scram-common.c

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/*-------------------------------------------------------------------------
 * scram-common.c
 *      Shared frontend/backend code for SCRAM authentication
 *
 * This contains the common low-level functions needed in both frontend and
 * backend, for implement the Salted Challenge Response Authentication
 * Mechanism (SCRAM), per IETF's RFC 5802.
 *
 * Portions Copyright (c) 2017-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/common/scram-common.c
 *
 *-------------------------------------------------------------------------
 */
#ifndef FRONTEND
#include "postgres.h"
#else
#include "postgres_fe.h"
#endif

#include "common/base64.h"
#include "common/hmac.h"
#include "common/scram-common.h"
#include "port/pg_bswap.h"

/*
 * Calculate SaltedPassword.
 *
 * The password should already be normalized by SASLprep.  Returns 0 on
 * success, -1 on failure.
 */
int
scram_SaltedPassword(const char *password,
                     const char *salt, int saltlen, int iterations,
                     uint8 *result)
{
    int         password_len = strlen(password);
    uint32      one = pg_hton32(1);
    int         i,
                j;
    uint8       Ui[SCRAM_KEY_LEN];
    uint8       Ui_prev[SCRAM_KEY_LEN];
    pg_hmac_ctx *hmac_ctx = pg_hmac_create(PG_SHA256);

    if (hmac_ctx == NULL)
        return -1;

    /*
     * Iterate hash calculation of HMAC entry using given salt.  This is
     * essentially PBKDF2 (see RFC2898) with HMAC() as the pseudorandom
     * function.
     */

    /* First iteration */
    if (pg_hmac_init(hmac_ctx, (uint8 *) password, password_len) < 0 ||
        pg_hmac_update(hmac_ctx, (uint8 *) salt, saltlen) < 0 ||
        pg_hmac_update(hmac_ctx, (uint8 *) &one, sizeof(uint32)) < 0 ||
        pg_hmac_final(hmac_ctx, Ui_prev, sizeof(Ui_prev)) < 0)
    {
        pg_hmac_free(hmac_ctx);
        return -1;
    }

    memcpy(result, Ui_prev, SCRAM_KEY_LEN);

    /* Subsequent iterations */
    for (i = 2; i <= iterations; i++)
    {
        if (pg_hmac_init(hmac_ctx, (uint8 *) password, password_len) < 0 ||
            pg_hmac_update(hmac_ctx, (uint8 *) Ui_prev, SCRAM_KEY_LEN) < 0 ||
            pg_hmac_final(hmac_ctx, Ui, sizeof(Ui)) < 0)
        {
            pg_hmac_free(hmac_ctx);
            return -1;
        }

        for (j = 0; j < SCRAM_KEY_LEN; j++)
            result[j] ^= Ui[j];
        memcpy(Ui_prev, Ui, SCRAM_KEY_LEN);
    }

    pg_hmac_free(hmac_ctx);
    return 0;
}


/*
 * Calculate SHA-256 hash for a NULL-terminated string. (The NULL terminator is
 * not included in the hash).  Returns 0 on success, -1 on failure.
 */
int
scram_H(const uint8 *input, int len, uint8 *result)
{
    pg_cryptohash_ctx *ctx;

    ctx = pg_cryptohash_create(PG_SHA256);
    if (ctx == NULL)
        return -1;

    if (pg_cryptohash_init(ctx) < 0 ||
        pg_cryptohash_update(ctx, input, len) < 0 ||
        pg_cryptohash_final(ctx, result, SCRAM_KEY_LEN) < 0)
    {
        pg_cryptohash_free(ctx);
        return -1;
    }

    pg_cryptohash_free(ctx);
    return 0;
}

/*
 * Calculate ClientKey.  Returns 0 on success, -1 on failure.
 */
int
scram_ClientKey(const uint8 *salted_password, uint8 *result)
{
    pg_hmac_ctx *ctx = pg_hmac_create(PG_SHA256);

    if (ctx == NULL)
        return -1;

    if (pg_hmac_init(ctx, salted_password, SCRAM_KEY_LEN) < 0 ||
        pg_hmac_update(ctx, (uint8 *) "Client Key", strlen("Client Key")) < 0 ||
        pg_hmac_final(ctx, result, SCRAM_KEY_LEN) < 0)
    {
        pg_hmac_free(ctx);
        return -1;
    }

    pg_hmac_free(ctx);
    return 0;
}

/*
 * Calculate ServerKey.  Returns 0 on success, -1 on failure.
 */
int
scram_ServerKey(const uint8 *salted_password, uint8 *result)
{
    pg_hmac_ctx *ctx = pg_hmac_create(PG_SHA256);

    if (ctx == NULL)
        return -1;

    if (pg_hmac_init(ctx, salted_password, SCRAM_KEY_LEN) < 0 ||
        pg_hmac_update(ctx, (uint8 *) "Server Key", strlen("Server Key")) < 0 ||
        pg_hmac_final(ctx, result, SCRAM_KEY_LEN) < 0)
    {
        pg_hmac_free(ctx);
        return -1;
    }

    pg_hmac_free(ctx);
    return 0;
}


/*
 * Construct a SCRAM secret, for storing in pg_authid.rolpassword.
 *
 * The password should already have been processed with SASLprep, if necessary!
 *
 * If iterations is 0, default number of iterations is used.  The result is
 * palloc'd or malloc'd, so caller is responsible for freeing it.
 */
char *
scram_build_secret(const char *salt, int saltlen, int iterations,
                   const char *password)
{
    uint8       salted_password[SCRAM_KEY_LEN];
    uint8       stored_key[SCRAM_KEY_LEN];
    uint8       server_key[SCRAM_KEY_LEN];
    char       *result;
    char       *p;
    int         maxlen;
    int         encoded_salt_len;
    int         encoded_stored_len;
    int         encoded_server_len;
    int         encoded_result;

    if (iterations <= 0)
        iterations = SCRAM_DEFAULT_ITERATIONS;

    /* Calculate StoredKey and ServerKey */
    if (scram_SaltedPassword(password, salt, saltlen, iterations,
                             salted_password) < 0 ||
        scram_ClientKey(salted_password, stored_key) < 0 ||
        scram_H(stored_key, SCRAM_KEY_LEN, stored_key) < 0 ||
        scram_ServerKey(salted_password, server_key) < 0)
    {
#ifdef FRONTEND
        return NULL;
#else
        elog(ERROR, "could not calculate stored key and server key");
#endif
    }

    /*----------
     * The format is:
     * SCRAM-SHA-256$<iteration count>:<salt>$<StoredKey>:<ServerKey>
     *----------
     */
    encoded_salt_len = pg_b64_enc_len(saltlen);
    encoded_stored_len = pg_b64_enc_len(SCRAM_KEY_LEN);
    encoded_server_len = pg_b64_enc_len(SCRAM_KEY_LEN);

    maxlen = strlen("SCRAM-SHA-256") + 1
        + 10 + 1                /* iteration count */
        + encoded_salt_len + 1  /* Base64-encoded salt */
        + encoded_stored_len + 1    /* Base64-encoded StoredKey */
        + encoded_server_len + 1;   /* Base64-encoded ServerKey */

#ifdef FRONTEND
    result = malloc(maxlen);
    if (!result)
        return NULL;
#else
    result = palloc(maxlen);
#endif

    p = result + sprintf(result, "SCRAM-SHA-256$%d:", iterations);

    /* salt */
    encoded_result = pg_b64_encode(salt, saltlen, p, encoded_salt_len);
    if (encoded_result < 0)
    {
#ifdef FRONTEND
        free(result);
        return NULL;
#else
        elog(ERROR, "could not encode salt");
#endif
    }
    p += encoded_result;
    *(p++) = '$';

    /* stored key */
    encoded_result = pg_b64_encode((char *) stored_key, SCRAM_KEY_LEN, p,
                                   encoded_stored_len);
    if (encoded_result < 0)
    {
#ifdef FRONTEND
        free(result);
        return NULL;
#else
        elog(ERROR, "could not encode stored key");
#endif
    }

    p += encoded_result;
    *(p++) = ':';

    /* server key */
    encoded_result = pg_b64_encode((char *) server_key, SCRAM_KEY_LEN, p,
                                   encoded_server_len);
    if (encoded_result < 0)
    {
#ifdef FRONTEND
        free(result);
        return NULL;
#else
        elog(ERROR, "could not encode server key");
#endif
    }

    p += encoded_result;
    *(p++) = '\0';

    Assert(p - result <= maxlen);

    return result;
}