*output == NULL)
                goto error;

            *outputlen = strlen(*output);
            *done = false;
            state->state = FE_SCRAM_PROOF_SENT;
            break;

        case FE_SCRAM_PROOF_SENT:
            /* Receive server signature */
            if (!read_server_final_message(state, input))
                goto error;

            /*
             * Verify server signature, to make sure we're talking to the
             * genuine server.
             */
            if (!verify_server_signature(state, success))
            {
                appendPQExpBufferStr(&conn->errorMessage,
                                     libpq_gettext("could not verify server signature\n"));
                goto error;
            }

            if (!*success)
            {
                appendPQExpBufferStr(&conn->errorMessage,
                                     libpq_gettext("incorrect server signature\n"));
            }
            *done = true;
            state->state = FE_SCRAM_FINISHED;
            break;

        default:
            /* shouldn't happen */
            appendPQExpBufferStr(&conn->errorMessage,
                                 libpq_gettext("invalid SCRAM exchange state\n"));
            goto error;
    }
    return;

error:
    *done = true;
    *success = false;
}

/*
 * Read value for an attribute part of a SCRAM message.
 *
 * The buffer at **input is destructively modified, and *input is
 * advanced over the "attr=value" string and any following comma.
 *
 * On failure, append an error message to *errorMessage and return NULL.
 */
static char *
read_attr_value(char **input, char attr, PQExpBuffer errorMessage)
{
    char       *begin = *input;
    char       *end;

    if (*begin != attr)
    {
        appendPQExpBuffer(errorMessage,
                          libpq_gettext("malformed SCRAM message (attribute \"%c\" expected)\n"),
                          attr);
        return NULL;
    }
    begin++;

    if (*begin != '=')
    {
        appendPQExpBuffer(errorMessage,
                          libpq_gettext("malformed SCRAM message (expected character \"=\" for attribute \"%c\")\n"),
                          attr);
        return NULL;
    }
    begin++;

    end = begin;
    while (*end && *end != ',')
        end++;

    if (*end)
    {
        *end = '\0';
        *input = end + 1;
    }
    else
        *input = end;

    return begin;
}

/*
 * Build the first exchange message sent by the client.
 */
static char *
build_client_first_message(fe_scram_state *state)
{
    PGconn     *conn = state->conn;
    char        raw_nonce[SCRAM_RAW_NONCE_LEN + 1];
    char       *result;
    int         channel_info_len;
    int         encoded_len;
    PQExpBufferData buf;

    /*
     * Generate a "raw" nonce.  This is converted to ASCII-printable form by
     * base64-encoding it.
     */
    if (!pg_strong_random(raw_nonce, SCRAM_RAW_NONCE_LEN))
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("could not generate nonce\n"));
        return NULL;
    }

    encoded_len = pg_b64_enc_len(SCRAM_RAW_NONCE_LEN);
    /* don't forget the zero-terminator */
    state->client_nonce = malloc(encoded_len + 1);
    if (state->client_nonce == NULL)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return NULL;
    }
    encoded_len = pg_b64_encode(raw_nonce, SCRAM_RAW_NONCE_LEN,
                                state->client_nonce, encoded_len);
    if (encoded_len < 0)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("could not encode nonce\n"));
        return NULL;
    }
    state->client_nonce[encoded_len] = '\0';

    /*
     * Generate message.  The username is left empty as the backend uses the
     * value provided by the startup packet.  Also, as this username is not
     * prepared with SASLprep, the message parsing would fail if it includes
     * '=' or ',' characters.
     */

    initPQExpBuffer(&buf);

    /*
     * First build the gs2-header with channel binding information.
     */
    if (strcmp(state->sasl_mechanism, SCRAM_SHA_256_PLUS_NAME) == 0)
    {
        Assert(conn->ssl_in_use);
        appendPQExpBufferStr(&buf, "p=tls-server-end-point");
    }
#ifdef HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
    else if (conn->channel_binding[0] != 'd' && /* disable */
             conn->ssl_in_use)
    {
        /*
         * Client supports channel binding, but thinks the server does not.
         */
        appendPQExpBufferChar(&buf, 'y');
    }
#endif
    else
    {
        /*
         * Client does not support channel binding, or has disabled it.
         */
        appendPQExpBufferChar(&buf, 'n');
    }

    if (PQExpBufferDataBroken(buf))
        goto oom_error;

    channel_info_len = buf.len;

    appendPQExpBuffer(&buf, ",,n=,r=%s", state->client_nonce);
    if (PQExpBufferDataBroken(buf))
        goto oom_error;

    /*
     * The first message content needs to be saved without channel binding
     * information.
     */
    state->client_first_message_bare = strdup(buf.data + channel_info_len + 2);
    if (!state->client_first_message_bare)
        goto oom_error;

    result = strdup(buf.data);
    if (result == NULL)
        goto oom_error;

    termPQExpBuffer(&buf);
    return result;

oom_error:
    termPQExpBuffer(&buf);
    appendPQExpBufferStr(&conn->errorMessage,
                         libpq_gettext("out of memory\n"));
    return NULL;
}

/*
 * Build the final exchange message sent from the client.
 */
static char *
build_client_final_message(fe_scram_state *state)
{
    PQExpBufferData buf;
    PGconn     *conn = state->conn;
    uint8       client_proof[SCRAM_KEY_LEN];
    char       *result;
    int         encoded_len;

    initPQExpBuffer(&buf);

    /*
     * Construct client-final-message-without-proof.  We need to remember it
     * for verifying the server proof in the final step of authentication.
     *
     * The channel binding flag handling (p/y/n) must be consistent with
     * build_client_first_message(), because the server will check that it's
     * the same flag both times.
     */
    if (strcmp(state->sasl_mechanism, SCRAM_SHA_256_PLUS_NAME) == 0)
    {
#ifdef HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
        char       *cbind_data = NULL;
        size_t      cbind_data_len = 0;
        size_t      cbind_header_len;
        char       *cbind_input;
        size_t      cbind_input_len;
        int         encoded_cbind_len;

        /* Fetch hash data of server's SSL certificate */
        cbind_data =
            pgtls_get_peer_certificate_hash(state->conn,
                                            &cbind_data_len);
        if (cbind_data == NULL)
        {
            /* error message is already set on error */
            termPQExpBuffer(&buf);
            return NULL;
        }

        appendPQExpBufferStr(&buf, "c=");

        /* p=type,, */
        cbind_header_len = strlen("p=tls-server-end-point,,");
        cbind_input_len = cbind_header_len + cbind_data_len;
        cbind_input = malloc(cbind_input_len);
        if (!cbind_input)
        {
            free(cbind_data);
            goto oom_error;
        }
        memcpy(cbind_input, "p=tls-server-end-point,,", cbind_header_len);
        memcpy(cbind_input + cbind_header_len, cbind_data, cbind_data_len);

        encoded_cbind_len = pg_b64_enc_len(cbind_input_len);
        if (!enlargePQExpBuffer(&buf, encoded_cbind_len))
        {
            free(cbind_data);
            free(cbind_input);
            goto oom_error;
        }
        encoded_cbind_len = pg_b64_encode(cbind_input, cbind_input_len,
                                          buf.data + buf.len,
                                          encoded_cbind_len);
        if (encoded_cbind_len < 0)
        {
            free(cbind_data);
            free(cbind_input);
            termPQExpBuffer(&buf);
            appendPQExpBufferStr(&conn->errorMessage,
                                 "could not encode cbind data for channel binding\n");
            return NULL;
        }
        buf.len += encoded_cbind_len;
        buf.data[buf.len] = '\0';

        free(cbind_data);
        free(cbind_input);
#else
        /*
         * Chose channel binding, but the SSL library doesn't support it.
         * Shouldn't happen.
         */
        termPQExpBuffer(&buf);
        appendPQExpBufferStr(&conn->errorMessage,
                             "channel binding not supported by this build\n");
        return NULL;
#endif                          /* HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH */
    }
#ifdef HAVE_PGTLS_GET_PEER_CERTIFICATE_HASH
    else if (conn->channel_binding[0] != 'd' && /* disable */
             conn->ssl_in_use)
        appendPQExpBufferStr(&buf, "c=eSws");   /* base64 of "y,," */
#endif
    else
        appendPQExpBufferStr(&buf, "c=biws");   /* base64 of "n,," */

    if (PQExpBufferDataBroken(buf))
        goto oom_error;

    appendPQExpBuffer(&buf, ",r=%s", state->nonce);
    if (PQExpBufferDataBroken(buf))
        goto oom_error;

    state->client_final_message_without_proof = strdup(buf.data);
    if (state->client_final_message_without_proof == NULL)
        goto oom_error;

    /* Append proof to it, to form client-final-message. */
    if (!calculate_client_proof(state,
                                state->client_final_message_without_proof,
                                client_proof))
    {
        termPQExpBuffer(&buf);
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("could not calculate client proof\n"));
        return NULL;
    }

    appendPQExpBufferStr(&buf, ",p=");
    encoded_len = pg_b64_enc_len(SCRAM_KEY_LEN);
    if (!enlargePQExpBuffer(&buf, encoded_len))
        goto oom_error;
    encoded_len = pg_b64_encode((char *) client_proof,
                                SCRAM_KEY_LEN,
                                buf.data + buf.len,
                                encoded_len);
    if (encoded_len < 0)
    {
        termPQExpBuffer(&buf);
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("could not encode client proof\n"));
        return NULL;
    }
    buf.len += encoded_len;
    buf.data[buf.len] = '\0';

    result = strdup(buf.data);
    if (result == NULL)
        goto oom_error;

    termPQExpBuffer(&buf);
    return result;

oom_error:
    termPQExpBuffer(&buf);
    appendPQExpBufferStr(&conn->errorMessage,
                         libpq_gettext("out of memory\n"));
    return NULL;
}

/*
 * Read the first exchange message coming from the server.
 */
static bool
read_server_first_message(fe_scram_state *state, char *input)
{
    PGconn     *conn = state->conn;
    char       *iterations_str;
    char       *endptr;
    char       *encoded_salt;
    char       *nonce;
    int         decoded_salt_len;

    state->server_first_message = strdup(input);
    if (state->server_first_message == NULL)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return false;
    }

    /* parse the message */
    nonce = read_attr_value(&input, 'r',
                            &conn->errorMessage);
    if (nonce == NULL)
    {
        /* read_attr_value() has appended an error string */
        return false;
    }

    /* Verify immediately that the server used our part of the nonce */
    if (strlen(nonce) < strlen(state->client_nonce) ||
        memcmp(nonce, state->client_nonce, strlen(state->client_nonce)) != 0)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("invalid SCRAM response (nonce mismatch)\n"));
        return false;
    }

    state->nonce = strdup(nonce);
    if (state->nonce == NULL)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return false;
    }

    encoded_salt = read_attr_value(&input, 's', &conn->errorMessage);
    if (encoded_salt == NULL)
    {
        /* read_attr_value() has appended an error string */
        return false;
    }
    decoded_salt_len = pg_b64_dec_len(strlen(encoded_salt));
    state->salt = malloc(decoded_salt_len);
    if (state->salt == NULL)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return false;
    }
    state->saltlen = pg_b64_decode(encoded_salt,
                                   strlen(encoded_salt),
                                   state->salt,
                                   decoded_salt_len);
    if (state->saltlen < 0)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("malformed SCRAM message (invalid salt)\n"));
        return false;
    }

    iterations_str = read_attr_value(&input, 'i', &conn->errorMessage);
    if (iterations_str == NULL)
    {
        /* read_attr_value() has appended an error string */
        return false;
    }
    state->iterations = strtol(iterations_str, &endptr, 10);
    if (*endptr != '\0' || state->iterations < 1)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("malformed SCRAM message (invalid iteration count)\n"));
        return false;
    }

    if (*input != '\0')
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("malformed SCRAM message (garbage at end of server-first-message)\n"));

    return true;
}

/*
 * Read the final exchange message coming from the server.
 */
static bool
read_server_final_message(fe_scram_state *state, char *input)
{
    PGconn     *conn = state->conn;
    char       *encoded_server_signature;
    char       *decoded_server_signature;
    int         server_signature_len;

    state->server_final_message = strdup(input);
    if (!state->server_final_message)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return false;
    }

    /* Check for error result. */
    if (*input == 'e')
    {
        char       *errmsg = read_attr_value(&input, 'e',
                                             &conn->errorMessage);

        if (errmsg == NULL)
        {
            /* read_attr_value() has appended an error message */
            return false;
        }
        appendPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("error received from server in SCRAM exchange: %s\n"),
                          errmsg);
        return false;
    }

    /* Parse the message. */
    encoded_server_signature = read_attr_value(&input, 'v',
                                               &conn->errorMessage);
    if (encoded_server_signature == NULL)
    {
        /* read_attr_value() has appended an error message */
        return false;
    }

    if (*input != '\0')
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("malformed SCRAM message (garbage at end of server-final-message)\n"));

    server_signature_len = pg_b64_dec_len(strlen(encoded_server_signature));
    decoded_server_signature = malloc(server_signature_len);
    if (!decoded_server_signature)
    {
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("out of memory\n"));
        return false;
    }

    server_signature_len = pg_b64_decode(encoded_server_signature,
                                         strlen(encoded_server_signature),
                                         decoded_server_signature,
                                         server_signature_len);
    if (server_signature_len != SCRAM_KEY_LEN)
    {
        free(decoded_server_signature);
        appendPQExpBufferStr(&conn->errorMessage,
                             libpq_gettext("malformed SCRAM message (invalid server signature)\n"));
        return false;
    }
    memcpy(state->ServerSignature, decoded_server_signature, SCRAM_KEY_LEN);
    free(decoded_server_signature);

    return true;
}

/*
 * Calculate the client proof, part of the final exchange message sent
 * by the client.  Returns true on success, false on failure.
 */
static bool
calculate_client_proof(fe_scram_state *state,
                       const char *client_final_message_without_proof,
                       uint8 *result)
{
    uint8       StoredKey[SCRAM_KEY_LEN];
    uint8       ClientKey[SCRAM_KEY_LEN];
    uint8       ClientSignature[SCRAM_KEY_LEN];
    int         i;
    pg_hmac_ctx *ctx;

    ctx = pg_hmac_create(PG_SHA256);
    if (ctx == NULL)
        return false;

    /*
     * Calculate SaltedPassword, and store it in 'state' so that we can reuse
     * it later in verify_server_signature.
     */
    if (scram_SaltedPassword(state->password, state->salt, state->saltlen,
                             state->iterations, state->SaltedPassword) < 0 ||
        scram_ClientKey(state->SaltedPassword, ClientKey) < 0 ||
        scram_H(ClientKey, SCRAM_KEY_LEN, StoredKey) < 0 ||
        pg_hmac_init(ctx, StoredKey, SCRAM_KEY_LEN) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) state->client_first_message_bare,
                       strlen(state->client_first_message_bare)) < 0 ||
        pg_hmac_update(ctx, (uint8 *) ",", 1) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) state->server_first_message,
                       strlen(state->server_first_message)) < 0 ||
        pg_hmac_update(ctx, (uint8 *) ",", 1) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) client_final_message_without_proof,
                       strlen(client_final_message_without_proof)) < 0 ||
        pg_hmac_final(ctx, ClientSignature, sizeof(ClientSignature)) < 0)
    {
        pg_hmac_free(ctx);
        return false;
    }

    for (i = 0; i < SCRAM_KEY_LEN; i++)
        result[i] = ClientKey[i] ^ ClientSignature[i];

    pg_hmac_free(ctx);
    return true;
}

/*
 * Validate the server signature, received as part of the final exchange
 * message received from the server.  *match tracks if the server signature
 * matched or not. Returns true if the server signature got verified, and
 * false for a processing error.
 */
static bool
verify_server_signature(fe_scram_state *state, bool *match)
{
    uint8       expected_ServerSignature[SCRAM_KEY_LEN];
    uint8       ServerKey[SCRAM_KEY_LEN];
    pg_hmac_ctx *ctx;

    ctx = pg_hmac_create(PG_SHA256);
    if (ctx == NULL)
        return false;

    if (scram_ServerKey(state->SaltedPassword, ServerKey) < 0 ||
    /* calculate ServerSignature */
        pg_hmac_init(ctx, ServerKey, SCRAM_KEY_LEN) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) state->client_first_message_bare,
                       strlen(state->client_first_message_bare)) < 0 ||
        pg_hmac_update(ctx, (uint8 *) ",", 1) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) state->server_first_message,
                       strlen(state->server_first_message)) < 0 ||
        pg_hmac_update(ctx, (uint8 *) ",", 1) < 0 ||
        pg_hmac_update(ctx,
                       (uint8 *) state->client_final_message_without_proof,
                       strlen(state->client_final_message_without_proof)) < 0 ||
        pg_hmac_final(ctx, expected_ServerSignature,
                      sizeof(expected_ServerSignature)) < 0)
    {
        pg_hmac_free(ctx);
        return false;
    }

    pg_hmac_free(ctx);

    /* signature processed, so now check after it */
    if (memcmp(expected_ServerSignature, state->ServerSignature, SCRAM_KEY_LEN) != 0)
        *match = false;
    else
        *match = true;

    return true;
}

/*
 * Build a new SCRAM secret.
 */
char *
pg_fe_scram_build_secret(const char *password)
{
    char       *prep_password;
    pg_saslprep_rc rc;
    char        saltbuf[SCRAM_DEFAULT_SALT_LEN];
    char       *result;

    /*
     * Normalize the password with SASLprep.  If that doesn't work, because
     * the password isn't valid UTF-8 or contains prohibited characters, just
     * proceed with the original password.  (See comments at top of file.)
     */
    rc = pg_saslprep(password, &prep_password);
    if (rc == SASLPREP_OOM)
        return NULL;
    if (rc == SASLPREP_SUCCESS)
        password = (const char *) prep_password;

    /* Generate a random salt */
    if (!pg_strong_random(saltbuf, SCRAM_DEFAULT_SALT_LEN))
    {
        if (prep_password)
            free(prep_password);
        return NULL;
    }

    result = scram_build_secret(saltbuf, SCRAM_DEFAULT_SALT_LEN,
                                SCRAM_DEFAULT_ITERATIONS, password);

    if (prep_password)
        free(prep_password);

    return result;
}