auth-scram.c

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/*-------------------------------------------------------------------------
 *
 * auth-scram.c
 *    Server-side implementation of the SASL SCRAM-SHA-256 mechanism.
 *
 * See the following RFCs for more details:
 * - RFC 5802: https://tools.ietf.org/html/rfc5802
 * - RFC 5803: https://tools.ietf.org/html/rfc5803
 * - RFC 7677: https://tools.ietf.org/html/rfc7677
 *
 * Here are some differences:
 *
 * - Username from the authentication exchange is not used. The client
 *   should send an empty string as the username.
 *
 * - If the password isn't valid UTF-8, or contains characters prohibited
 *   by the SASLprep profile, we skip the SASLprep pre-processing and use
 *   the raw bytes in calculating the hash.
 *
 * - If channel binding is used, the channel binding type is always
 *   "tls-server-end-point".  The spec says the default is "tls-unique"
 *   (RFC 5802, section 6.1. Default Channel Binding), but there are some
 *   problems with that.  Firstly, not all SSL libraries provide an API to
 *   get the TLS Finished message, required to use "tls-unique".  Secondly,
 *   "tls-unique" is not specified for TLS v1.3, and as of this writing,
 *   it's not clear if there will be a replacement.  We could support both
 *   "tls-server-end-point" and "tls-unique", but for our use case,
 *   "tls-unique" doesn't really have any advantages.  The main advantage
 *   of "tls-unique" would be that it works even if the server doesn't
 *   have a certificate, but PostgreSQL requires a server certificate
 *   whenever SSL is used, anyway.
 *
 *
 * The password stored in pg_authid consists of the iteration count, salt,
 * StoredKey and ServerKey.
 *
 * SASLprep usage
 * --------------
 *
 * One notable difference to the SCRAM specification is that while the
 * specification dictates that the password is in UTF-8, and prohibits
 * certain characters, we are more lenient.  If the password isn't a valid
 * UTF-8 string, or contains prohibited characters, the raw bytes are used
 * to calculate the hash instead, without SASLprep processing.  This is
 * because PostgreSQL supports other encodings too, and the encoding being
 * used during authentication is undefined (client_encoding isn't set until
 * after authentication).  In effect, we try to interpret the password as
 * UTF-8 and apply SASLprep processing, but if it looks invalid, we assume
 * that it's in some other encoding.
 *
 * In the worst case, we misinterpret a password that's in a different
 * encoding as being Unicode, because it happens to consists entirely of
 * valid UTF-8 bytes, and we apply Unicode normalization to it.  As long
 * as we do that consistently, that will not lead to failed logins.
 * Fortunately, the UTF-8 byte sequences that are ignored by SASLprep
 * don't correspond to any commonly used characters in any of the other
 * supported encodings, so it should not lead to any significant loss in
 * entropy, even if the normalization is incorrectly applied to a
 * non-UTF-8 password.
 *
 * Error handling
 * --------------
 *
 * Don't reveal user information to an unauthenticated client.  We don't
 * want an attacker to be able to probe whether a particular username is
 * valid.  In SCRAM, the server has to read the salt and iteration count
 * from the user's stored secret, and send it to the client.  To avoid
 * revealing whether a user exists, when the client tries to authenticate
 * with a username that doesn't exist, or doesn't have a valid SCRAM
 * secret in pg_authid, we create a fake salt and iteration count
 * on-the-fly, and proceed with the authentication with that.  In the end,
 * we'll reject the attempt, as if an incorrect password was given.  When
 * we are performing a "mock" authentication, the 'doomed' flag in
 * scram_state is set.
 *
 * In the error messages, avoid printing strings from the client, unless
 * you check that they are pure ASCII.  We don't want an unauthenticated
 * attacker to be able to spam the logs with characters that are not valid
 * to the encoding being used, whatever that is.  We cannot avoid that in
 * general, after logging in, but let's do what we can here.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/libpq/auth-scram.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/xlog.h"
#include "catalog/pg_authid.h"
#include "catalog/pg_control.h"
#include "common/base64.h"
#include "common/hmac.h"
#include "common/saslprep.h"
#include "common/scram-common.h"
#include "common/sha2.h"
#include "libpq/auth.h"
#include "libpq/crypt.h"
#include "libpq/sasl.h"
#include "libpq/scram.h"
#include "miscadmin.h"
#include "utils/builtins.h"
#include "utils/timestamp.h"

static void scram_get_mechanisms(Port *port, StringInfo buf);
static void *scram_init(Port *port, const char *selected_mech,
                        const char *shadow_pass);
static int  scram_exchange(void *opaq, const char *input, int inputlen,
                           char **output, int *outputlen, char **logdetail);

/* Mechanism declaration */
const pg_be_sasl_mech pg_be_scram_mech = {
    scram_get_mechanisms,
    scram_init,
    scram_exchange
};

/*
 * Status data for a SCRAM authentication exchange.  This should be kept
 * internal to this file.
 */
typedef enum
{
    SCRAM_AUTH_INIT,
    SCRAM_AUTH_SALT_SENT,
    SCRAM_AUTH_FINISHED
} scram_state_enum;

typedef struct
{
    scram_state_enum state;

    const char *username;       /* username from startup packet */

    Port       *port;
    bool        channel_binding_in_use;

    int         iterations;
    char       *salt;           /* base64-encoded */
    uint8       StoredKey[SCRAM_KEY_LEN];
    uint8       ServerKey[SCRAM_KEY_LEN];

    /* Fields of the first message from client */
    char        cbind_flag;
    char       *client_first_message_bare;
    char       *client_username;
    char       *client_nonce;

    /* Fields from the last message from client */
    char       *client_final_message_without_proof;
    char       *client_final_nonce;
    char        ClientProof[SCRAM_KEY_LEN];

    /* Fields generated in the server */
    char       *server_first_message;
    char       *server_nonce;

    /*
     * If something goes wrong during the authentication, or we are performing
     * a "mock" authentication (see comments at top of file), the 'doomed'
     * flag is set.  A reason for the failure, for the server log, is put in
     * 'logdetail'.
     */
    bool        doomed;
    char       *logdetail;
} scram_state;

static void read_client_first_message(scram_state *state, const char *input);
static void read_client_final_message(scram_state *state, const char *input);
static char *build_server_first_message(scram_state *state);
static char *build_server_final_message(scram_state *state);
static bool verify_client_proof(scram_state *state);
static bool verify_final_nonce(scram_state *state);
static void mock_scram_secret(const char *username, int *iterations,
                              char **salt, uint8 *stored_key, uint8 *server_key);
static bool is_scram_printable(char *p);
static char *sanitize_char(char c);
static char *sanitize_str(const char *s);
static char *scram_mock_salt(const char *username);

/*
 * Get a list of SASL mechanisms that this module supports.
 *
 * For the convenience of building the FE/BE packet that lists the
 * mechanisms, the names are appended to the given StringInfo buffer,
 * separated by '\0' bytes.
 */
static void
scram_get_mechanisms(Port *port, StringInfo buf)
{
    /*
     * Advertise the mechanisms in decreasing order of importance.  So the
     * channel-binding variants go first, if they are supported.  Channel
     * binding is only supported with SSL, and only if the SSL implementation
     * has a function to get the certificate's hash.
     */
#ifdef HAVE_BE_TLS_GET_CERTIFICATE_HASH
    if (port->ssl_in_use)
    {
        appendStringInfoString(buf, SCRAM_SHA_256_PLUS_NAME);
        appendStringInfoChar(buf, '\0');
    }
#endif
    appendStringInfoString(buf, SCRAM_SHA_256_NAME);
    appendStringInfoChar(buf, '\0');
}

/*
 * Initialize a new SCRAM authentication exchange status tracker.  This
 * needs to be called before doing any exchange.  It will be filled later
 * after the beginning of the exchange with authentication information.
 *
 * 'selected_mech' identifies the SASL mechanism that the client selected.
 * It should be one of the mechanisms that we support, as returned by
 * scram_get_mechanisms().
 *
 * 'shadow_pass' is the role's stored secret, from pg_authid.rolpassword.
 * The username was provided by the client in the startup message, and is
 * available in port->user_name.  If 'shadow_pass' is NULL, we still perform
 * an authentication exchange, but it will fail, as if an incorrect password
 * was given.
 */
static void *
scram_init(Port *port, const char *selected_mech, const char *shadow_pass)
{
    scram_state *state;
    bool        got_secret;

    state = (scram_state *) palloc0(sizeof(scram_state));
    state->port = port;
    state->state = SCRAM_AUTH_INIT;

    /*
     * Parse the selected mechanism.
     *
     * Note that if we don't support channel binding, either because the SSL
     * implementation doesn't support it or we're not using SSL at all, we
     * would not have advertised the PLUS variant in the first place.  If the
     * client nevertheless tries to select it, it's a protocol violation like
     * selecting any other SASL mechanism we don't support.
     */
#ifdef HAVE_BE_TLS_GET_CERTIFICATE_HASH
    if (strcmp(selected_mech, SCRAM_SHA_256_PLUS_NAME) == 0 && port->ssl_in_use)
        state->channel_binding_in_use = true;
    else
#endif
    if (strcmp(selected_mech, SCRAM_SHA_256_NAME) == 0)
        state->channel_binding_in_use = false;
    else
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("client selected an invalid SASL authentication mechanism")));

    /*
     * Parse the stored secret.
     */
    if (shadow_pass)
    {
        int         password_type = get_password_type(shadow_pass);

        if (password_type == PASSWORD_TYPE_SCRAM_SHA_256)
        {
            if (parse_scram_secret(shadow_pass, &state->iterations, &state->salt,
                                   state->StoredKey, state->ServerKey))
                got_secret = true;
            else
            {
                /*
                 * The password looked like a SCRAM secret, but could not be
                 * parsed.
                 */
                ereport(LOG,
                        (errmsg("invalid SCRAM secret for user \"%s\"",
                                state->port->user_name)));
                got_secret = false;
            }
        }
        else
        {
            /*
             * The user doesn't have SCRAM secret. (You cannot do SCRAM
             * authentication with an MD5 hash.)
             */
            state->logdetail = psprintf(_("User \"%s\" does not have a valid SCRAM secret."),
                                        state->port->user_name);
            got_secret = false;
        }
    }
    else
    {
        /*
         * The caller requested us to perform a dummy authentication.  This is
         * considered normal, since the caller requested it, so don't set log
         * detail.
         */
        got_secret = false;
    }

    /*
     * If the user did not have a valid SCRAM secret, we still go through the
     * motions with a mock one, and fail as if the client supplied an
     * incorrect password.  This is to avoid revealing information to an
     * attacker.
     */
    if (!got_secret)
    {
        mock_scram_secret(state->port->user_name, &state->iterations,
                          &state->salt, state->StoredKey, state->ServerKey);
        state->doomed = true;
    }

    return state;
}

/*
 * Continue a SCRAM authentication exchange.
 *
 * 'input' is the SCRAM payload sent by the client.  On the first call,
 * 'input' contains the "Initial Client Response" that the client sent as
 * part of the SASLInitialResponse message, or NULL if no Initial Client
 * Response was given.  (The SASL specification distinguishes between an
 * empty response and non-existing one.)  On subsequent calls, 'input'
 * cannot be NULL.  For convenience in this function, the caller must
 * ensure that there is a null terminator at input[inputlen].
 *
 * The next message to send to client is saved in 'output', for a length
 * of 'outputlen'.  In the case of an error, optionally store a palloc'd
 * string at *logdetail that will be sent to the postmaster log (but not
 * the client).
 */
static int
scram_exchange(void *opaq, const char *input, int inputlen,
               char **output, int *outputlen, char **logdetail)
{
    scram_state *state = (scram_state *) opaq;
    int         result;

    *output = NULL;

    /*
     * If the client didn't include an "Initial Client Response" in the
     * SASLInitialResponse message, send an empty challenge, to which the
     * client will respond with the same data that usually comes in the
     * Initial Client Response.
     */
    if (input == NULL)
    {
        Assert(state->state == SCRAM_AUTH_INIT);

        *output = pstrdup("");
        *outputlen = 0;
        return PG_SASL_EXCHANGE_CONTINUE;
    }

    /*
     * Check that the input length agrees with the string length of the input.
     * We can ignore inputlen after this.
     */
    if (inputlen == 0)
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("The message is empty.")));
    if (inputlen != strlen(input))
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Message length does not match input length.")));

    switch (state->state)
    {
        case SCRAM_AUTH_INIT:

            /*
             * Initialization phase.  Receive the first message from client
             * and be sure that it parsed correctly.  Then send the challenge
             * to the client.
             */
            read_client_first_message(state, input);

            /* prepare message to send challenge */
            *output = build_server_first_message(state);

            state->state = SCRAM_AUTH_SALT_SENT;
            result = PG_SASL_EXCHANGE_CONTINUE;
            break;

        case SCRAM_AUTH_SALT_SENT:

            /*
             * Final phase for the server.  Receive the response to the
             * challenge previously sent, verify, and let the client know that
             * everything went well (or not).
             */
            read_client_final_message(state, input);

            if (!verify_final_nonce(state))
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("invalid SCRAM response"),
                         errdetail("Nonce does not match.")));

            /*
             * Now check the final nonce and the client proof.
             *
             * If we performed a "mock" authentication that we knew would fail
             * from the get go, this is where we fail.
             *
             * The SCRAM specification includes an error code,
             * "invalid-proof", for authentication failure, but it also allows
             * erroring out in an application-specific way.  We choose to do
             * the latter, so that the error message for invalid password is
             * the same for all authentication methods.  The caller will call
             * ereport(), when we return PG_SASL_EXCHANGE_FAILURE with no
             * output.
             *
             * NB: the order of these checks is intentional.  We calculate the
             * client proof even in a mock authentication, even though it's
             * bound to fail, to thwart timing attacks to determine if a role
             * with the given name exists or not.
             */
            if (!verify_client_proof(state) || state->doomed)
            {
                result = PG_SASL_EXCHANGE_FAILURE;
                break;
            }

            /* Build final message for client */
            *output = build_server_final_message(state);

            /* Success! */
            result = PG_SASL_EXCHANGE_SUCCESS;
            state->state = SCRAM_AUTH_FINISHED;
            break;

        default:
            elog(ERROR, "invalid SCRAM exchange state");
            result = PG_SASL_EXCHANGE_FAILURE;
    }

    if (result == PG_SASL_EXCHANGE_FAILURE && state->logdetail && logdetail)
        *logdetail = state->logdetail;

    if (*output)
        *outputlen = strlen(*output);

    return result;
}

/*
 * Construct a SCRAM secret, for storing in pg_authid.rolpassword.
 *
 * The result is palloc'd, so caller is responsible for freeing it.
 */
char *
pg_be_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_SUCCESS)
        password = (const char *) prep_password;

    /* Generate random salt */
    if (!pg_strong_random(saltbuf, SCRAM_DEFAULT_SALT_LEN))
        ereport(ERROR,
                (errcode(ERRCODE_INTERNAL_ERROR),
                 errmsg("could not generate random salt")));

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

    if (prep_password)
        pfree(prep_password);

    return result;
}

/*
 * Verify a plaintext password against a SCRAM secret.  This is used when
 * performing plaintext password authentication for a user that has a SCRAM
 * secret stored in pg_authid.
 */
bool
scram_verify_plain_password(const char *username, const char *password,
                            const char *secret)
{
    char       *encoded_salt;
    char       *salt;
    int         saltlen;
    int         iterations;
    uint8       salted_password[SCRAM_KEY_LEN];
    uint8       stored_key[SCRAM_KEY_LEN];
    uint8       server_key[SCRAM_KEY_LEN];
    uint8       computed_key[SCRAM_KEY_LEN];
    char       *prep_password;
    pg_saslprep_rc rc;

    if (!parse_scram_secret(secret, &iterations, &encoded_salt,
                            stored_key, server_key))
    {
        /*
         * The password looked like a SCRAM secret, but could not be parsed.
         */
        ereport(LOG,
                (errmsg("invalid SCRAM secret for user \"%s\"", username)));
        return false;
    }

    saltlen = pg_b64_dec_len(strlen(encoded_salt));
    salt = palloc(saltlen);
    saltlen = pg_b64_decode(encoded_salt, strlen(encoded_salt), salt,
                            saltlen);
    if (saltlen < 0)
    {
        ereport(LOG,
                (errmsg("invalid SCRAM secret for user \"%s\"", username)));
        return false;
    }

    /* Normalize the password */
    rc = pg_saslprep(password, &prep_password);
    if (rc == SASLPREP_SUCCESS)
        password = prep_password;

    /* Compute Server Key based on the user-supplied plaintext password */
    if (scram_SaltedPassword(password, salt, saltlen, iterations,
                             salted_password) < 0 ||
        scram_ServerKey(salted_password, computed_key) < 0)
    {
        elog(ERROR, "could not compute server key");
    }

    if (prep_password)
        pfree(prep_password);

    /*
     * Compare the secret's Server Key with the one computed from the
     * user-supplied password.
     */
    return memcmp(computed_key, server_key, SCRAM_KEY_LEN) == 0;
}


/*
 * Parse and validate format of given SCRAM secret.
 *
 * On success, the iteration count, salt, stored key, and server key are
 * extracted from the secret, and returned to the caller.  For 'stored_key'
 * and 'server_key', the caller must pass pre-allocated buffers of size
 * SCRAM_KEY_LEN.  Salt is returned as a base64-encoded, null-terminated
 * string.  The buffer for the salt is palloc'd by this function.
 *
 * Returns true if the SCRAM secret has been parsed, and false otherwise.
 */
bool
parse_scram_secret(const char *secret, int *iterations, char **salt,
                   uint8 *stored_key, uint8 *server_key)
{
    char       *v;
    char       *p;
    char       *scheme_str;
    char       *salt_str;
    char       *iterations_str;
    char       *storedkey_str;
    char       *serverkey_str;
    int         decoded_len;
    char       *decoded_salt_buf;
    char       *decoded_stored_buf;
    char       *decoded_server_buf;

    /*
     * The secret is of form:
     *
     * SCRAM-SHA-256$<iterations>:<salt>$<storedkey>:<serverkey>
     */
    v = pstrdup(secret);
    if ((scheme_str = strtok(v, "$")) == NULL)
        goto invalid_secret;
    if ((iterations_str = strtok(NULL, ":")) == NULL)
        goto invalid_secret;
    if ((salt_str = strtok(NULL, "$")) == NULL)
        goto invalid_secret;
    if ((storedkey_str = strtok(NULL, ":")) == NULL)
        goto invalid_secret;
    if ((serverkey_str = strtok(NULL, "")) == NULL)
        goto invalid_secret;

    /* Parse the fields */
    if (strcmp(scheme_str, "SCRAM-SHA-256") != 0)
        goto invalid_secret;

    errno = 0;
    *iterations = strtol(iterations_str, &p, 10);
    if (*p || errno != 0)
        goto invalid_secret;

    /*
     * Verify that the salt is in Base64-encoded format, by decoding it,
     * although we return the encoded version to the caller.
     */
    decoded_len = pg_b64_dec_len(strlen(salt_str));
    decoded_salt_buf = palloc(decoded_len);
    decoded_len = pg_b64_decode(salt_str, strlen(salt_str),
                                decoded_salt_buf, decoded_len);
    if (decoded_len < 0)
        goto invalid_secret;
    *salt = pstrdup(salt_str);

    /*
     * Decode StoredKey and ServerKey.
     */
    decoded_len = pg_b64_dec_len(strlen(storedkey_str));
    decoded_stored_buf = palloc(decoded_len);
    decoded_len = pg_b64_decode(storedkey_str, strlen(storedkey_str),
                                decoded_stored_buf, decoded_len);
    if (decoded_len != SCRAM_KEY_LEN)
        goto invalid_secret;
    memcpy(stored_key, decoded_stored_buf, SCRAM_KEY_LEN);

    decoded_len = pg_b64_dec_len(strlen(serverkey_str));
    decoded_server_buf = palloc(decoded_len);
    decoded_len = pg_b64_decode(serverkey_str, strlen(serverkey_str),
                                decoded_server_buf, decoded_len);
    if (decoded_len != SCRAM_KEY_LEN)
        goto invalid_secret;
    memcpy(server_key, decoded_server_buf, SCRAM_KEY_LEN);

    return true;

invalid_secret:
    *salt = NULL;
    return false;
}

/*
 * Generate plausible SCRAM secret parameters for mock authentication.
 *
 * In a normal authentication, these are extracted from the secret
 * stored in the server.  This function generates values that look
 * realistic, for when there is no stored secret.
 *
 * Like in parse_scram_secret(), for 'stored_key' and 'server_key', the
 * caller must pass pre-allocated buffers of size SCRAM_KEY_LEN, and
 * the buffer for the salt is palloc'd by this function.
 */
static void
mock_scram_secret(const char *username, int *iterations, char **salt,
                  uint8 *stored_key, uint8 *server_key)
{
    char       *raw_salt;
    char       *encoded_salt;
    int         encoded_len;

    /*
     * Generate deterministic salt.
     *
     * Note that we cannot reveal any information to an attacker here so the
     * error messages need to remain generic.  This should never fail anyway
     * as the salt generated for mock authentication uses the cluster's nonce
     * value.
     */
    raw_salt = scram_mock_salt(username);
    if (raw_salt == NULL)
        elog(ERROR, "could not encode salt");

    encoded_len = pg_b64_enc_len(SCRAM_DEFAULT_SALT_LEN);
    /* don't forget the zero-terminator */
    encoded_salt = (char *) palloc(encoded_len + 1);
    encoded_len = pg_b64_encode(raw_salt, SCRAM_DEFAULT_SALT_LEN, encoded_salt,
                                encoded_len);

    if (encoded_len < 0)
        elog(ERROR, "could not encode salt");
    encoded_salt[encoded_len] = '\0';

    *salt = encoded_salt;
    *iterations = SCRAM_DEFAULT_ITERATIONS;

    /* StoredKey and ServerKey are not used in a doomed authentication */
    memset(stored_key, 0, SCRAM_KEY_LEN);
    memset(server_key, 0, SCRAM_KEY_LEN);
}

/*
 * Read the value in a given SCRAM exchange message for given attribute.
 */
static char *
read_attr_value(char **input, char attr)
{
    char       *begin = *input;
    char       *end;

    if (*begin != attr)
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Expected attribute \"%c\" but found \"%s\".",
                           attr, sanitize_char(*begin))));
    begin++;

    if (*begin != '=')
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Expected character \"=\" for attribute \"%c\".", attr)));
    begin++;

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

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

    return begin;
}

static bool
is_scram_printable(char *p)
{
    /*------
     * Printable characters, as defined by SCRAM spec: (RFC 5802)
     *
     *  printable       = %x21-2B / %x2D-7E
     *                    ;; Printable ASCII except ",".
     *                    ;; Note that any "printable" is also
     *                    ;; a valid "value".
     *------
     */
    for (; *p; p++)
    {
        if (*p < 0x21 || *p > 0x7E || *p == 0x2C /* comma */ )
            return false;
    }
    return true;
}

/*
 * Convert an arbitrary byte to printable form.  For error messages.
 *
 * If it's a printable ASCII character, print it as a single character.
 * otherwise, print it in hex.
 *
 * The returned pointer points to a static buffer.
 */
static char *
sanitize_char(char c)
{
    static char buf[5];

    if (c >= 0x21 && c <= 0x7E)
        snprintf(buf, sizeof(buf), "'%c'", c);
    else
        snprintf(buf, sizeof(buf), "0x%02x", (unsigned char) c);
    return buf;
}

/*
 * Convert an arbitrary string to printable form, for error messages.
 *
 * Anything that's not a printable ASCII character is replaced with
 * '?', and the string is truncated at 30 characters.
 *
 * The returned pointer points to a static buffer.
 */
static char *
sanitize_str(const char *s)
{
    static char buf[30 + 1];
    int         i;

    for (i = 0; i < sizeof(buf) - 1; i++)
    {
        char        c = s[i];

        if (c == '\0')
            break;

        if (c >= 0x21 && c <= 0x7E)
            buf[i] = c;
        else
            buf[i] = '?';
    }
    buf[i] = '\0';
    return buf;
}

/*
 * Read the next attribute and value in a SCRAM exchange message.
 *
 * The attribute character is set in *attr_p, the attribute value is the
 * return value.
 */
static char *
read_any_attr(char **input, char *attr_p)
{
    char       *begin = *input;
    char       *end;
    char        attr = *begin;

    if (attr == '\0')
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Attribute expected, but found end of string.")));

    /*------
     * attr-val        = ALPHA "=" value
     *                   ;; Generic syntax of any attribute sent
     *                   ;; by server or client
     *------
     */
    if (!((attr >= 'A' && attr <= 'Z') ||
          (attr >= 'a' && attr <= 'z')))
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Attribute expected, but found invalid character \"%s\".",
                           sanitize_char(attr))));
    if (attr_p)
        *attr_p = attr;
    begin++;

    if (*begin != '=')
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Expected character \"=\" for attribute \"%c\".", attr)));
    begin++;

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

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

    return begin;
}

/*
 * Read and parse the first message from client in the context of a SCRAM
 * authentication exchange message.
 *
 * At this stage, any errors will be reported directly with ereport(ERROR).
 */
static void
read_client_first_message(scram_state *state, const char *input)
{
    char       *p = pstrdup(input);
    char       *channel_binding_type;


    /*------
     * The syntax for the client-first-message is: (RFC 5802)
     *
     * saslname        = 1*(value-safe-char / "=2C" / "=3D")
     *                   ;; Conforms to <value>.
     *
     * authzid         = "a=" saslname
     *                   ;; Protocol specific.
     *
     * cb-name         = 1*(ALPHA / DIGIT / "." / "-")
     *                    ;; See RFC 5056, Section 7.
     *                    ;; E.g., "tls-server-end-point" or
     *                    ;; "tls-unique".
     *
     * gs2-cbind-flag  = ("p=" cb-name) / "n" / "y"
     *                   ;; "n" -> client doesn't support channel binding.
     *                   ;; "y" -> client does support channel binding
     *                   ;;        but thinks the server does not.
     *                   ;; "p" -> client requires channel binding.
     *                   ;; The selected channel binding follows "p=".
     *
     * gs2-header      = gs2-cbind-flag "," [ authzid ] ","
     *                   ;; GS2 header for SCRAM
     *                   ;; (the actual GS2 header includes an optional
     *                   ;; flag to indicate that the GSS mechanism is not
     *                   ;; "standard", but since SCRAM is "standard", we
     *                   ;; don't include that flag).
     *
     * username        = "n=" saslname
     *                   ;; Usernames are prepared using SASLprep.
     *
     * reserved-mext  = "m=" 1*(value-char)
     *                   ;; Reserved for signaling mandatory extensions.
     *                   ;; The exact syntax will be defined in
     *                   ;; the future.
     *
     * nonce           = "r=" c-nonce [s-nonce]
     *                   ;; Second part provided by server.
     *
     * c-nonce         = printable
     *
     * client-first-message-bare =
     *                   [reserved-mext ","]
     *                   username "," nonce ["," extensions]
     *
     * client-first-message =
     *                   gs2-header client-first-message-bare
     *
     * For example:
     * n,,n=user,r=fyko+d2lbbFgONRv9qkxdawL
     *
     * The "n,," in the beginning means that the client doesn't support
     * channel binding, and no authzid is given.  "n=user" is the username.
     * However, in PostgreSQL the username is sent in the startup packet, and
     * the username in the SCRAM exchange is ignored.  libpq always sends it
     * as an empty string.  The last part, "r=fyko+d2lbbFgONRv9qkxdawL" is
     * the client nonce.
     *------
     */

    /*
     * Read gs2-cbind-flag.  (For details see also RFC 5802 Section 6 "Channel
     * Binding".)
     */
    state->cbind_flag = *p;
    switch (*p)
    {
        case 'n':

            /*
             * The client does not support channel binding or has simply
             * decided to not use it.  In that case just let it go.
             */
            if (state->channel_binding_in_use)
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("malformed SCRAM message"),
                         errdetail("The client selected SCRAM-SHA-256-PLUS, but the SCRAM message does not include channel binding data.")));

            p++;
            if (*p != ',')
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("malformed SCRAM message"),
                         errdetail("Comma expected, but found character \"%s\".",
                                   sanitize_char(*p))));
            p++;
            break;
        case 'y':

            /*
             * The client supports channel binding and thinks that the server
             * does not.  In this case, the server must fail authentication if
             * it supports channel binding.
             */
            if (state->channel_binding_in_use)
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("malformed SCRAM message"),
                         errdetail("The client selected SCRAM-SHA-256-PLUS, but the SCRAM message does not include channel binding data.")));

#ifdef HAVE_BE_TLS_GET_CERTIFICATE_HASH
            if (state->port->ssl_in_use)
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
                         errmsg("SCRAM channel binding negotiation error"),
                         errdetail("The client supports SCRAM channel binding but thinks the server does not.  "
                                   "However, this server does support channel binding.")));
#endif
            p++;
            if (*p != ',')
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("malformed SCRAM message"),
                         errdetail("Comma expected, but found character \"%s\".",
                                   sanitize_char(*p))));
            p++;
            break;
        case 'p':

            /*
             * The client requires channel binding.  Channel binding type
             * follows, e.g., "p=tls-server-end-point".
             */
            if (!state->channel_binding_in_use)
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("malformed SCRAM message"),
                         errdetail("The client selected SCRAM-SHA-256 without channel binding, but the SCRAM message includes channel binding data.")));

            channel_binding_type = read_attr_value(&p, 'p');

            /*
             * The only channel binding type we support is
             * tls-server-end-point.
             */
            if (strcmp(channel_binding_type, "tls-server-end-point") != 0)
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("unsupported SCRAM channel-binding type \"%s\"",
                                sanitize_str(channel_binding_type))));
            break;
        default:
            ereport(ERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("malformed SCRAM message"),
                     errdetail("Unexpected channel-binding flag \"%s\".",
                               sanitize_char(*p))));
    }

    /*
     * Forbid optional authzid (authorization identity).  We don't support it.
     */
    if (*p == 'a')
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("client uses authorization identity, but it is not supported")));
    if (*p != ',')
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Unexpected attribute \"%s\" in client-first-message.",
                           sanitize_char(*p))));
    p++;

    state->client_first_message_bare = pstrdup(p);

    /*
     * Any mandatory extensions would go here.  We don't support any.
     *
     * RFC 5802 specifies error code "e=extensions-not-supported" for this,
     * but it can only be sent in the server-final message.  We prefer to fail
     * immediately (which the RFC also allows).
     */
    if (*p == 'm')
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("client requires an unsupported SCRAM extension")));

    /*
     * Read username.  Note: this is ignored.  We use the username from the
     * startup message instead, still it is kept around if provided as it
     * proves to be useful for debugging purposes.
     */
    state->client_username = read_attr_value(&p, 'n');

    /* read nonce and check that it is made of only printable characters */
    state->client_nonce = read_attr_value(&p, 'r');
    if (!is_scram_printable(state->client_nonce))
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("non-printable characters in SCRAM nonce")));

    /*
     * There can be any number of optional extensions after this.  We don't
     * support any extensions, so ignore them.
     */
    while (*p != '\0')
        read_any_attr(&p, NULL);

    /* success! */
}

/*
 * Verify the final nonce contained in the last message received from
 * client in an exchange.
 */
static bool
verify_final_nonce(scram_state *state)
{
    int         client_nonce_len = strlen(state->client_nonce);
    int         server_nonce_len = strlen(state->server_nonce);
    int         final_nonce_len = strlen(state->client_final_nonce);

    if (final_nonce_len != client_nonce_len + server_nonce_len)
        return false;
    if (memcmp(state->client_final_nonce, state->client_nonce, client_nonce_len) != 0)
        return false;
    if (memcmp(state->client_final_nonce + client_nonce_len, state->server_nonce, server_nonce_len) != 0)
        return false;

    return true;
}

/*
 * Verify the client proof contained in the last message received from
 * client in an exchange.  Returns true if the verification is a success,
 * or false for a failure.
 */
static bool
verify_client_proof(scram_state *state)
{
    uint8       ClientSignature[SCRAM_KEY_LEN];
    uint8       ClientKey[SCRAM_KEY_LEN];
    uint8       client_StoredKey[SCRAM_KEY_LEN];
    pg_hmac_ctx *ctx = pg_hmac_create(PG_SHA256);
    int         i;

    /*
     * Calculate ClientSignature.  Note that we don't log directly a failure
     * here even when processing the calculations as this could involve a mock
     * authentication.
     */
    if (pg_hmac_init(ctx, state->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 *) state->client_final_message_without_proof,
                       strlen(state->client_final_message_without_proof)) < 0 ||
        pg_hmac_final(ctx, ClientSignature, sizeof(ClientSignature)) < 0)
    {
        elog(ERROR, "could not calculate client signature");
    }

    pg_hmac_free(ctx);

    /* Extract the ClientKey that the client calculated from the proof */
    for (i = 0; i < SCRAM_KEY_LEN; i++)
        ClientKey[i] = state->ClientProof[i] ^ ClientSignature[i];

    /* Hash it one more time, and compare with StoredKey */
    if (scram_H(ClientKey, SCRAM_KEY_LEN, client_StoredKey) < 0)
        elog(ERROR, "could not hash stored key");

    if (memcmp(client_StoredKey, state->StoredKey, SCRAM_KEY_LEN) != 0)
        return false;

    return true;
}

/*
 * Build the first server-side message sent to the client in a SCRAM
 * communication exchange.
 */
static char *
build_server_first_message(scram_state *state)
{
    /*------
     * The syntax for the server-first-message is: (RFC 5802)
     *
     * server-first-message =
     *                   [reserved-mext ","] nonce "," salt ","
     *                   iteration-count ["," extensions]
     *
     * nonce           = "r=" c-nonce [s-nonce]
     *                   ;; Second part provided by server.
     *
     * c-nonce         = printable
     *
     * s-nonce         = printable
     *
     * salt            = "s=" base64
     *
     * iteration-count = "i=" posit-number
     *                   ;; A positive number.
     *
     * Example:
     *
     * r=fyko+d2lbbFgONRv9qkxdawL3rfcNHYJY1ZVvWVs7j,s=QSXCR+Q6sek8bf92,i=4096
     *------
     */

    /*
     * Per the spec, the nonce may consist of any printable ASCII characters.
     * For convenience, however, we don't use the whole range available,
     * rather, we generate some random bytes, and base64 encode them.
     */
    char        raw_nonce[SCRAM_RAW_NONCE_LEN];
    int         encoded_len;

    if (!pg_strong_random(raw_nonce, SCRAM_RAW_NONCE_LEN))
        ereport(ERROR,
                (errcode(ERRCODE_INTERNAL_ERROR),
                 errmsg("could not generate random nonce")));

    encoded_len = pg_b64_enc_len(SCRAM_RAW_NONCE_LEN);
    /* don't forget the zero-terminator */
    state->server_nonce = palloc(encoded_len + 1);
    encoded_len = pg_b64_encode(raw_nonce, SCRAM_RAW_NONCE_LEN,
                                state->server_nonce, encoded_len);
    if (encoded_len < 0)
        ereport(ERROR,
                (errcode(ERRCODE_INTERNAL_ERROR),
                 errmsg("could not encode random nonce")));
    state->server_nonce[encoded_len] = '\0';

    state->server_first_message =
        psprintf("r=%s%s,s=%s,i=%d",
                 state->client_nonce, state->server_nonce,
                 state->salt, state->iterations);

    return pstrdup(state->server_first_message);
}


/*
 * Read and parse the final message received from client.
 */
static void
read_client_final_message(scram_state *state, const char *input)
{
    char        attr;
    char       *channel_binding;
    char       *value;
    char       *begin,
               *proof;
    char       *p;
    char       *client_proof;
    int         client_proof_len;

    begin = p = pstrdup(input);

    /*------
     * The syntax for the server-first-message is: (RFC 5802)
     *
     * gs2-header      = gs2-cbind-flag "," [ authzid ] ","
     *                   ;; GS2 header for SCRAM
     *                   ;; (the actual GS2 header includes an optional
     *                   ;; flag to indicate that the GSS mechanism is not
     *                   ;; "standard", but since SCRAM is "standard", we
     *                   ;; don't include that flag).
     *
     * cbind-input   = gs2-header [ cbind-data ]
     *                   ;; cbind-data MUST be present for
     *                   ;; gs2-cbind-flag of "p" and MUST be absent
     *                   ;; for "y" or "n".
     *
     * channel-binding = "c=" base64
     *                   ;; base64 encoding of cbind-input.
     *
     * proof           = "p=" base64
     *
     * client-final-message-without-proof =
     *                   channel-binding "," nonce [","
     *                   extensions]
     *
     * client-final-message =
     *                   client-final-message-without-proof "," proof
     *------
     */

    /*
     * Read channel binding.  This repeats the channel-binding flags and is
     * then followed by the actual binding data depending on the type.
     */
    channel_binding = read_attr_value(&p, 'c');
    if (state->channel_binding_in_use)
    {
#ifdef HAVE_BE_TLS_GET_CERTIFICATE_HASH
        const char *cbind_data = NULL;
        size_t      cbind_data_len = 0;
        size_t      cbind_header_len;
        char       *cbind_input;
        size_t      cbind_input_len;
        char       *b64_message;
        int         b64_message_len;

        Assert(state->cbind_flag == 'p');

        /* Fetch hash data of server's SSL certificate */
        cbind_data = be_tls_get_certificate_hash(state->port,
                                                 &cbind_data_len);

        /* should not happen */
        if (cbind_data == NULL || cbind_data_len == 0)
            elog(ERROR, "could not get server certificate hash");

        cbind_header_len = strlen("p=tls-server-end-point,,");  /* p=type,, */
        cbind_input_len = cbind_header_len + cbind_data_len;
        cbind_input = palloc(cbind_input_len);
        snprintf(cbind_input, cbind_input_len, "p=tls-server-end-point,,");
        memcpy(cbind_input + cbind_header_len, cbind_data, cbind_data_len);

        b64_message_len = pg_b64_enc_len(cbind_input_len);
        /* don't forget the zero-terminator */
        b64_message = palloc(b64_message_len + 1);
        b64_message_len = pg_b64_encode(cbind_input, cbind_input_len,
                                        b64_message, b64_message_len);
        if (b64_message_len < 0)
            elog(ERROR, "could not encode channel binding data");
        b64_message[b64_message_len] = '\0';

        /*
         * Compare the value sent by the client with the value expected by the
         * server.
         */
        if (strcmp(channel_binding, b64_message) != 0)
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION),
                     errmsg("SCRAM channel binding check failed")));
#else
        /* shouldn't happen, because we checked this earlier already */
        elog(ERROR, "channel binding not supported by this build");
#endif
    }
    else
    {
        /*
         * If we are not using channel binding, the binding data is expected
         * to always be "biws", which is "n,," base64-encoded, or "eSws",
         * which is "y,,".  We also have to check whether the flag is the same
         * one that the client originally sent.
         */
        if (!(strcmp(channel_binding, "biws") == 0 && state->cbind_flag == 'n') &&
            !(strcmp(channel_binding, "eSws") == 0 && state->cbind_flag == 'y'))
            ereport(ERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("unexpected SCRAM channel-binding attribute in client-final-message")));
    }

    state->client_final_nonce = read_attr_value(&p, 'r');

    /* ignore optional extensions, read until we find "p" attribute */
    do
    {
        proof = p - 1;
        value = read_any_attr(&p, &attr);
    } while (attr != 'p');

    client_proof_len = pg_b64_dec_len(strlen(value));
    client_proof = palloc(client_proof_len);
    if (pg_b64_decode(value, strlen(value), client_proof,
                      client_proof_len) != SCRAM_KEY_LEN)
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Malformed proof in client-final-message.")));
    memcpy(state->ClientProof, client_proof, SCRAM_KEY_LEN);
    pfree(client_proof);

    if (*p != '\0')
        ereport(ERROR,
                (errcode(ERRCODE_PROTOCOL_VIOLATION),
                 errmsg("malformed SCRAM message"),
                 errdetail("Garbage found at the end of client-final-message.")));

    state->client_final_message_without_proof = palloc(proof - begin + 1);
    memcpy(state->client_final_message_without_proof, input, proof - begin);
    state->client_final_message_without_proof[proof - begin] = '\0';
}

/*
 * Build the final server-side message of an exchange.
 */
static char *
build_server_final_message(scram_state *state)
{
    uint8       ServerSignature[SCRAM_KEY_LEN];
    char       *server_signature_base64;
    int         siglen;
    pg_hmac_ctx *ctx = pg_hmac_create(PG_SHA256);

    /* calculate ServerSignature */
    if (pg_hmac_init(ctx, state->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, ServerSignature, sizeof(ServerSignature)) < 0)
    {
        elog(ERROR, "could not calculate server signature");
    }

    pg_hmac_free(ctx);

    siglen = pg_b64_enc_len(SCRAM_KEY_LEN);
    /* don't forget the zero-terminator */
    server_signature_base64 = palloc(siglen + 1);
    siglen = pg_b64_encode((const char *) ServerSignature,
                           SCRAM_KEY_LEN, server_signature_base64,
                           siglen);
    if (siglen < 0)
        elog(ERROR, "could not encode server signature");
    server_signature_base64[siglen] = '\0';

    /*------
     * The syntax for the server-final-message is: (RFC 5802)
     *
     * verifier        = "v=" base64
     *                   ;; base-64 encoded ServerSignature.
     *
     * server-final-message = (server-error / verifier)
     *                   ["," extensions]
     *
     *------
     */
    return psprintf("v=%s", server_signature_base64);
}


/*
 * Deterministically generate salt for mock authentication, using a SHA256
 * hash based on the username and a cluster-level secret key.  Returns a
 * pointer to a static buffer of size SCRAM_DEFAULT_SALT_LEN, or NULL.
 */
static char *
scram_mock_salt(const char *username)
{
    pg_cryptohash_ctx *ctx;
    static uint8 sha_digest[PG_SHA256_DIGEST_LENGTH];
    char       *mock_auth_nonce = GetMockAuthenticationNonce();

    /*
     * Generate salt using a SHA256 hash of the username and the cluster's
     * mock authentication nonce.  (This works as long as the salt length is
     * not larger than the SHA256 digest length.  If the salt is smaller, the
     * caller will just ignore the extra data.)
     */
    StaticAssertStmt(PG_SHA256_DIGEST_LENGTH >= SCRAM_DEFAULT_SALT_LEN,
                     "salt length greater than SHA256 digest length");

    ctx = pg_cryptohash_create(PG_SHA256);
    if (pg_cryptohash_init(ctx) < 0 ||
        pg_cryptohash_update(ctx, (uint8 *) username, strlen(username)) < 0 ||
        pg_cryptohash_update(ctx, (uint8 *) mock_auth_nonce, MOCK_AUTH_NONCE_LEN) < 0 ||
        pg_cryptohash_final(ctx, sha_digest, sizeof(sha_digest)) < 0)
    {
        pg_cryptohash_free(ctx);
        return NULL;
    }
    pg_cryptohash_free(ctx);

    return (char *) sha_digest;
}