fe-auth.c File Reference

#include "postgres_fe.h"
#include <unistd.h>
#include <fcntl.h>
#include <sys/param.h>
#include <sys/socket.h>
#include <netdb.h>
#include <pwd.h>
#include "common/md5.h"
#include "common/scram-common.h"
#include "libpq-fe.h"
#include "fe-auth.h"

Include dependency graph for fe-auth.c:

Go to the source code of this file.

Macros
#define	MAX_ALGORITHM_NAME_LEN   50

Functions
static int	pg_SASL_init (PGconn *conn, int payloadlen)
static int	pg_SASL_continue (PGconn *conn, int payloadlen, bool final)
static int	pg_local_sendauth (PGconn *conn)
static int	pg_password_sendauth (PGconn *conn, const char *password, AuthRequest areq)
int	pg_fe_sendauth (AuthRequest areq, int payloadlen, PGconn *conn)
char *	pg_fe_getauthname (PQExpBuffer errorMessage)
char *	PQencryptPassword (const char *passwd, const char *user)
char *	PQencryptPasswordConn (PGconn *conn, const char *passwd, const char *user, const char *algorithm)

Macro Definition Documentation

MAX_ALGORITHM_NAME_LEN

#define MAX_ALGORITHM_NAME_LEN   50

Referenced by PQencryptPasswordConn().

Function Documentation

pg_fe_getauthname()

char* pg_fe_getauthname

(

PQExpBuffer

errorMessage

)

Definition at line 1043 of file fe-auth.c.

References libpq_gettext, name, pglock_thread, pgunlock_thread, pqGetpwuid(), pqStrerror(), printfPQExpBuffer(), and username.

Referenced by connectOptions2(), and conninfo_add_defaults().

{
    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
    uid_t       user_id = geteuid();
    char        pwdbuf[BUFSIZ];
    struct passwd pwdstr;
    struct passwd *pw = NULL;
    int         pwerr;
#endif

    /*
     * Some users are using configure --enable-thread-safety-force, so we
     * might as well do the locking within our library to protect
     * pqGetpwuid(). In fact, application developers can use getpwuid() in
     * their application if they use the locking call we provide, or install
     * their own locking function using PQregisterThreadLock().
     */
    pglock_thread();

#ifdef WIN32
    if (GetUserName(username, &namesize))
        name = username;
    else if (errorMessage)
        printfPQExpBuffer(errorMessage,
                          libpq_gettext("user name lookup failure: error code %lu\n"),
                          GetLastError());
#else
    pwerr = pqGetpwuid(user_id, &pwdstr, pwdbuf, sizeof(pwdbuf), &pw);
    if (pw != NULL)
        name = pw->pw_name;
    else if (errorMessage)
    {
        if (pwerr != 0)
            printfPQExpBuffer(errorMessage,
                              libpq_gettext("could not look up local user ID %d: %s\n"),
                              (int) user_id,
                              pqStrerror(pwerr, pwdbuf, sizeof(pwdbuf)));
        else
            printfPQExpBuffer(errorMessage,
                              libpq_gettext("local user with ID %d does not exist\n"),
                              (int) user_id);
    }
#endif

    if (name)
    {
        result = strdup(name);
        if (result == NULL && errorMessage)
            printfPQExpBuffer(errorMessage,
                              libpq_gettext("out of memory\n"));
    }

    pgunlock_thread();

    return result;
}

pg_fe_sendauth()

int pg_fe_sendauth	(	AuthRequest	areq,
		int	payloadlen,
		PGconn *	conn
	)

Definition at line 836 of file fe-auth.c.

References AUTH_REQ_CRYPT, AUTH_REQ_GSS, AUTH_REQ_GSS_CONT, AUTH_REQ_KRB4, AUTH_REQ_KRB5, AUTH_REQ_MD5, AUTH_REQ_OK, AUTH_REQ_PASSWORD, AUTH_REQ_SASL, AUTH_REQ_SASL_CONT, AUTH_REQ_SASL_FIN, AUTH_REQ_SCM_CREDS, AUTH_REQ_SSPI, pg_conn::connhost, pg_conn::errorMessage, PQExpBufferData::len, libpq_gettext, password, pg_conn_host::password, pg_conn::password_needed, pg_local_sendauth(), pg_password_sendauth(), pg_SASL_continue(), pg_SASL_init(), pg_strcasecmp(), pglock_thread, pg_conn::pgpass, pgunlock_thread, PQnoPasswordSupplied, printfPQExpBuffer(), pg_conn::sasl_state, STATUS_ERROR, STATUS_OK, and pg_conn::whichhost.

Referenced by PQconnectPoll().

{
    switch (areq)
    {
        case AUTH_REQ_OK:
            break;

        case AUTH_REQ_KRB4:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("Kerberos 4 authentication not supported\n"));
            return STATUS_ERROR;

        case AUTH_REQ_KRB5:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("Kerberos 5 authentication not supported\n"));
            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:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("GSSAPI authentication not supported\n"));
            return STATUS_ERROR;
#endif                          /* defined(ENABLE_GSS) || defined(ENABLE_SSPI) */

#ifdef ENABLE_SSPI
        case AUTH_REQ_SSPI:

            /*
             * SSPI has it's 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_GSSAPI above, so don't duplicate the case label in
             * that case.
             */
#if !defined(ENABLE_GSS)
        case AUTH_REQ_SSPI:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("SSPI authentication not supported\n"));
            return STATUS_ERROR;
#endif                          /* !define(ENABLE_GSSAPI) */
#endif                          /* ENABLE_SSPI */


        case AUTH_REQ_CRYPT:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("Crypt authentication not supported\n"));
            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')
                {
                    printfPQExpBuffer(&conn->errorMessage,
                                      PQnoPasswordSupplied);
                    return STATUS_ERROR;
                }
                if (pg_password_sendauth(conn, password, areq) != STATUS_OK)
                {
                    printfPQExpBuffer(&conn->errorMessage,
                                      "fe_sendauth: error sending password authentication\n");
                    return STATUS_ERROR;
                }
                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)
            {
                printfPQExpBuffer(&conn->errorMessage,
                                  "fe_sendauth: invalid authentication request from server: AUTH_REQ_SASL_CONT without AUTH_REQ_SASL\n");
                return STATUS_ERROR;
            }
            if (pg_SASL_continue(conn, payloadlen,
                                 (areq == AUTH_REQ_SASL_FIN)) != STATUS_OK)
            {
                /* Use error message, if set already */
                if (conn->errorMessage.len == 0)
                    printfPQExpBuffer(&conn->errorMessage,
                                      "fe_sendauth: error in SASL authentication\n");
                return STATUS_ERROR;
            }
            break;

        case AUTH_REQ_SCM_CREDS:
            if (pg_local_sendauth(conn) != STATUS_OK)
                return STATUS_ERROR;
            break;

        default:
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("authentication method %u not supported\n"), areq);
            return STATUS_ERROR;
    }

    return STATUS_OK;
}

pg_local_sendauth()

static int pg_local_sendauth ( PGconn *  conn )

static

Definition at line 708 of file fe-auth.c.

References buf, pg_conn::errorMessage, libpq_gettext, pqStrerror(), printfPQExpBuffer(), pg_conn::sock, STATUS_ERROR, and STATUS_OK.

Referenced by pg_fe_sendauth().

{
#ifdef HAVE_STRUCT_CMSGCRED
    char        buf;
    struct iovec iov;
    struct msghdr msg;
    struct cmsghdr *cmsg;
    union
    {
        struct cmsghdr hdr;
        unsigned char buf[CMSG_SPACE(sizeof(struct cmsgcred))];
    }           cmsgbuf;

    /*
     * The backend doesn't care what we send here, but it wants exactly one
     * character to force recvmsg() to block and wait for us.
     */
    buf = '\0';
    iov.iov_base = &buf;
    iov.iov_len = 1;

    memset(&msg, 0, sizeof(msg));
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    /* We must set up a message that will be filled in by kernel */
    memset(&cmsgbuf, 0, sizeof(cmsgbuf));
    msg.msg_control = &cmsgbuf.buf;
    msg.msg_controllen = sizeof(cmsgbuf.buf);
    cmsg = CMSG_FIRSTHDR(&msg);
    cmsg->cmsg_len = CMSG_LEN(sizeof(struct cmsgcred));
    cmsg->cmsg_level = SOL_SOCKET;
    cmsg->cmsg_type = SCM_CREDS;

    if (sendmsg(conn->sock, &msg, 0) == -1)
    {
        char        sebuf[256];

        printfPQExpBuffer(&conn->errorMessage,
                          "pg_local_sendauth: sendmsg: %s\n",
                          pqStrerror(errno, sebuf, sizeof(sebuf)));
        return STATUS_ERROR;
    }
    return STATUS_OK;
#else
    printfPQExpBuffer(&conn->errorMessage,
                      libpq_gettext("SCM_CRED authentication method not supported\n"));
    return STATUS_ERROR;
#endif
}

pg_password_sendauth()

static int pg_password_sendauth ( PGconn *  conn, const char *  password, AuthRequest  areq  )

static

Definition at line 760 of file fe-auth.c.

References AUTH_REQ_MD5, AUTH_REQ_PASSWORD, pg_conn::errorMessage, free, libpq_gettext, malloc, MD5_PASSWD_LEN, password, pg_md5_encrypt(), PG_PROTOCOL_MAJOR, pg_conn::pguser, pqGetnchar(), pqPacketSend(), printfPQExpBuffer(), pg_conn::pversion, and STATUS_ERROR.

Referenced by pg_fe_sendauth().

{
    int         ret;
    char       *crypt_pwd = NULL;
    const char *pwd_to_send;
    char        md5Salt[4];

    /* Read the salt from the AuthenticationMD5 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;

                /* Allocate enough space for two MD5 hashes */
                crypt_pwd = malloc(2 * (MD5_PASSWD_LEN + 1));
                if (!crypt_pwd)
                {
                    printfPQExpBuffer(&conn->errorMessage,
                                      libpq_gettext("out of memory\n"));
                    return STATUS_ERROR;
                }

                crypt_pwd2 = crypt_pwd + MD5_PASSWD_LEN + 1;
                if (!pg_md5_encrypt(password, conn->pguser,
                                    strlen(conn->pguser), crypt_pwd2))
                {
                    free(crypt_pwd);
                    return STATUS_ERROR;
                }
                if (!pg_md5_encrypt(crypt_pwd2 + strlen("md5"), md5Salt,
                                    4, crypt_pwd))
                {
                    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;
    }
    /* Packet has a message type as of protocol 3.0 */
    if (PG_PROTOCOL_MAJOR(conn->pversion) >= 3)
        ret = pqPacketSend(conn, 'p', pwd_to_send, strlen(pwd_to_send) + 1);
    else
        ret = pqPacketSend(conn, 0, pwd_to_send, strlen(pwd_to_send) + 1);
    if (crypt_pwd)
        free(crypt_pwd);
    return ret;
}

pg_SASL_continue()

static int pg_SASL_continue ( PGconn *  conn, int  payloadlen, bool  final  )

static

Definition at line 638 of file fe-auth.c.

References pg_conn::errorMessage, free, libpq_gettext, malloc, output(), pg_fe_scram_exchange(), pqGetnchar(), pqPacketSend(), printfPQExpBuffer(), pg_conn::sasl_state, STATUS_ERROR, STATUS_OK, and success.

Referenced by pg_fe_sendauth().

{
    char       *output;
    int         outputlen;
    bool        done;
    bool        success;
    int         res;
    char       *challenge;

    /* Read the SASL challenge from the AuthenticationSASLContinue message. */
    challenge = malloc(payloadlen + 1);
    if (!challenge)
    {
        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("out of memory allocating SASL buffer (%d)\n"),
                          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';

    pg_fe_scram_exchange(conn->sasl_state,
                         challenge, payloadlen,
                         &output, &outputlen,
                         &done, &success);
    free(challenge);            /* don't need the input anymore */

    if (final && !done)
    {
        if (outputlen != 0)
            free(output);

        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("AuthenticationSASLFinal received from server, but SASL authentication was not completed\n"));
        return STATUS_ERROR;
    }
    if (outputlen != 0)
    {
        /*
         * Send the SASL response to the server.
         */
        res = pqPacketSend(conn, 'p', output, outputlen);
        free(output);

        if (res != STATUS_OK)
            return STATUS_ERROR;
    }

    if (done && !success)
        return STATUS_ERROR;

    return STATUS_OK;
}

pg_SASL_init()

static int pg_SASL_init ( PGconn *  conn, int  payloadlen  )

static

Definition at line 486 of file fe-auth.c.

References pg_conn::connhost, PQExpBufferData::data, error(), pg_conn::errorMessage, free, initPQExpBuffer(), libpq_gettext, password, pg_conn_host::password, pg_conn::password_needed, pg_fe_scram_exchange(), pg_fe_scram_init(), pg_conn::pgpass, PQExpBufferDataBroken, pqFlush(), pqGets(), PQnoPasswordSupplied, pqPutInt(), pqPutMsgEnd(), pqPutMsgStart(), pqPutnchar(), pqPuts(), printfPQExpBuffer(), pg_conn::sasl_state, pg_conn::scram_channel_binding, SCRAM_SHA_256_NAME, SCRAM_SHA_256_PLUS_NAME, pg_conn::ssl_in_use, STATUS_ERROR, STATUS_OK, success, termPQExpBuffer(), and pg_conn::whichhost.

Referenced by pg_fe_sendauth().

{
    char       *initialresponse = NULL;
    int         initialresponselen;
    bool        done;
    bool        success;
    const char *selected_mechanism;
    PQExpBufferData mechanism_buf;
    char       *password;

    initPQExpBuffer(&mechanism_buf);

    if (conn->sasl_state)
    {
        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("duplicate SASL authentication request\n"));
        goto error;
    }

    /*
     * Parse the list of SASL authentication mechanisms in the
     * AuthenticationSASL message, and select the best mechanism that we
     * support.  SCRAM-SHA-256-PLUS and SCRAM-SHA-256 are the only ones
     * supported at the moment, listed by order of decreasing importance.
     */
    selected_mechanism = NULL;
    for (;;)
    {
        if (pqGets(&mechanism_buf, conn))
        {
            printfPQExpBuffer(&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.  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 (conn->ssl_in_use &&
            conn->scram_channel_binding &&
            strlen(conn->scram_channel_binding) > 0 &&
            strcmp(mechanism_buf.data, SCRAM_SHA_256_PLUS_NAME) == 0)
            selected_mechanism = SCRAM_SHA_256_PLUS_NAME;
        else if (strcmp(mechanism_buf.data, SCRAM_SHA_256_NAME) == 0 &&
                 !selected_mechanism)
            selected_mechanism = SCRAM_SHA_256_NAME;
    }

    if (!selected_mechanism)
    {
        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("none of the server's SASL authentication mechanisms are supported\n"));
        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.  Currently, all supported SASL mechanisms require a
     * password, so we can just go ahead here without further distinction.
     */
    conn->password_needed = true;
    password = conn->connhost[conn->whichhost].password;
    if (password == NULL)
        password = conn->pgpass;
    if (password == NULL || password[0] == '\0')
    {
        printfPQExpBuffer(&conn->errorMessage,
                          PQnoPasswordSupplied);
        goto error;
    }

    /*
     * 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 = pg_fe_scram_init(conn,
                                        password,
                                        selected_mechanism);
    if (!conn->sasl_state)
        goto oom_error;

    /* Get the mechanism-specific Initial Client Response, if any */
    pg_fe_scram_exchange(conn->sasl_state,
                         NULL, -1,
                         &initialresponse, &initialresponselen,
                         &done, &success);

    if (done && !success)
        goto error;

    /*
     * Build a SASLInitialResponse message, and send it.
     */
    if (pqPutMsgStart('p', true, 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);
    if (initialresponse)
        free(initialresponse);

    return STATUS_OK;

error:
    termPQExpBuffer(&mechanism_buf);
    if (initialresponse)
        free(initialresponse);
    return STATUS_ERROR;

oom_error:
    termPQExpBuffer(&mechanism_buf);
    if (initialresponse)
        free(initialresponse);
    printfPQExpBuffer(&conn->errorMessage,
                      libpq_gettext("out of memory\n"));
    return STATUS_ERROR;
}

PQencryptPassword()

char* PQencryptPassword	(	const char *	passwd,
		const char *	user
	)

Definition at line 1117 of file fe-auth.c.

References free, malloc, MD5_PASSWD_LEN, and pg_md5_encrypt().

{
    char       *crypt_pwd;

    crypt_pwd = malloc(MD5_PASSWD_LEN + 1);
    if (!crypt_pwd)
        return NULL;

    if (!pg_md5_encrypt(passwd, user, strlen(user), crypt_pwd))
    {
        free(crypt_pwd);
        return NULL;
    }

    return crypt_pwd;
}

PQencryptPasswordConn()

char* PQencryptPasswordConn	(	PGconn *	conn,
		const char *	passwd,
		const char *	user,
		const char *	algorithm
	)

Definition at line 1159 of file fe-auth.c.

References pg_conn::errorMessage, free, libpq_gettext, malloc, MAX_ALGORITHM_NAME_LEN, MD5_PASSWD_LEN, pg_fe_scram_build_verifier(), pg_md5_encrypt(), PGRES_TUPLES_OK, PQclear(), PQexec(), PQgetvalue(), PQnfields(), PQntuples(), PQresultStatus(), printfPQExpBuffer(), and val.

Referenced by exec_command_password(), and main().

{
#define MAX_ALGORITHM_NAME_LEN 50
    char        algobuf[MAX_ALGORITHM_NAME_LEN + 1];
    char       *crypt_pwd = NULL;

    if (!conn)
        return NULL;

    /* 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);
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("unexpected shape of result set returned for SHOW\n"));
            return NULL;
        }
        val = PQgetvalue(res, 0, 0);

        if (strlen(val) > MAX_ALGORITHM_NAME_LEN)
        {
            PQclear(res);
            printfPQExpBuffer(&conn->errorMessage,
                              libpq_gettext("password_encryption value too long\n"));
            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)
    {
        crypt_pwd = pg_fe_scram_build_verifier(passwd);
    }
    else if (strcmp(algorithm, "md5") == 0)
    {
        crypt_pwd = malloc(MD5_PASSWD_LEN + 1);
        if (crypt_pwd)
        {
            if (!pg_md5_encrypt(passwd, user, strlen(user), crypt_pwd))
            {
                free(crypt_pwd);
                crypt_pwd = NULL;
            }
        }
    }
    else
    {
        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("unrecognized password encryption algorithm \"%s\"\n"),
                          algorithm);
        return NULL;
    }

    if (!crypt_pwd)
        printfPQExpBuffer(&conn->errorMessage,
                          libpq_gettext("out of memory\n"));

    return crypt_pwd;
}