int ssl3_send_server_key_exchange(SSL *s)
{
#ifndef OPENSSL_NO_RSA
    unsigned char *q;
    int j, num;
    RSA *rsa;
    unsigned char md_buf[MD5_DIGEST_LENGTH + SHA_DIGEST_LENGTH];
    unsigned int u;
#endif
#ifndef OPENSSL_NO_DH
    DH *dh = NULL, *dhp;
#endif
#ifndef OPENSSL_NO_ECDH
    EC_KEY *ecdh = NULL, *ecdhp;
    unsigned char *encodedPoint = NULL;
    int encodedlen = 0;
    int curve_id = 0;
    BN_CTX *bn_ctx = NULL;
#endif
    EVP_PKEY *pkey;
    const EVP_MD *md = NULL;
    unsigned char *p, *d;
    int al, i;
    unsigned long type;
    int n;
    CERT *cert;
    BIGNUM *r[4];
    int nr[4], kn;
    BUF_MEM *buf;
    EVP_MD_CTX md_ctx;

    EVP_MD_CTX_init(&md_ctx);
    if (s->state == SSL3_ST_SW_KEY_EXCH_A) {
        type = s->s3->tmp.new_cipher->algorithm_mkey;
        cert = s->cert;

        buf = s->init_buf;

        r[0] = r[1] = r[2] = r[3] = NULL;
        n = 0;
#ifndef OPENSSL_NO_RSA
        if (type & SSL_kRSA) {
            rsa = cert->rsa_tmp;
            if ((rsa == NULL) && (s->cert->rsa_tmp_cb != NULL)) {
                rsa = s->cert->rsa_tmp_cb(s,
                                          SSL_C_IS_EXPORT(s->s3->
                                                          tmp.new_cipher),
                                          SSL_C_EXPORT_PKEYLENGTH(s->s3->
                                                                  tmp.new_cipher));
                if (rsa == NULL) {
                    al = SSL_AD_HANDSHAKE_FAILURE;
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                           SSL_R_ERROR_GENERATING_TMP_RSA_KEY);
                    goto f_err;
                }
                RSA_up_ref(rsa);
                cert->rsa_tmp = rsa;
            }
            if (rsa == NULL) {
                al = SSL_AD_HANDSHAKE_FAILURE;
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_MISSING_TMP_RSA_KEY);
                goto f_err;
            }
            r[0] = rsa->n;
            r[1] = rsa->e;
            s->s3->tmp.use_rsa_tmp = 1;
        } else
#endif
#ifndef OPENSSL_NO_DH
        if (type & SSL_kEDH) {
            dhp = cert->dh_tmp;
            if ((dhp == NULL) && (s->cert->dh_tmp_cb != NULL))
                dhp = s->cert->dh_tmp_cb(s,
                                         SSL_C_IS_EXPORT(s->s3->
                                                         tmp.new_cipher),
                                         SSL_C_EXPORT_PKEYLENGTH(s->s3->
                                                                 tmp.new_cipher));
            if (dhp == NULL) {
                al = SSL_AD_HANDSHAKE_FAILURE;
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_MISSING_TMP_DH_KEY);
                goto f_err;
            }

            if (s->s3->tmp.dh != NULL) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       ERR_R_INTERNAL_ERROR);
                goto err;
            }

            if ((dh = DHparams_dup(dhp)) == NULL) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
                goto err;
            }

            s->s3->tmp.dh = dh;
            if ((dhp->pub_key == NULL ||
                 dhp->priv_key == NULL ||
                 (s->options & SSL_OP_SINGLE_DH_USE))) {
                if (!DH_generate_key(dh)) {
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
                    goto err;
                }
            } else {
                dh->pub_key = BN_dup(dhp->pub_key);
                dh->priv_key = BN_dup(dhp->priv_key);
                if ((dh->pub_key == NULL) || (dh->priv_key == NULL)) {
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_DH_LIB);
                    goto err;
                }
            }
            r[0] = dh->p;
            r[1] = dh->g;
            r[2] = dh->pub_key;
        } else
#endif
#ifndef OPENSSL_NO_ECDH
        if (type & SSL_kEECDH) {
            const EC_GROUP *group;

            ecdhp = cert->ecdh_tmp;
            if (s->cert->ecdh_tmp_auto) {
                /* Get NID of appropriate shared curve */
                int nid = tls1_shared_curve(s, -2);
                if (nid != NID_undef)
                    ecdhp = EC_KEY_new_by_curve_name(nid);
            } else if ((ecdhp == NULL) && s->cert->ecdh_tmp_cb) {
                ecdhp = s->cert->ecdh_tmp_cb(s,
                                             SSL_C_IS_EXPORT(s->s3->
                                                             tmp.new_cipher),
                                             SSL_C_EXPORT_PKEYLENGTH(s->
                                                                     s3->tmp.new_cipher));
            }
            if (ecdhp == NULL) {
                al = SSL_AD_HANDSHAKE_FAILURE;
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_MISSING_TMP_ECDH_KEY);
                goto f_err;
            }

            if (s->s3->tmp.ecdh != NULL) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       ERR_R_INTERNAL_ERROR);
                goto err;
            }

            /* Duplicate the ECDH structure. */
            if (ecdhp == NULL) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_ECDH_LIB);
                goto err;
            }
            if (s->cert->ecdh_tmp_auto)
                ecdh = ecdhp;
            else if ((ecdh = EC_KEY_dup(ecdhp)) == NULL) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_ECDH_LIB);
                goto err;
            }

            s->s3->tmp.ecdh = ecdh;
            if ((EC_KEY_get0_public_key(ecdh) == NULL) ||
                (EC_KEY_get0_private_key(ecdh) == NULL) ||
                (s->options & SSL_OP_SINGLE_ECDH_USE)) {
                if (!EC_KEY_generate_key(ecdh)) {
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                           ERR_R_ECDH_LIB);
                    goto err;
                }
            }

            if (((group = EC_KEY_get0_group(ecdh)) == NULL) ||
                (EC_KEY_get0_public_key(ecdh) == NULL) ||
                (EC_KEY_get0_private_key(ecdh) == NULL)) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_ECDH_LIB);
                goto err;
            }

            if (SSL_C_IS_EXPORT(s->s3->tmp.new_cipher) &&
                (EC_GROUP_get_degree(group) > 163)) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_ECGROUP_TOO_LARGE_FOR_CIPHER);
                goto err;
            }

            /*
             * XXX: For now, we only support ephemeral ECDH keys over named
             * (not generic) curves. For supported named curves, curve_id is
             * non-zero.
             */
            if ((curve_id =
                 tls1_ec_nid2curve_id(EC_GROUP_get_curve_name(group)))
                == 0) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_UNSUPPORTED_ELLIPTIC_CURVE);
                goto err;
            }

            /*
             * Encode the public key. First check the size of encoding and
             * allocate memory accordingly.
             */
            encodedlen = EC_POINT_point2oct(group,
                                            EC_KEY_get0_public_key(ecdh),
                                            POINT_CONVERSION_UNCOMPRESSED,
                                            NULL, 0, NULL);

            encodedPoint = (unsigned char *)
                OPENSSL_malloc(encodedlen * sizeof(unsigned char));
            bn_ctx = BN_CTX_new();
            if ((encodedPoint == NULL) || (bn_ctx == NULL)) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       ERR_R_MALLOC_FAILURE);
                goto err;
            }

            encodedlen = EC_POINT_point2oct(group,
                                            EC_KEY_get0_public_key(ecdh),
                                            POINT_CONVERSION_UNCOMPRESSED,
                                            encodedPoint, encodedlen, bn_ctx);

            if (encodedlen == 0) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_R_ECDH_LIB);
                goto err;
            }

            BN_CTX_free(bn_ctx);
            bn_ctx = NULL;

            /*
             * XXX: For now, we only support named (not generic) curves in
             * ECDH ephemeral key exchanges. In this situation, we need four
             * additional bytes to encode the entire ServerECDHParams
             * structure.
             */
            n = 4 + encodedlen;

            /*
             * We'll generate the serverKeyExchange message explicitly so we
             * can set these to NULLs
             */
            r[0] = NULL;
            r[1] = NULL;
            r[2] = NULL;
            r[3] = NULL;
        } else
#endif                          /* !OPENSSL_NO_ECDH */
#ifndef OPENSSL_NO_PSK
        if (type & SSL_kPSK) {
            /*
             * reserve size for record length and PSK identity hint
             */
            n += 2 + strlen(s->ctx->psk_identity_hint);
        } else
#endif                          /* !OPENSSL_NO_PSK */
#ifndef OPENSSL_NO_SRP
        if (type & SSL_kSRP) {
            if ((s->srp_ctx.N == NULL) ||
                (s->srp_ctx.g == NULL) ||
                (s->srp_ctx.s == NULL) || (s->srp_ctx.B == NULL)) {
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_MISSING_SRP_PARAM);
                goto err;
            }
            r[0] = s->srp_ctx.N;
            r[1] = s->srp_ctx.g;
            r[2] = s->srp_ctx.s;
            r[3] = s->srp_ctx.B;
        } else
#endif
        {
            al = SSL_AD_HANDSHAKE_FAILURE;
            SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                   SSL_R_UNKNOWN_KEY_EXCHANGE_TYPE);
            goto f_err;
        }
        for (i = 0; i < 4 && r[i] != NULL; i++) {
            nr[i] = BN_num_bytes(r[i]);
#ifndef OPENSSL_NO_SRP
            if ((i == 2) && (type & SSL_kSRP))
                n += 1 + nr[i];
            else
#endif
                n += 2 + nr[i];
        }

        if (!(s->s3->tmp.new_cipher->algorithm_auth & (SSL_aNULL | SSL_aSRP))
            && !(s->s3->tmp.new_cipher->algorithm_mkey & SSL_kPSK)) {
            if ((pkey = ssl_get_sign_pkey(s, s->s3->tmp.new_cipher, &md))
                == NULL) {
                al = SSL_AD_DECODE_ERROR;
                goto f_err;
            }
            kn = EVP_PKEY_size(pkey);
        } else {
            pkey = NULL;
            kn = 0;
        }

        if (!BUF_MEM_grow_clean(buf, n + SSL_HM_HEADER_LENGTH(s) + kn)) {
            SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_LIB_BUF);
            goto err;
        }
        d = p = ssl_handshake_start(s);

        for (i = 0; i < 4 && r[i] != NULL; i++) {
#ifndef OPENSSL_NO_SRP
            if ((i == 2) && (type & SSL_kSRP)) {
                *p = nr[i];
                p++;
            } else
#endif
                s2n(nr[i], p);
            BN_bn2bin(r[i], p);
            p += nr[i];
        }

#ifndef OPENSSL_NO_ECDH
        if (type & SSL_kEECDH) {
            /*
             * XXX: For now, we only support named (not generic) curves. In
             * this situation, the serverKeyExchange message has: [1 byte
             * CurveType], [2 byte CurveName] [1 byte length of encoded
             * point], followed by the actual encoded point itself
             */
            *p = NAMED_CURVE_TYPE;
            p += 1;
            *p = 0;
            p += 1;
            *p = curve_id;
            p += 1;
            *p = encodedlen;
            p += 1;
            memcpy((unsigned char *)p,
                   (unsigned char *)encodedPoint, encodedlen);
            OPENSSL_free(encodedPoint);
            encodedPoint = NULL;
            p += encodedlen;
        }
#endif

#ifndef OPENSSL_NO_PSK
        if (type & SSL_kPSK) {
            /* copy PSK identity hint */
            s2n(strlen(s->ctx->psk_identity_hint), p);
            strncpy((char *)p, s->ctx->psk_identity_hint,
                    strlen(s->ctx->psk_identity_hint));
            p += strlen(s->ctx->psk_identity_hint);
        }
#endif

        /* not anonymous */
        if (pkey != NULL) {
            /*
             * n is the length of the params, they start at &(d[4]) and p
             * points to the space at the end.
             */
#ifndef OPENSSL_NO_RSA
            if (pkey->type == EVP_PKEY_RSA && !SSL_USE_SIGALGS(s)) {
                q = md_buf;
                j = 0;
                for (num = 2; num > 0; num--) {
                    EVP_MD_CTX_set_flags(&md_ctx,
                                         EVP_MD_CTX_FLAG_NON_FIPS_ALLOW);
                    EVP_DigestInit_ex(&md_ctx, (num == 2)
                                      ? s->ctx->md5 : s->ctx->sha1, NULL);
                    EVP_DigestUpdate(&md_ctx, &(s->s3->client_random[0]),
                                     SSL3_RANDOM_SIZE);
                    EVP_DigestUpdate(&md_ctx, &(s->s3->server_random[0]),
                                     SSL3_RANDOM_SIZE);
                    EVP_DigestUpdate(&md_ctx, d, n);
                    EVP_DigestFinal_ex(&md_ctx, q, (unsigned int *)&i);
                    q += i;
                    j += i;
                }
                if (RSA_sign(NID_md5_sha1, md_buf, j,
                             &(p[2]), &u, pkey->pkey.rsa) <= 0) {
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_LIB_RSA);
                    goto err;
                }
                s2n(u, p);
                n += u + 2;
            } else
#endif
            if (md) {
                /* send signature algorithm */
                if (SSL_USE_SIGALGS(s)) {
                    if (!tls12_get_sigandhash(p, pkey, md)) {
                        /* Should never happen */
                        al = SSL_AD_INTERNAL_ERROR;
                        SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                               ERR_R_INTERNAL_ERROR);
                        goto f_err;
                    }
                    p += 2;
                }
#ifdef SSL_DEBUG
                fprintf(stderr, "Using hash %s\n", EVP_MD_name(md));
#endif
                EVP_SignInit_ex(&md_ctx, md, NULL);
                EVP_SignUpdate(&md_ctx, &(s->s3->client_random[0]),
                               SSL3_RANDOM_SIZE);
                EVP_SignUpdate(&md_ctx, &(s->s3->server_random[0]),
                               SSL3_RANDOM_SIZE);
                EVP_SignUpdate(&md_ctx, d, n);
                if (!EVP_SignFinal(&md_ctx, &(p[2]),
                                   (unsigned int *)&i, pkey)) {
                    SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE, ERR_LIB_EVP);
                    goto err;
                }
                s2n(i, p);
                n += i + 2;
                if (SSL_USE_SIGALGS(s))
                    n += 2;
            } else {
                /* Is this error check actually needed? */
                al = SSL_AD_HANDSHAKE_FAILURE;
                SSLerr(SSL_F_SSL3_SEND_SERVER_KEY_EXCHANGE,
                       SSL_R_UNKNOWN_PKEY_TYPE);
                goto f_err;
            }
        }

        ssl_set_handshake_header(s, SSL3_MT_SERVER_KEY_EXCHANGE, n);
    }

    s->state = SSL3_ST_SW_KEY_EXCH_B;
    EVP_MD_CTX_cleanup(&md_ctx);
    return ssl_do_write(s);
 f_err:
    ssl3_send_alert(s, SSL3_AL_FATAL, al);
 err:
#ifndef OPENSSL_NO_ECDH
    if (encodedPoint != NULL)
        OPENSSL_free(encodedPoint);
    BN_CTX_free(bn_ctx);
#endif
    EVP_MD_CTX_cleanup(&md_ctx);
    s->state = SSL_ST_ERR;
    return (-1);
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.