fe-auth.c

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/*-------------------------------------------------------------------------
 *
 * fe-auth.c
 *     The front-end (client) authorization routines
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/interfaces/libpq/fe-auth.c
 *
 *-------------------------------------------------------------------------
 */
 
/*
 * INTERFACE ROUTINES
 *     frontend (client) routines:
 *      pg_fe_sendauth          send authentication information
 *      pg_fe_getauthname       get user's name according to the client side
 *                              of the authentication system
 */
 
#include "postgres_fe.h"
 
#ifdef WIN32
#include "win32.h"
#else
#include <unistd.h>
#include <fcntl.h>
#include <limits.h>
#include <sys/param.h>          /* for MAXHOSTNAMELEN on most */
#include <sys/socket.h>
#ifdef HAVE_SYS_UCRED_H
#include <sys/ucred.h>
#endif
#ifndef  MAXHOSTNAMELEN
#include <netdb.h>              /* for MAXHOSTNAMELEN on some */
#endif
#endif
 
#include "common/md5.h"
#include "common/scram-common.h"
#include "fe-auth.h"
#include "fe-auth-sasl.h"
#include "libpq-fe.h"
 
#ifdef ENABLE_GSS
/*
 * GSSAPI authentication system.
 */
 
#include "fe-gssapi-common.h"
 
/*
 * Continue GSS authentication with next token as needed.
 */
static int
pg_GSS_continue(PGconn *conn, int payloadlen)
{
    OM_uint32   maj_stat,
                min_stat,
                lmin_s,
                gss_flags = GSS_C_MUTUAL_FLAG;
    gss_buffer_desc ginbuf;
    gss_buffer_desc goutbuf;
 
    /*
     * On first call, there's no input token. On subsequent calls, read the
     * input token into a GSS buffer.
     */
    if (conn->gctx != GSS_C_NO_CONTEXT)
    {
        ginbuf.length = payloadlen;
        ginbuf.value = malloc(payloadlen);
        if (!ginbuf.value)
        {
            libpq_append_conn_error(conn, "out of memory allocating GSSAPI buffer (%d)",
                                    payloadlen);
            return STATUS_ERROR;
        }
        if (pqGetnchar(ginbuf.value, payloadlen, conn))
        {
            /*
             * Shouldn't happen, because the caller should've ensured that the
             * whole message is already in the input buffer.
             */
            free(ginbuf.value);
            return STATUS_ERROR;
        }
    }
    else
    {
        ginbuf.length = 0;
        ginbuf.value = NULL;
    }
 
    /* Only try to acquire credentials if GSS delegation isn't disabled. */
    if (!pg_GSS_have_cred_cache(&conn->gcred))
        conn->gcred = GSS_C_NO_CREDENTIAL;
 
    if (conn->gssdelegation && conn->gssdelegation[0] == '1')
        gss_flags |= GSS_C_DELEG_FLAG;
 
    maj_stat = gss_init_sec_context(&min_stat,
                                    conn->gcred,
                                    &conn->gctx,
                                    conn->gtarg_nam,
                                    GSS_C_NO_OID,
                                    gss_flags,
                                    0,
                                    GSS_C_NO_CHANNEL_BINDINGS,
                                    (ginbuf.value == NULL) ? GSS_C_NO_BUFFER : &ginbuf,
                                    NULL,
                                    &goutbuf,
                                    NULL,
                                    NULL);
 
    free(ginbuf.value);
 
    if (goutbuf.length != 0)
    {
        /*
         * GSS generated data to send to the server. We don't care if it's the
         * first or subsequent packet, just send the same kind of password
         * packet.
         */
        if (pqPacketSend(conn, PqMsg_GSSResponse,
                         goutbuf.value, goutbuf.length) != STATUS_OK)
        {
            gss_release_buffer(&lmin_s, &goutbuf);
            return STATUS_ERROR;
        }
    }
    gss_release_buffer(&lmin_s, &goutbuf);
 
    if (maj_stat != GSS_S_COMPLETE && maj_stat != GSS_S_CONTINUE_NEEDED)
    {
        pg_GSS_error(libpq_gettext("GSSAPI continuation error"),
                     conn,
                     maj_stat, min_stat);
        gss_release_name(&lmin_s, &conn->gtarg_nam);
        if (conn->gctx)
            gss_delete_sec_context(&lmin_s, &conn->gctx, GSS_C_NO_BUFFER);
        return STATUS_ERROR;
    }
 
    if (maj_stat == GSS_S_COMPLETE)
    {
        conn->client_finished_auth = true;
        gss_release_name(&lmin_s, &conn->gtarg_nam);
        conn->gssapi_used = true;
    }
 
    return STATUS_OK;
}
 
/*
 * Send initial GSS authentication token
 */
static int
pg_GSS_startup(PGconn *conn, int payloadlen)
{
    int         ret;
    char       *host = conn->connhost[conn->whichhost].host;
 
    if (!(host && host[0] != '\0'))
    {
        libpq_append_conn_error(conn, "host name must be specified");
        return STATUS_ERROR;
    }
 
    if (conn->gctx)
    {
        libpq_append_conn_error(conn, "duplicate GSS authentication request");
        return STATUS_ERROR;
    }
 
    ret = pg_GSS_load_servicename(conn);
    if (ret != STATUS_OK)
        return ret;
 
    /*
     * Initial packet is the same as a continuation packet with no initial
     * context.
     */
    conn->gctx = GSS_C_NO_CONTEXT;
 
    return pg_GSS_continue(conn, payloadlen);
}
#endif                          /* ENABLE_GSS */
 
 
#ifdef ENABLE_SSPI
/*
 * SSPI authentication system (Windows only)
 */
 
static void
pg_SSPI_error(PGconn *conn, const char *mprefix, SECURITY_STATUS r)
{
    char        sysmsg[256];
 
    if (FormatMessage(FORMAT_MESSAGE_IGNORE_INSERTS |
                      FORMAT_MESSAGE_FROM_SYSTEM,
                      NULL, r, 0,
                      sysmsg, sizeof(sysmsg), NULL) == 0)
        appendPQExpBuffer(&conn->errorMessage, "%s: SSPI error %x\n",
                          mprefix, (unsigned int) r);
    else
        appendPQExpBuffer(&conn->errorMessage, "%s: %s (%x)\n",
                          mprefix, sysmsg, (unsigned int) r);
}
 
/*
 * Continue SSPI authentication with next token as needed.
 */
static int
pg_SSPI_continue(PGconn *conn, int payloadlen)
{
    SECURITY_STATUS r;
    CtxtHandle  newContext;
    ULONG       contextAttr;
    SecBufferDesc inbuf;
    SecBufferDesc outbuf;
    SecBuffer   OutBuffers[1];
    SecBuffer   InBuffers[1];
    char       *inputbuf = NULL;
 
    if (conn->sspictx != NULL)
    {
        /*
         * On runs other than the first we have some data to send. Put this
         * data in a SecBuffer type structure.
         */
        inputbuf = malloc(payloadlen);
        if (!inputbuf)
        {
            libpq_append_conn_error(conn, "out of memory allocating SSPI buffer (%d)",
                                    payloadlen);
            return STATUS_ERROR;
        }
        if (pqGetnchar(inputbuf, payloadlen, conn))
        {
            /*
             * Shouldn't happen, because the caller should've ensured that the
             * whole message is already in the input buffer.
             */
            free(inputbuf);
            return STATUS_ERROR;
        }
 
        inbuf.ulVersion = SECBUFFER_VERSION;
        inbuf.cBuffers = 1;
        inbuf.pBuffers = InBuffers;
        InBuffers[0].pvBuffer = inputbuf;
        InBuffers[0].cbBuffer = payloadlen;
        InBuffers[0].BufferType = SECBUFFER_TOKEN;
    }
 
    OutBuffers[0].pvBuffer = NULL;
    OutBuffers[0].BufferType = SECBUFFER_TOKEN;
    OutBuffers[0].cbBuffer = 0;
    outbuf.cBuffers = 1;
    outbuf.pBuffers = OutBuffers;
    outbuf.ulVersion = SECBUFFER_VERSION;
 
    r = InitializeSecurityContext(conn->sspicred,
                                  conn->sspictx,
                                  conn->sspitarget,
                                  ISC_REQ_ALLOCATE_MEMORY,
                                  0,
                                  SECURITY_NETWORK_DREP,
                                  (conn->sspictx == NULL) ? NULL : &inbuf,
                                  0,
                                  &newContext,
                                  &outbuf,
                                  &contextAttr,
                                  NULL);
 
    /* we don't need the input anymore */
    free(inputbuf);
 
    if (r != SEC_E_OK && r != SEC_I_CONTINUE_NEEDED)
    {
        pg_SSPI_error(conn, libpq_gettext("SSPI continuation error"), r);
 
        return STATUS_ERROR;
    }
 
    if (conn->sspictx == NULL)
    {
        /* On first run, transfer retrieved context handle */
        conn->sspictx = malloc(sizeof(CtxtHandle));
        if (conn->sspictx == NULL)
        {
            libpq_append_conn_error(conn, "out of memory");
            return STATUS_ERROR;
        }
        memcpy(conn->sspictx, &newContext, sizeof(CtxtHandle));
    }
 
    /*
     * If SSPI returned any data to be sent to the server (as it normally
     * would), send this data as a password packet.
     */
    if (outbuf.cBuffers > 0)
    {
        if (outbuf.cBuffers != 1)
        {
            /*
             * This should never happen, at least not for Kerberos
             * authentication. Keep check in case it shows up with other
             * authentication methods later.
             */
            appendPQExpBufferStr(&conn->errorMessage,
                                 "SSPI returned invalid number of output buffers\n");
            return STATUS_ERROR;
        }
 
        /*
         * If the negotiation is complete, there may be zero bytes to send.
         * The server is at this point not expecting any more data, so don't
         * send it.
         */
        if (outbuf.pBuffers[0].cbBuffer > 0)
        {
            if (pqPacketSend(conn, PqMsg_GSSResponse,
                             outbuf.pBuffers[0].pvBuffer, outbuf.pBuffers[0].cbBuffer))
            {
                FreeContextBuffer(outbuf.pBuffers[0].pvBuffer);
                return STATUS_ERROR;
            }
        }
        FreeContextBuffer(outbuf.pBuffers[0].pvBuffer);
    }
 
    if (r == SEC_E_OK)
        conn->client_finished_auth = true;
 
    /* Cleanup is handled by the code in freePGconn() */
    return STATUS_OK;
}
 
/*
 * Send initial SSPI authentication token.
 * If use_negotiate is 0, use kerberos authentication package which is
 * compatible with Unix. If use_negotiate is 1, use the negotiate package
 * which supports both kerberos and NTLM, but is not compatible with Unix.
 */
static int
pg_SSPI_startup(PGconn *conn, int use_negotiate, int payloadlen)
{
    SECURITY_STATUS r;
    TimeStamp   expire;
    char       *host = conn->connhost[conn->whichhost].host;
 
    if (conn->sspictx)
    {
        libpq_append_conn_error(conn, "duplicate SSPI authentication request");
        return STATUS_ERROR;
    }
 
    /*
     * Retrieve credentials handle
     */
    conn->sspicred = malloc(sizeof(CredHandle));
    if (conn->sspicred == NULL)
    {
        libpq_append_conn_error(conn, "out of memory");
        return STATUS_ERROR;
    }
 
    r = AcquireCredentialsHandle(NULL,
                                 use_negotiate ? "negotiate" : "kerberos",
                                 SECPKG_CRED_OUTBOUND,
                                 NULL,
                                 NULL,
                                 NULL,
                                 NULL,
                                 conn->sspicred,
                                 &expire);
    if (r != SEC_E_OK)
    {
        pg_SSPI_error(conn, libpq_gettext("could not acquire SSPI credentials"), r);
        free(conn->sspicred);
        conn->sspicred = NULL;
        return STATUS_ERROR;
    }
 
    /*
     * Compute target principal name. SSPI has a different format from GSSAPI,
     * but not more complex. We can skip the @REALM part, because Windows will
     * fill that in for us automatically.
     */
    if (!(host && host[0] != '\0'))
    {
        libpq_append_conn_error(conn, "host name must be specified");
        return STATUS_ERROR;
    }
    conn->sspitarget = malloc(strlen(conn->krbsrvname) + strlen(host) + 2);
    if (!conn->sspitarget)
    {
        libpq_append_conn_error(conn, "out of memory");
        return STATUS_ERROR;
    }
    sprintf(conn->sspitarget, "%s/%s", conn->krbsrvname, host);
 
    /*
     * Indicate that we're in SSPI authentication mode to make sure that
     * pg_SSPI_continue is called next time in the negotiation.
     */
    conn->usesspi = 1;
 
    return pg_SSPI_continue(conn, payloadlen);
}
#endif                          /* ENABLE_SSPI */
 
/*
 * Initialize SASL authentication exchange.
 */
static int
pg_SASL_init(PGconn *conn, int payloadlen)
{
    char       *initialresponse = NULL;
    int         initialresponselen;
    const char *selected_mechanism;
    PQExpBufferData mechanism_buf;
    char       *password = NULL;
    SASLStatus  status;
 
    initPQExpBuffer(&mechanism_buf);
 
    if (conn->channel_binding[0] == 'r' &&  /* require */
        !conn->ssl_in_use)
    {
        libpq_append_conn_error(conn, "channel binding required, but SSL not in use");
        goto error;
    }
 
    if (conn->sasl_state)
    {
        libpq_append_conn_error(conn, "duplicate SASL authentication request");
        goto error;
    }
 
    /*
     * Parse the list of SASL authentication mechanisms in the
     * AuthenticationSASL message, and select the best mechanism that we
     * support. Mechanisms are listed by order of decreasing importance.
     */
    selected_mechanism = NULL;
    for (;;)
    {
        if (pqGets(&mechanism_buf, conn))
        {
            appendPQExpBufferStr(&conn->errorMessage,
                                 "fe_sendauth: invalid authentication request from server: invalid list of authentication mechanisms\n");
            goto error;
        }
        if (PQExpBufferDataBroken(mechanism_buf))
            goto oom_error;
 
        /* An empty string indicates end of list */
        if (mechanism_buf.data[0] == '\0')
            break;
 
        /*
         * Select the mechanism to use.  Pick SCRAM-SHA-256-PLUS over anything
         * else if a channel binding type is set and if the client supports it
         * (and did not set channel_binding=disable). Pick SCRAM-SHA-256 if
         * nothing else has already been picked.  If we add more mechanisms, a
         * more refined priority mechanism might become necessary.
         */
        if (strcmp(mechanism_buf.data, SCRAM_SHA_256_PLUS_NAME) == 0)
        {
            if (conn->ssl_in_use)
            {
                /* The server has offered SCRAM-SHA-256-PLUS. */
 
#ifdef USE_SSL
                /*
                 * The client supports channel binding, which is chosen if
                 * channel_binding is not disabled.
                 */
                if (conn->channel_binding[0] != 'd')    /* disable */
                {
                    selected_mechanism = SCRAM_SHA_256_PLUS_NAME;
                    conn->sasl = &pg_scram_mech;
                    conn->password_needed = true;
                }
#else
                /*
                 * The client does not support channel binding.  If it is
                 * required, complain immediately instead of the error below
                 * which would be confusing as the server is publishing
                 * SCRAM-SHA-256-PLUS.
                 */
                if (conn->channel_binding[0] == 'r')    /* require */
                {
                    libpq_append_conn_error(conn, "channel binding is required, but client does not support it");
                    goto error;
                }
#endif
            }
            else
            {
                /*
                 * The server offered SCRAM-SHA-256-PLUS, but the connection
                 * is not SSL-encrypted. That's not sane. Perhaps SSL was
                 * stripped by a proxy? There's no point in continuing,
                 * because the server will reject the connection anyway if we
                 * try authenticate without channel binding even though both
                 * the client and server supported it. The SCRAM exchange
                 * checks for that, to prevent downgrade attacks.
                 */
                libpq_append_conn_error(conn, "server offered SCRAM-SHA-256-PLUS authentication over a non-SSL connection");
                goto error;
            }
        }
        else if (strcmp(mechanism_buf.data, SCRAM_SHA_256_NAME) == 0 &&
                 !selected_mechanism)
        {
            selected_mechanism = SCRAM_SHA_256_NAME;
            conn->sasl = &pg_scram_mech;
            conn->password_needed = true;
        }
    }
 
    if (!selected_mechanism)
    {
        libpq_append_conn_error(conn, "none of the server's SASL authentication mechanisms are supported");
        goto error;
    }
 
    if (conn->channel_binding[0] == 'r' &&  /* require */
        strcmp(selected_mechanism, SCRAM_SHA_256_PLUS_NAME) != 0)
    {
        libpq_append_conn_error(conn, "channel binding is required, but server did not offer an authentication method that supports channel binding");
        goto error;
    }
 
    /*
     * Now that the SASL mechanism has been chosen for the exchange,
     * initialize its state information.
     */
 
    /*
     * First, select the password to use for the exchange, complaining if
     * there isn't one and the selected SASL mechanism needs it.
     */
    if (conn->password_needed)
    {
        password = conn->connhost[conn->whichhost].password;
        if (password == NULL)
            password = conn->pgpass;
        if (password == NULL || password[0] == '\0')
        {
            appendPQExpBufferStr(&conn->errorMessage,
                                 PQnoPasswordSupplied);
            goto error;
        }
    }
 
    Assert(conn->sasl);
 
    /*
     * Initialize the SASL state information with all the information gathered
     * during the initial exchange.
     *
     * Note: Only tls-unique is supported for the moment.
     */
    conn->sasl_state = conn->sasl->init(conn,
                                        password,
                                        selected_mechanism);
    if (!conn->sasl_state)
        goto oom_error;
 
    /* Get the mechanism-specific Initial Client Response, if any */
    status = conn->sasl->exchange(conn->sasl_state,
                                  NULL, -1,
                                  &initialresponse, &initialresponselen);
 
    if (status == SASL_FAILED)
        goto error;
 
    /*
     * Build a SASLInitialResponse message, and send it.
     */
    if (pqPutMsgStart(PqMsg_SASLInitialResponse, conn))
        goto error;
    if (pqPuts(selected_mechanism, conn))
        goto error;
    if (initialresponse)
    {
        if (pqPutInt(initialresponselen, 4, conn))
            goto error;
        if (pqPutnchar(initialresponse, initialresponselen, conn))
            goto error;
    }
    if (pqPutMsgEnd(conn))
        goto error;
    if (pqFlush(conn))
        goto error;
 
    termPQExpBuffer(&mechanism_buf);
    free(initialresponse);
 
    return STATUS_OK;
 
error:
    termPQExpBuffer(&mechanism_buf);
    free(initialresponse);
    return STATUS_ERROR;
 
oom_error:
    termPQExpBuffer(&mechanism_buf);
    free(initialresponse);
    libpq_append_conn_error(conn, "out of memory");
    return STATUS_ERROR;
}
 
/*
 * Exchange a message for SASL communication protocol with the backend.
 * This should be used after calling pg_SASL_init to set up the status of
 * the protocol.
 */
static int
pg_SASL_continue(PGconn *conn, int payloadlen, bool final)
{
    char       *output;
    int         outputlen;
    int         res;
    char       *challenge;
    SASLStatus  status;
 
    /* Read the SASL challenge from the AuthenticationSASLContinue message. */
    challenge = malloc(payloadlen + 1);
    if (!challenge)
    {
        libpq_append_conn_error(conn, "out of memory allocating SASL buffer (%d)",
                                payloadlen);
        return STATUS_ERROR;
    }
 
    if (pqGetnchar(challenge, payloadlen, conn))
    {
        free(challenge);
        return STATUS_ERROR;
    }
    /* For safety and convenience, ensure the buffer is NULL-terminated. */
    challenge[payloadlen] = '\0';
 
    status = conn->sasl->exchange(conn->sasl_state,
                                  challenge, payloadlen,
                                  &output, &outputlen);
    free(challenge);            /* don't need the input anymore */
 
    if (final && status == SASL_CONTINUE)
    {
        if (outputlen != 0)
            free(output);
 
        libpq_append_conn_error(conn, "AuthenticationSASLFinal received from server, but SASL authentication was not completed");
        return STATUS_ERROR;
    }
 
    /*
     * If the exchange is not completed yet, we need to make sure that the
     * SASL mechanism has generated a message to send back.
     */
    if (output == NULL && status == SASL_CONTINUE)
    {
        libpq_append_conn_error(conn, "no client response found after SASL exchange success");
        return STATUS_ERROR;
    }
 
    /*
     * SASL allows zero-length responses, so this check uses "output" and not
     * "outputlen" to allow the case of an empty message.
     */
    if (output)
    {
        /*
         * Send the SASL response to the server.
         */
        res = pqPacketSend(conn, PqMsg_SASLResponse, output, outputlen);
        free(output);
 
        if (res != STATUS_OK)
            return STATUS_ERROR;
    }
 
    if (status == SASL_FAILED)
        return STATUS_ERROR;
 
    return STATUS_OK;
}
 
static int
pg_password_sendauth(PGconn *conn, const char *password, AuthRequest areq)
{
    int         ret;
    char       *crypt_pwd = NULL;
    const char *pwd_to_send;
    char        md5Salt[4];
 
    /* Read the salt from the AuthenticationMD5Password message. */
    if (areq == AUTH_REQ_MD5)
    {
        if (pqGetnchar(md5Salt, 4, conn))
            return STATUS_ERROR;    /* shouldn't happen */
    }
 
    /* Encrypt the password if needed. */
 
    switch (areq)
    {
        case AUTH_REQ_MD5:
            {
                char       *crypt_pwd2;
                const char *errstr = NULL;
 
                /* Allocate enough space for two MD5 hashes */
                crypt_pwd = malloc(2 * (MD5_PASSWD_LEN + 1));
                if (!crypt_pwd)
                {
                    libpq_append_conn_error(conn, "out of memory");
                    return STATUS_ERROR;
                }
 
                crypt_pwd2 = crypt_pwd + MD5_PASSWD_LEN + 1;
                if (!pg_md5_encrypt(password, conn->pguser,
                                    strlen(conn->pguser), crypt_pwd2,
                                    &errstr))
                {
                    libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
                    free(crypt_pwd);
                    return STATUS_ERROR;
                }
                if (!pg_md5_encrypt(crypt_pwd2 + strlen("md5"), md5Salt,
                                    4, crypt_pwd, &errstr))
                {
                    libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
                    free(crypt_pwd);
                    return STATUS_ERROR;
                }
 
                pwd_to_send = crypt_pwd;
                break;
            }
        case AUTH_REQ_PASSWORD:
            pwd_to_send = password;
            break;
        default:
            return STATUS_ERROR;
    }
    ret = pqPacketSend(conn, PqMsg_PasswordMessage,
                       pwd_to_send, strlen(pwd_to_send) + 1);
    free(crypt_pwd);
    return ret;
}
 
/*
 * Translate a disallowed AuthRequest code into an error message.
 */
static const char *
auth_method_description(AuthRequest areq)
{
    switch (areq)
    {
        case AUTH_REQ_PASSWORD:
            return libpq_gettext("server requested a cleartext password");
        case AUTH_REQ_MD5:
            return libpq_gettext("server requested a hashed password");
        case AUTH_REQ_GSS:
        case AUTH_REQ_GSS_CONT:
            return libpq_gettext("server requested GSSAPI authentication");
        case AUTH_REQ_SSPI:
            return libpq_gettext("server requested SSPI authentication");
        case AUTH_REQ_SASL:
        case AUTH_REQ_SASL_CONT:
        case AUTH_REQ_SASL_FIN:
            return libpq_gettext("server requested SASL authentication");
    }
 
    return libpq_gettext("server requested an unknown authentication type");
}
 
/*
 * Convenience macro for checking the allowed_auth_methods bitmask.  Caller
 * must ensure that type is not greater than 31 (high bit of the bitmask).
 */
#define auth_method_allowed(conn, type) \
    (((conn)->allowed_auth_methods & (1 << (type))) != 0)
 
/*
 * Verify that the authentication request is expected, given the connection
 * parameters. This is especially important when the client wishes to
 * authenticate the server before any sensitive information is exchanged.
 */
static bool
check_expected_areq(AuthRequest areq, PGconn *conn)
{
    bool        result = true;
    const char *reason = NULL;
 
    StaticAssertDecl((sizeof(conn->allowed_auth_methods) * CHAR_BIT) > AUTH_REQ_MAX,
                     "AUTH_REQ_MAX overflows the allowed_auth_methods bitmask");
 
    if (conn->sslcertmode[0] == 'r' /* require */
        && areq == AUTH_REQ_OK)
    {
        /*
         * Trade off a little bit of complexity to try to get these error
         * messages as precise as possible.
         */
        if (!conn->ssl_cert_requested)
        {
            libpq_append_conn_error(conn, "server did not request an SSL certificate");
            return false;
        }
        else if (!conn->ssl_cert_sent)
        {
            libpq_append_conn_error(conn, "server accepted connection without a valid SSL certificate");
            return false;
        }
    }
 
    /*
     * If the user required a specific auth method, or specified an allowed
     * set, then reject all others here, and make sure the server actually
     * completes an authentication exchange.
     */
    if (conn->require_auth)
    {
        switch (areq)
        {
            case AUTH_REQ_OK:
 
                /*
                 * Check to make sure we've actually finished our exchange (or
                 * else that the user has allowed an authentication-less
                 * connection).
                 *
                 * If the user has allowed both SCRAM and unauthenticated
                 * (trust) connections, then this check will silently accept
                 * partial SCRAM exchanges, where a misbehaving server does
                 * not provide its verifier before sending an OK.  This is
                 * consistent with historical behavior, but it may be a point
                 * to revisit in the future, since it could allow a server
                 * that doesn't know the user's password to silently harvest
                 * material for a brute force attack.
                 */
                if (!conn->auth_required || conn->client_finished_auth)
                    break;
 
                /*
                 * No explicit authentication request was made by the server
                 * -- or perhaps it was made and not completed, in the case of
                 * SCRAM -- but there is one special case to check.  If the
                 * user allowed "gss", then a GSS-encrypted channel also
                 * satisfies the check.
                 */
#ifdef ENABLE_GSS
                if (auth_method_allowed(conn, AUTH_REQ_GSS) && conn->gssenc)
                {
                    /*
                     * If implicit GSS auth has already been performed via GSS
                     * encryption, we don't need to have performed an
                     * AUTH_REQ_GSS exchange.  This allows require_auth=gss to
                     * be combined with gssencmode, since there won't be an
                     * explicit authentication request in that case.
                     */
                }
                else
#endif
                {
                    reason = libpq_gettext("server did not complete authentication");
                    result = false;
                }
 
                break;
 
            case AUTH_REQ_PASSWORD:
            case AUTH_REQ_MD5:
            case AUTH_REQ_GSS:
            case AUTH_REQ_GSS_CONT:
            case AUTH_REQ_SSPI:
            case AUTH_REQ_SASL:
            case AUTH_REQ_SASL_CONT:
            case AUTH_REQ_SASL_FIN:
 
                /*
                 * We don't handle these with the default case, to avoid
                 * bit-shifting past the end of the allowed_auth_methods mask
                 * if the server sends an unexpected AuthRequest.
                 */
                result = auth_method_allowed(conn, areq);
                break;
 
            default:
                result = false;
                break;
        }
    }
 
    if (!result)
    {
        if (!reason)
            reason = auth_method_description(areq);
 
        libpq_append_conn_error(conn, "authentication method requirement \"%s\" failed: %s",
                                conn->require_auth, reason);
        return result;
    }
 
    /*
     * When channel_binding=require, we must protect against two cases: (1) we
     * must not respond to non-SASL authentication requests, which might leak
     * information such as the client's password; and (2) even if we receive
     * AUTH_REQ_OK, we still must ensure that channel binding has happened in
     * order to authenticate the server.
     */
    if (conn->channel_binding[0] == 'r' /* require */ )
    {
        switch (areq)
        {
            case AUTH_REQ_SASL:
            case AUTH_REQ_SASL_CONT:
            case AUTH_REQ_SASL_FIN:
                break;
            case AUTH_REQ_OK:
                if (!conn->sasl || !conn->sasl->channel_bound(conn->sasl_state))
                {
                    libpq_append_conn_error(conn, "channel binding required, but server authenticated client without channel binding");
                    result = false;
                }
                break;
            default:
                libpq_append_conn_error(conn, "channel binding required but not supported by server's authentication request");
                result = false;
                break;
        }
    }
 
    return result;
}
 
/*
 * pg_fe_sendauth
 *      client demux routine for processing an authentication request
 *
 * The server has sent us an authentication challenge (or OK). Send an
 * appropriate response. The caller has ensured that the whole message is
 * now in the input buffer, and has already read the type and length of
 * it. We are responsible for reading any remaining extra data, specific
 * to the authentication method. 'payloadlen' is the remaining length in
 * the message.
 */
int
pg_fe_sendauth(AuthRequest areq, int payloadlen, PGconn *conn)
{
    int         oldmsglen;
 
    if (!check_expected_areq(areq, conn))
        return STATUS_ERROR;
 
    switch (areq)
    {
        case AUTH_REQ_OK:
            break;
 
        case AUTH_REQ_KRB4:
            libpq_append_conn_error(conn, "Kerberos 4 authentication not supported");
            return STATUS_ERROR;
 
        case AUTH_REQ_KRB5:
            libpq_append_conn_error(conn, "Kerberos 5 authentication not supported");
            return STATUS_ERROR;
 
#if defined(ENABLE_GSS) || defined(ENABLE_SSPI)
        case AUTH_REQ_GSS:
#if !defined(ENABLE_SSPI)
            /* no native SSPI, so use GSSAPI library for it */
        case AUTH_REQ_SSPI:
#endif
            {
                int         r;
 
                pglock_thread();
 
                /*
                 * If we have both GSS and SSPI support compiled in, use SSPI
                 * support by default. This is overridable by a connection
                 * string parameter. Note that when using SSPI we still leave
                 * the negotiate parameter off, since we want SSPI to use the
                 * GSSAPI kerberos protocol. For actual SSPI negotiate
                 * protocol, we use AUTH_REQ_SSPI.
                 */
#if defined(ENABLE_GSS) && defined(ENABLE_SSPI)
                if (conn->gsslib && (pg_strcasecmp(conn->gsslib, "gssapi") == 0))
                    r = pg_GSS_startup(conn, payloadlen);
                else
                    r = pg_SSPI_startup(conn, 0, payloadlen);
#elif defined(ENABLE_GSS) && !defined(ENABLE_SSPI)
                r = pg_GSS_startup(conn, payloadlen);
#elif !defined(ENABLE_GSS) && defined(ENABLE_SSPI)
                r = pg_SSPI_startup(conn, 0, payloadlen);
#endif
                if (r != STATUS_OK)
                {
                    /* Error message already filled in. */
                    pgunlock_thread();
                    return STATUS_ERROR;
                }
                pgunlock_thread();
            }
            break;
 
        case AUTH_REQ_GSS_CONT:
            {
                int         r;
 
                pglock_thread();
#if defined(ENABLE_GSS) && defined(ENABLE_SSPI)
                if (conn->usesspi)
                    r = pg_SSPI_continue(conn, payloadlen);
                else
                    r = pg_GSS_continue(conn, payloadlen);
#elif defined(ENABLE_GSS) && !defined(ENABLE_SSPI)
                r = pg_GSS_continue(conn, payloadlen);
#elif !defined(ENABLE_GSS) && defined(ENABLE_SSPI)
                r = pg_SSPI_continue(conn, payloadlen);
#endif
                if (r != STATUS_OK)
                {
                    /* Error message already filled in. */
                    pgunlock_thread();
                    return STATUS_ERROR;
                }
                pgunlock_thread();
            }
            break;
#else                           /* defined(ENABLE_GSS) || defined(ENABLE_SSPI) */
            /* No GSSAPI *or* SSPI support */
        case AUTH_REQ_GSS:
        case AUTH_REQ_GSS_CONT:
            libpq_append_conn_error(conn, "GSSAPI authentication not supported");
            return STATUS_ERROR;
#endif                          /* defined(ENABLE_GSS) || defined(ENABLE_SSPI) */
 
#ifdef ENABLE_SSPI
        case AUTH_REQ_SSPI:
 
            /*
             * SSPI has its own startup message so libpq can decide which
             * method to use. Indicate to pg_SSPI_startup that we want SSPI
             * negotiation instead of Kerberos.
             */
            pglock_thread();
            if (pg_SSPI_startup(conn, 1, payloadlen) != STATUS_OK)
            {
                /* Error message already filled in. */
                pgunlock_thread();
                return STATUS_ERROR;
            }
            pgunlock_thread();
            break;
#else
 
            /*
             * No SSPI support. However, if we have GSSAPI but not SSPI
             * support, AUTH_REQ_SSPI will have been handled in the codepath
             * for AUTH_REQ_GSS above, so don't duplicate the case label in
             * that case.
             */
#if !defined(ENABLE_GSS)
        case AUTH_REQ_SSPI:
            libpq_append_conn_error(conn, "SSPI authentication not supported");
            return STATUS_ERROR;
#endif                          /* !define(ENABLE_GSS) */
#endif                          /* ENABLE_SSPI */
 
 
        case AUTH_REQ_CRYPT:
            libpq_append_conn_error(conn, "Crypt authentication not supported");
            return STATUS_ERROR;
 
        case AUTH_REQ_MD5:
        case AUTH_REQ_PASSWORD:
            {
                char       *password;
 
                conn->password_needed = true;
                password = conn->connhost[conn->whichhost].password;
                if (password == NULL)
                    password = conn->pgpass;
                if (password == NULL || password[0] == '\0')
                {
                    appendPQExpBufferStr(&conn->errorMessage,
                                         PQnoPasswordSupplied);
                    return STATUS_ERROR;
                }
                if (pg_password_sendauth(conn, password, areq) != STATUS_OK)
                {
                    appendPQExpBufferStr(&conn->errorMessage,
                                         "fe_sendauth: error sending password authentication\n");
                    return STATUS_ERROR;
                }
 
                /* We expect no further authentication requests. */
                conn->client_finished_auth = true;
                break;
            }
 
        case AUTH_REQ_SASL:
 
            /*
             * The request contains the name (as assigned by IANA) of the
             * authentication mechanism.
             */
            if (pg_SASL_init(conn, payloadlen) != STATUS_OK)
            {
                /* pg_SASL_init already set the error message */
                return STATUS_ERROR;
            }
            break;
 
        case AUTH_REQ_SASL_CONT:
        case AUTH_REQ_SASL_FIN:
            if (conn->sasl_state == NULL)
            {
                appendPQExpBufferStr(&conn->errorMessage,
                                     "fe_sendauth: invalid authentication request from server: AUTH_REQ_SASL_CONT without AUTH_REQ_SASL\n");
                return STATUS_ERROR;
            }
            oldmsglen = conn->errorMessage.len;
            if (pg_SASL_continue(conn, payloadlen,
                                 (areq == AUTH_REQ_SASL_FIN)) != STATUS_OK)
            {
                /* Use this message if pg_SASL_continue didn't supply one */
                if (conn->errorMessage.len == oldmsglen)
                    appendPQExpBufferStr(&conn->errorMessage,
                                         "fe_sendauth: error in SASL authentication\n");
                return STATUS_ERROR;
            }
            break;
 
        default:
            libpq_append_conn_error(conn, "authentication method %u not supported", areq);
            return STATUS_ERROR;
    }
 
    return STATUS_OK;
}
 
 
/*
 * pg_fe_getusername
 *
 * Returns a pointer to malloc'd space containing the name of the
 * specified user_id.  If there is an error, return NULL, and append
 * a suitable error message to *errorMessage if that's not NULL.
 *
 * Caution: on Windows, the user_id argument is ignored, and we always
 * fetch the current user's name.
 */
char *
pg_fe_getusername(uid_t user_id, PQExpBuffer errorMessage)
{
    char       *result = NULL;
    const char *name = NULL;
 
#ifdef WIN32
    /* Microsoft recommends buffer size of UNLEN+1, where UNLEN = 256 */
    char        username[256 + 1];
    DWORD       namesize = sizeof(username);
#else
    char        pwdbuf[BUFSIZ];
#endif
 
#ifdef WIN32
    if (GetUserName(username, &namesize))
        name = username;
    else if (errorMessage)
        libpq_append_error(errorMessage,
                           "user name lookup failure: error code %lu",
                           GetLastError());
#else
    if (pg_get_user_name(user_id, pwdbuf, sizeof(pwdbuf)))
        name = pwdbuf;
    else if (errorMessage)
        appendPQExpBuffer(errorMessage, "%s\n", pwdbuf);
#endif
 
    if (name)
    {
        result = strdup(name);
        if (result == NULL && errorMessage)
            libpq_append_error(errorMessage, "out of memory");
    }
 
    return result;
}
 
/*
 * pg_fe_getauthname
 *
 * Returns a pointer to malloc'd space containing whatever name the user
 * has authenticated to the system.  If there is an error, return NULL,
 * and append a suitable error message to *errorMessage if that's not NULL.
 */
char *
pg_fe_getauthname(PQExpBuffer errorMessage)
{
#ifdef WIN32
    return pg_fe_getusername(0, errorMessage);
#else
    return pg_fe_getusername(geteuid(), errorMessage);
#endif
}
 
 
/*
 * PQencryptPassword -- exported routine to encrypt a password with MD5
 *
 * This function is equivalent to calling PQencryptPasswordConn with
 * "md5" as the encryption method, except that this doesn't require
 * a connection object.  This function is deprecated, use
 * PQencryptPasswordConn instead.
 */
char *
PQencryptPassword(const char *passwd, const char *user)
{
    char       *crypt_pwd;
    const char *errstr = NULL;
 
    crypt_pwd = malloc(MD5_PASSWD_LEN + 1);
    if (!crypt_pwd)
        return NULL;
 
    if (!pg_md5_encrypt(passwd, user, strlen(user), crypt_pwd, &errstr))
    {
        free(crypt_pwd);
        return NULL;
    }
 
    return crypt_pwd;
}
 
/*
 * PQencryptPasswordConn -- exported routine to encrypt a password
 *
 * This is intended to be used by client applications that wish to send
 * commands like ALTER USER joe PASSWORD 'pwd'.  The password need not
 * be sent in cleartext if it is encrypted on the client side.  This is
 * good because it ensures the cleartext password won't end up in logs,
 * pg_stat displays, etc.  We export the function so that clients won't
 * be dependent on low-level details like whether the encryption is MD5
 * or something else.
 *
 * Arguments are a connection object, the cleartext password, the SQL
 * name of the user it is for, and a string indicating the algorithm to
 * use for encrypting the password.  If algorithm is NULL, this queries
 * the server for the current 'password_encryption' value.  If you wish
 * to avoid that, e.g. to avoid blocking, you can execute
 * 'show password_encryption' yourself before calling this function, and
 * pass it as the algorithm.
 *
 * Return value is a malloc'd string.  The client may assume the string
 * doesn't contain any special characters that would require escaping.
 * On error, an error message is stored in the connection object, and
 * returns NULL.
 */
char *
PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
                      const char *algorithm)
{
#define MAX_ALGORITHM_NAME_LEN 50
    char        algobuf[MAX_ALGORITHM_NAME_LEN + 1];
    char       *crypt_pwd = NULL;
 
    if (!conn)
        return NULL;
 
    pqClearConnErrorState(conn);
 
    /* If no algorithm was given, ask the server. */
    if (algorithm == NULL)
    {
        PGresult   *res;
        char       *val;
 
        res = PQexec(conn, "show password_encryption");
        if (res == NULL)
        {
            /* PQexec() should've set conn->errorMessage already */
            return NULL;
        }
        if (PQresultStatus(res) != PGRES_TUPLES_OK)
        {
            /* PQexec() should've set conn->errorMessage already */
            PQclear(res);
            return NULL;
        }
        if (PQntuples(res) != 1 || PQnfields(res) != 1)
        {
            PQclear(res);
            libpq_append_conn_error(conn, "unexpected shape of result set returned for SHOW");
            return NULL;
        }
        val = PQgetvalue(res, 0, 0);
 
        if (strlen(val) > MAX_ALGORITHM_NAME_LEN)
        {
            PQclear(res);
            libpq_append_conn_error(conn, "\"password_encryption\" value too long");
            return NULL;
        }
        strcpy(algobuf, val);
        PQclear(res);
 
        algorithm = algobuf;
    }
 
    /*
     * Also accept "on" and "off" as aliases for "md5", because
     * password_encryption was a boolean before PostgreSQL 10.  We refuse to
     * send the password in plaintext even if it was "off".
     */
    if (strcmp(algorithm, "on") == 0 ||
        strcmp(algorithm, "off") == 0)
        algorithm = "md5";
 
    /*
     * Ok, now we know what algorithm to use
     */
    if (strcmp(algorithm, "scram-sha-256") == 0)
    {
        const char *errstr = NULL;
 
        crypt_pwd = pg_fe_scram_build_secret(passwd,
                                             conn->scram_sha_256_iterations,
                                             &errstr);
        if (!crypt_pwd)
            libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
    }
    else if (strcmp(algorithm, "md5") == 0)
    {
        crypt_pwd = malloc(MD5_PASSWD_LEN + 1);
        if (crypt_pwd)
        {
            const char *errstr = NULL;
 
            if (!pg_md5_encrypt(passwd, user, strlen(user), crypt_pwd, &errstr))
            {
                libpq_append_conn_error(conn, "could not encrypt password: %s", errstr);
                free(crypt_pwd);
                crypt_pwd = NULL;
            }
        }
        else
            libpq_append_conn_error(conn, "out of memory");
    }
    else
    {
        libpq_append_conn_error(conn, "unrecognized password encryption algorithm \"%s\"",
                                algorithm);
        return NULL;
    }
 
    return crypt_pwd;
}
 
/*
 * PQchangePassword -- exported routine to change a password
 *
 * This is intended to be used by client applications that wish to
 * change the password for a user. The password is not sent in
 * cleartext because it is encrypted on the client side. This is
 * good because it ensures the cleartext password is never known by
 * the server, and therefore won't end up in logs, pg_stat displays,
 * etc. The password encryption is performed by PQencryptPasswordConn(),
 * which is passed a NULL for the algorithm argument. Hence encryption
 * is done according to the server's password_encryption
 * setting. We export the function so that clients won't be dependent
 * on the implementation specific details with respect to how the
 * server changes passwords.
 *
 * Arguments are a connection object, the SQL name of the target user,
 * and the cleartext password.
 *
 * Return value is the PGresult of the executed ALTER USER statement
 * or NULL if we never get there. The caller is responsible to PQclear()
 * the returned PGresult.
 *
 * PQresultStatus() should be called to check the return value for errors,
 * and PQerrorMessage() used to get more information about such errors.
 */
PGresult *
PQchangePassword(PGconn *conn, const char *user, const char *passwd)
{
    char       *encrypted_password = PQencryptPasswordConn(conn, passwd,
                                                           user, NULL);
 
    if (!encrypted_password)
    {
        /* PQencryptPasswordConn() already registered the error */
        return NULL;
    }
    else
    {
        char       *fmtpw = PQescapeLiteral(conn, encrypted_password,
                                            strlen(encrypted_password));
 
        /* no longer needed, so clean up now */
        PQfreemem(encrypted_password);
 
        if (!fmtpw)
        {
            /* PQescapeLiteral() already registered the error */
            return NULL;
        }
        else
        {
            char       *fmtuser = PQescapeIdentifier(conn, user, strlen(user));
 
            if (!fmtuser)
            {
                /* PQescapeIdentifier() already registered the error */
                PQfreemem(fmtpw);
                return NULL;
            }
            else
            {
                PQExpBufferData buf;
                PGresult   *res;
 
                initPQExpBuffer(&buf);
                printfPQExpBuffer(&buf, "ALTER USER %s PASSWORD %s",
                                  fmtuser, fmtpw);
 
                res = PQexec(conn, buf.data);
 
                /* clean up */
                termPQExpBuffer(&buf);
                PQfreemem(fmtuser);
                PQfreemem(fmtpw);
 
                return res;
            }
        }
    }
}