Fixed selection of ECDHE_RSA suites for pre-1.2 TLS versions.

[BearSSL] / src / ssl / ssl_hs_server.t0

\ Copyright (c) 2016 Thomas Pornin <pornin@bolet.org>
\
\ Permission is hereby granted, free of charge, to any person obtaining 
\ a copy of this software and associated documentation files (the
\ "Software"), to deal in the Software without restriction, including
\ without limitation the rights to use, copy, modify, merge, publish,
\ distribute, sublicense, and/or sell copies of the Software, and to
\ permit persons to whom the Software is furnished to do so, subject to
\ the following conditions:
\
\ The above copyright notice and this permission notice shall be 
\ included in all copies or substantial portions of the Software.
\
\ THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
\ EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
\ MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
\ NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
\ BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
\ ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
\ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
\ SOFTWARE.

\ ----------------------------------------------------------------------
\ Handshake processing code, for the server.
\ The common T0 code (ssl_hs_common.t0) shall be read first.

preamble {

/*
 * This macro evaluates to a pointer to the server context, under that
 * specific name. It must be noted that since the engine context is the
 * first field of the br_ssl_server_context structure ('eng'), then
 * pointers values of both types are interchangeable, modulo an
 * appropriate cast. This also means that "adresses" computed as offsets
 * within the structure work for both kinds of context.
 */
#define CTX  ((br_ssl_server_context *)ENG)

/*
 * Decrypt the pre-master secret (RSA key exchange).
 */
static void
do_rsa_decrypt(br_ssl_server_context *ctx, int prf_id,
        unsigned char *epms, size_t len)
{
        uint32_t x;
        unsigned char rpms[48];

        /*
         * Decrypt the PMS.
         */
        x = (*ctx->policy_vtable)->do_keyx(ctx->policy_vtable, epms, &len);

        /*
         * Set the first two bytes to the maximum supported client
         * protocol version. These bytes are used for version rollback
         * detection; forceing the two bytes will make the master secret
         * wrong if the bytes are not correct. This process is
         * recommended by RFC 5246 (section 7.4.7.1).
         */
        br_enc16be(epms, ctx->client_max_version);

        /*
         * Make a random PMS and copy it above the decrypted value if the
         * decryption failed. Note that we use a constant-time conditional
         * copy.
         */
        br_hmac_drbg_generate(&ctx->eng.rng, rpms, sizeof rpms);
        br_ccopy(x ^ 1, epms, rpms, sizeof rpms);

        /*
         * Compute master secret.
         */
        br_ssl_engine_compute_master(&ctx->eng, prf_id, epms, 48);

        /*
         * Clear the pre-master secret from RAM: it is normally a buffer
         * in the context, hence potentially long-lived.
         */
        memset(epms, 0, len);
}

/*
 * Common part for ECDH and ECDHE.
 */
static void
ecdh_common(br_ssl_server_context *ctx, int prf_id,
        unsigned char *xcoor, size_t xcoor_len, uint32_t ctl)
{
        unsigned char rpms[80];

        if (xcoor_len > sizeof rpms) {
                xcoor_len = sizeof rpms;
                ctl = 0;
        }

        /*
         * Make a random PMS and copy it above the decrypted value if the
         * decryption failed. Note that we use a constant-time conditional
         * copy.
         */
        br_hmac_drbg_generate(&ctx->eng.rng, rpms, xcoor_len);
        br_ccopy(ctl ^ 1, xcoor, rpms, xcoor_len);

        /*
         * Compute master secret.
         */
        br_ssl_engine_compute_master(&ctx->eng, prf_id, xcoor, xcoor_len);

        /*
         * Clear the pre-master secret from RAM: it is normally a buffer
         * in the context, hence potentially long-lived.
         */
        memset(xcoor, 0, xcoor_len);
}

/*
 * Do the ECDH key exchange (not ECDHE).
 */
static void
do_ecdh(br_ssl_server_context *ctx, int prf_id,
        unsigned char *cpoint, size_t cpoint_len)
{
        uint32_t x;

        /*
         * Finalise the key exchange.
         */
        x = (*ctx->policy_vtable)->do_keyx(ctx->policy_vtable,
                cpoint, &cpoint_len);
        ecdh_common(ctx, prf_id, cpoint, cpoint_len, x);
}

/*
 * Do the full static ECDH key exchange. When this function is called,
 * it has already been verified that the cipher suite uses ECDH (not ECDHE),
 * and the client's public key (from its certificate) has type EC and is
 * apt for key exchange.
 */
static void
do_static_ecdh(br_ssl_server_context *ctx, int prf_id)
{
        unsigned char cpoint[133];
        size_t cpoint_len;
        const br_x509_class **xc;
        const br_x509_pkey *pk;

        xc = ctx->eng.x509ctx;
        pk = (*xc)->get_pkey(xc, NULL);
        cpoint_len = pk->key.ec.qlen;
        if (cpoint_len > sizeof cpoint) {
                /*
                 * If the point is larger than our buffer then we need to
                 * restrict it. Length 2 is not a valid point length, so
                 * the ECDH will fail.
                 */
                cpoint_len = 2;
        }
        memcpy(cpoint, pk->key.ec.q, cpoint_len);
        do_ecdh(ctx, prf_id, cpoint, cpoint_len);
}

static size_t
hash_data(br_ssl_server_context *ctx,
        void *dst, int hash_id, const void *src, size_t len)
{
        const br_hash_class *hf;
        br_hash_compat_context hc;

        if (hash_id == 0) {
                unsigned char tmp[36];

                hf = br_multihash_getimpl(&ctx->eng.mhash, br_md5_ID);
                if (hf == NULL) {
                        return 0;
                }
                hf->init(&hc.vtable);
                hf->update(&hc.vtable, src, len);
                hf->out(&hc.vtable, tmp);
                hf = br_multihash_getimpl(&ctx->eng.mhash, br_sha1_ID);
                if (hf == NULL) {
                        return 0;
                }
                hf->init(&hc.vtable);
                hf->update(&hc.vtable, src, len);
                hf->out(&hc.vtable, tmp + 16);
                memcpy(dst, tmp, 36);
                return 36;
        } else {
                hf = br_multihash_getimpl(&ctx->eng.mhash, hash_id);
                if (hf == NULL) {
                        return 0;
                }
                hf->init(&hc.vtable);
                hf->update(&hc.vtable, src, len);
                hf->out(&hc.vtable, dst);
                return (hf->desc >> BR_HASHDESC_OUT_OFF) & BR_HASHDESC_OUT_MASK;
        }
}

/*
 * Do the ECDHE key exchange (part 1: generation of transient key, and
 * computing of the point to send to the client). Returned value is the
 * signature length (in bytes), or -x on error (with x being an error
 * code). The encoded point is written in the ecdhe_point[] context buffer
 * (length in ecdhe_point_len).
 */
static int
do_ecdhe_part1(br_ssl_server_context *ctx, int curve)
{
        unsigned algo_id;
        unsigned mask;
        const unsigned char *order;
        size_t olen, glen;
        size_t hv_len, sig_len;

        if (!((ctx->eng.iec->supported_curves >> curve) & 1)) {
                return -BR_ERR_INVALID_ALGORITHM;
        }
        ctx->eng.ecdhe_curve = curve;

        /*
         * Generate our private key. We need a non-zero random value
         * which is lower than the curve order, in a "large enough"
         * range. We force the top bit to 0 and bottom bit to 1, which
         * does the trick. Note that contrary to what happens in ECDSA,
         * this is not a problem if we do not cover the full range of
         * possible values.
         */
        order = ctx->eng.iec->order(curve, &olen);
        mask = 0xFF;
        while (mask >= order[0]) {
                mask >>= 1;
        }
        br_hmac_drbg_generate(&ctx->eng.rng, ctx->ecdhe_key, olen);
        ctx->ecdhe_key[0] &= mask;
        ctx->ecdhe_key[olen - 1] |= 0x01;
        ctx->ecdhe_key_len = olen;

        /*
         * Compute our ECDH point.
         */
        glen = ctx->eng.iec->mulgen(ctx->eng.ecdhe_point,
                ctx->ecdhe_key, olen, curve);
        ctx->eng.ecdhe_point_len = glen;

        /*
         * Assemble the message to be signed, and possibly hash it.
         */
        memcpy(ctx->eng.pad, ctx->eng.client_random, 32);
        memcpy(ctx->eng.pad + 32, ctx->eng.server_random, 32);
        ctx->eng.pad[64 + 0] = 0x03;
        ctx->eng.pad[64 + 1] = 0x00;
        ctx->eng.pad[64 + 2] = curve;
        ctx->eng.pad[64 + 3] = ctx->eng.ecdhe_point_len;
        memcpy(ctx->eng.pad + 64 + 4,
                ctx->eng.ecdhe_point, ctx->eng.ecdhe_point_len);
        hv_len = 64 + 4 + ctx->eng.ecdhe_point_len;
        algo_id = ctx->sign_hash_id;
        if (algo_id >= (unsigned)0xFF00) {
                hv_len = hash_data(ctx, ctx->eng.pad, algo_id & 0xFF,
                        ctx->eng.pad, hv_len);
                if (hv_len == 0) {
                        return -BR_ERR_INVALID_ALGORITHM;
                }
        }

        sig_len = (*ctx->policy_vtable)->do_sign(ctx->policy_vtable,
                algo_id, ctx->eng.pad, hv_len, sizeof ctx->eng.pad);
        return sig_len ? (int)sig_len : -BR_ERR_INVALID_ALGORITHM;
}

/*
 * Do the ECDHE key exchange (part 2: computation of the shared secret
 * from the point sent by the client).
 */
static void
do_ecdhe_part2(br_ssl_server_context *ctx, int prf_id,
        unsigned char *cpoint, size_t cpoint_len)
{
        int curve;
        uint32_t ctl;
        size_t xoff, xlen;

        curve = ctx->eng.ecdhe_curve;

        /*
         * Finalise the key exchange.
         */
        ctl = ctx->eng.iec->mul(cpoint, cpoint_len,
                ctx->ecdhe_key, ctx->ecdhe_key_len, curve);
        xoff = ctx->eng.iec->xoff(curve, &xlen);
        ecdh_common(ctx, prf_id, cpoint + xoff, xlen, ctl);

        /*
         * Clear the ECDHE private key. Forward Secrecy is achieved insofar
         * as that key does not get stolen, so we'd better destroy it
         * as soon as it ceases to be useful.
         */
        memset(ctx->ecdhe_key, 0, ctx->ecdhe_key_len);
}

/*
 * Offset for hash value within the pad (when obtaining all hash values,
 * in preparation for verification of the CertificateVerify message).
 * Order is MD5, SHA-1, SHA-224, SHA-256, SHA-384, SHA-512; last value
 * is used to get the total length.
 */
static const unsigned char HASH_PAD_OFF[] = { 0, 16, 36, 64, 96, 144, 208 };

/*
 * OID for hash functions in RSA signatures.
 */
static const unsigned char HASH_OID_SHA1[] = {
        0x05, 0x2B, 0x0E, 0x03, 0x02, 0x1A
};

static const unsigned char HASH_OID_SHA224[] = {
        0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
};

static const unsigned char HASH_OID_SHA256[] = {
        0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
};

static const unsigned char HASH_OID_SHA384[] = {
        0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02
};

static const unsigned char HASH_OID_SHA512[] = {
        0x09, 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
};

static const unsigned char *HASH_OID[] = {
        HASH_OID_SHA1,
        HASH_OID_SHA224,
        HASH_OID_SHA256,
        HASH_OID_SHA384,
        HASH_OID_SHA512
};

/*
 * Verify the signature in CertificateVerify. Returned value is 0 on
 * success, or a non-zero error code. Lack of implementation of the
 * designated signature algorithm is reported as a "bad signature"
 * error (because it means that the peer did not honour our advertised
 * set of supported signature algorithms).
 */
static int
verify_CV_sig(br_ssl_server_context *ctx, size_t sig_len)
{
        const br_x509_class **xc;
        const br_x509_pkey *pk;
        int id;

        id = ctx->hash_CV_id;
        xc = ctx->eng.x509ctx;
        pk = (*xc)->get_pkey(xc, NULL);
        if (pk->key_type == BR_KEYTYPE_RSA) {
                unsigned char tmp[64];
                const unsigned char *hash_oid;

                if (id == 0) {
                        hash_oid = NULL;
                } else {
                        hash_oid = HASH_OID[id - 2];
                }
                if (ctx->eng.irsavrfy == 0) {
                        return BR_ERR_BAD_SIGNATURE;
                }
                if (!ctx->eng.irsavrfy(ctx->eng.pad, sig_len,
                        hash_oid, ctx->hash_CV_len, &pk->key.rsa, tmp)
                        || memcmp(tmp, ctx->hash_CV, ctx->hash_CV_len) != 0)
                {
                        return BR_ERR_BAD_SIGNATURE;
                }
        } else {
                if (ctx->eng.iecdsa == 0) {
                        return BR_ERR_BAD_SIGNATURE;
                }
                if (!ctx->eng.iecdsa(ctx->eng.iec,
                        ctx->hash_CV, ctx->hash_CV_len,
                        &pk->key.ec, ctx->eng.pad, sig_len))
                {
                        return BR_ERR_BAD_SIGNATURE;
                }
        }
        return 0;
}

}

\ =======================================================================

: addr-ctx:
        next-word { field }
        "addr-" field + 0 1 define-word
        0 8191 "offsetof(br_ssl_server_context, " field + ")" + make-CX
        postpone literal postpone ; ;

addr-ctx: client_max_version
addr-ctx: client_suites
addr-ctx: client_suites_num
addr-ctx: hashes
addr-ctx: curves
addr-ctx: sign_hash_id

\ Get address and length of the client_suites[] buffer. Length is expressed
\ in bytes.
: addr-len-client_suites ( -- addr len )
        addr-client_suites
        CX 0 1023 { BR_MAX_CIPHER_SUITES * sizeof(br_suite_translated) } ;

\ Read the client SNI extension.
: read-client-sni ( lim -- lim )
        \ Open extension value.
        read16 open-elt

        \ Open ServerNameList.
        read16 open-elt

        \ Find if there is a name of type 0 (host_name) with a length
        \ that fits in our dedicated buffer.
        begin dup while
                read8 if
                        read-ignore-16
                else
                        read16
                        dup 255 <= if
                                dup addr-server_name + 0 swap set8
                                addr-server_name swap read-blob
                        else
                                skip-blob
                        then
                then
        repeat

        \ Close ServerNameList.
        close-elt

        \ Close extension value.
        close-elt ;

\ Set the new maximum fragment length. BEWARE: this shall be called only
\ after reading the ClientHello and before writing the ServerHello.
cc: set-max-frag-len ( len -- ) {
        size_t max_frag_len = T0_POP();

        br_ssl_engine_new_max_frag_len(ENG, max_frag_len);

        /*
         * We must adjust our own output limit. Since we call this only
         * after receiving a ClientHello and before beginning to send
         * the ServerHello, the next output record should be empty at
         * that point, so we can use max_frag_len as a limit.
         */
        if (ENG->hlen_out > max_frag_len) {
                ENG->hlen_out = max_frag_len;
        }
}

\ Read the client Max Frag Length extension.
: read-client-frag ( lim -- lim )
        \ Extension value must have length exactly 1 byte.
        read16 1 <> if ERR_BAD_FRAGLEN fail then
        read8

        \ The byte value must be 1, 2, 3 or 4.
        dup dup 0= swap 5 >= or if ERR_BAD_FRAGLEN fail then

        \ If our own maximum fragment length is greater, then we reduce
        \ our length.
        8 + dup addr-log_max_frag_len get8 < if
                dup 1 swap << set-max-frag-len
                dup addr-log_max_frag_len set8
                addr-peer_log_max_frag_len set8
        else
                drop
        then ;

\ Read the Secure Renegotiation extension from the client.
: read-client-reneg ( lim -- lim )
        \ Get value length.
        read16

        \ The "reneg" value is one of:
        \   0   on first handshake, client support is unknown
        \   1   client does not support secure renegotiation
        \   2   client supports secure renegotiation
        addr-reneg get8 case
                0 of
                        \ First handshake, value length shall be 1.
                        1 = ifnot ERR_BAD_SECRENEG fail then
                        read8 if ERR_BAD_SECRENEG fail then
                        2 addr-reneg set8
                endof
                2 of
                        \ Renegotiation, value shall consist of 13 bytes
                        \ (header + copy of the saved client "Finished").
                        13 = ifnot ERR_BAD_SECRENEG fail then
                        read8 12 = ifnot ERR_BAD_SECRENEG fail then
                        addr-pad 12 read-blob
                        addr-saved_finished addr-pad 12 memcmp ifnot
                                ERR_BAD_SECRENEG fail
                        then
                endof

                \ If "reneg" is 1 then the client is not supposed to support
                \ the extension, and it sends it nonetheless, which means
                \ foul play.
                ERR_BAD_SECRENEG fail
        endcase ;

\ Read the Signature Algorithms extension.
: read-signatures ( lim -- lim )
        \ Open extension value.
        read16 open-elt

        read-list-sign-algos addr-hashes set32

        \ Close extension value.
        close-elt ;

\ Read the Supported Curves extension.
: read-supported-curves ( lim -- lim )
        \ Open extension value.
        read16 open-elt

        \ Open list of curve identifiers.
        read16 open-elt

        \ Get all supported curves.
        0 addr-curves set32
        begin dup while
                read16 dup 32 < if
                        1 swap << addr-curves get32 or addr-curves set32
                else
                        drop
                then
        repeat
        close-elt
        close-elt ;

\ Read the ALPN extension from client.
: read-ALPN-from-client ( lim -- lim )
        \ If we do not have configured names, then we just ignore the
        \ extension.
        addr-protocol_names_num get16 ifnot read-ignore-16 ret then

        \ Open extension value.
        read16 open-elt

        \ Open list of protocol names.
        read16 open-elt

        \ Get all names and test for their support. We keep the one with
        \ the lowest index (because we apply server's preferences, as
        \ recommended by RFC 7301, section 3.2. We set the 'found' variable
        \ to -2 and use an unsigned comparison, making -2 a huge value.
        -2 { found }
        begin dup while
                read8 dup { len } addr-pad swap read-blob
                len test-protocol-name dup found u< if
                        >found
                else
                        drop
                then
        repeat

        \ End of extension.
        close-elt
        close-elt

        \ Write back found name index (or not). If no match was found,
        \ then we write -1 (0xFFFF) in the index value, not 0, so that
        \ the caller knows that we tried to match, and failed.
        found 1+ addr-selected_protocol set16 ;

\ Call policy handler to get cipher suite, hash function identifier and
\ certificate chain. Returned value is 0 (false) on failure.
cc: call-policy-handler ( -- bool ) {
        int x;
        br_ssl_server_choices choices;

        x = (*CTX->policy_vtable)->choose(
                CTX->policy_vtable, CTX, &choices);
        ENG->session.cipher_suite = choices.cipher_suite;
        CTX->sign_hash_id = choices.algo_id;
        ENG->chain = choices.chain;
        ENG->chain_len = choices.chain_len;
        T0_PUSHi(-(x != 0));
}

\ Check for a remembered session.
cc: check-resume ( -- bool ) {
        if (ENG->session.session_id_len == 32
                && CTX->cache_vtable != NULL && (*CTX->cache_vtable)->load(
                        CTX->cache_vtable, CTX, &ENG->session))
        {
                T0_PUSHi(-1);
        } else {
                T0_PUSH(0);
        }
}

\ Save the current session.
cc: save-session ( -- ) {
        if (CTX->cache_vtable != NULL) {
                (*CTX->cache_vtable)->save(
                        CTX->cache_vtable, CTX, &ENG->session);
        }
}

\ Read ClientHello. If the session is resumed, then -1 is returned.
: read-ClientHello ( -- resume )
        \ Get header, and check message type.
        read-handshake-header 1 = ifnot ERR_UNEXPECTED fail then

        \ Get maximum protocol version from client.
        read16 dup { client-version-max } addr-client_max_version set16

        \ Client random.
        addr-client_random 32 read-blob

        \ Client session ID.
        read8 dup 32 > if ERR_OVERSIZED_ID fail then
        dup addr-session_id_len set8
        addr-session_id swap read-blob

        \ Lookup session for resumption. We should do that here because
        \ we need to verify that the remembered cipher suite is still
        \ matched by this ClientHello.
        check-resume { resume }

        \ Cipher suites. We read all cipher suites from client, each time
        \ matching against our own list. We accumulate suites in the
        \ client_suites[] context buffer: we keep suites that are
        \ supported by both the client and the server (so the list size
        \ cannot exceed that of the server list), and we keep them in
        \ either client or server preference order (depending on the
        \ relevant flag).
        \
        \ We also need to identify the pseudo cipher suite for secure
        \ renegotiation here.
        read16 open-elt
        0 { reneg-scsv }
        0 { resume-suite }
        addr-len-client_suites dup2 bzero
        over + { css-off css-max }
        begin
                dup while
                read16 dup { suite }

                \ Check that when resuming a session, the requested
                \ suite is still valid.
                resume if
                        dup addr-cipher_suite get16 = if
                                -1 >resume-suite
                        then
                then

                \ Special handling for TLS_EMPTY_RENEGOTIATION_INFO_SCSV.
                \ This fake cipher suite may occur only in the first
                \ handshake.
                dup 0x00FF = if
                        addr-reneg get8 if ERR_BAD_SECRENEG fail then
                        -1 >reneg-scsv
                then

                \ Special handling for TLS_FALLBACK_SCSV. If the client
                \ maximum version is less than our own maximum version,
                \ then this is an undue downgrade. We mark it by setting
                \ the client max version to 0x10000.
                dup 0x5600 = if
                        client-version-max addr-version_min get16 >=
                        client-version-max addr-version_max get16 < and if
                                -1 >client-version-max
                        then
                then

                \ Test whether the suite is supported by the server.
                scan-suite dup 0< if
                        \ We do not support this cipher suite. Note
                        \ that this also covers the case of pseudo
                        \ cipher suites.
                        drop
                else
                        \ If we use server order, then we place the
                        \ suite at the computed offset; otherwise, we
                        \ append it to the list at the current place.
                        0 flag? if
                                2 << addr-client_suites + suite swap set16
                        else
                                drop
                                \ We need to test for list length because
                                \ the client list may have duplicates,
                                \ that we do not filter. Duplicates are
                                \ invalid so this is not a problem if we
                                \ reject such clients.
                                css-off css-max >= if
                                        ERR_BAD_HANDSHAKE fail
                                then
                                suite css-off set16
                                css-off 4 + >css-off
                        then
                then
        repeat
        drop

        \ Compression methods. We need method 0 (no compression).
        0 { ok-compression }
        read8 open-elt
        begin dup while
                read8 ifnot -1 >ok-compression then
        repeat
        close-elt

        \ Set default values for parameters that may be affected by
        \ extensions:
        \ -- server name is empty
        \ -- client is reputed to know RSA and ECDSA, both with SHA-1
        \ -- the default elliptic curve is P-256 (secp256r1, id = 23)
        0 addr-server_name set8
        0x0404 addr-hashes set32
        0x800000 addr-curves set32

        \ Process extensions, if any.
        dup if
                read16 open-elt
                begin dup while
                        read16 case
                                \ Server Name Indication.
                                0x0000 of
                                        read-client-sni
                                endof
                                \ Max Frag Length.
                                0x0001 of
                                        read-client-frag
                                endof
                                \ Secure Renegotiation.
                                0xFF01 of
                                        read-client-reneg
                                endof
                                \ Signature Algorithms.
                                0x000D of
                                        read-signatures
                                endof
                                \ Supported Curves.
                                0x000A of
                                        read-supported-curves
                                endof
                                \ Supported Point Formats.
                                \ We only support "uncompressed", that all
                                \ implementations are supposed to support,
                                \ so we can simply ignore that extension.
                                \ 0x000B of
                                \       read-ignore-16
                                \ endof

                                \ ALPN
                                0x0010 of
                                        read-ALPN-from-client
                                endof

                                \ Other extensions are ignored.
                                drop read-ignore-16 0
                        endcase
                repeat
                close-elt
        then

        \ Close message.
        close-elt

        \ Cancel session resumption if the cipher suite was not found.
        resume resume-suite and >resume

        \ Now check the received data. Since the client is expecting an
        \ answer, we can send an appropriate fatal alert on any error.

        \ Compute protocol version as the minimum of our maximum version,
        \ and the maximum version sent by the client. If that is less than
        \ 0x0300 (SSL-3.0), then fail. Otherwise, we may at least send an
        \ alert with that version. We still reject versions lower than our
        \ configured minimum.
        \ As a special case, in case of undue downgrade, we send a specific
        \ alert (see RFC 7507). Note that this case may happen only if
        \ we would otherwise accept the client's version.
        client-version-max 0< if
                addr-client_max_version get16 addr-version_out set16
                86 fail-alert
        then
        addr-version_max get16
        dup client-version-max > if drop client-version-max then
        dup 0x0300 < if ERR_BAD_VERSION fail then
        client-version-max addr-version_min get16 < if
                70 fail-alert
        then
        \ If resuming the session, then enforce the previously negotiated
        \ version (if still possible).
        resume if
                addr-version get16 client-version-max <= if
                        drop addr-version get16
                else
                        0 >resume
                then
        then
        dup addr-version set16
        dup addr-version_in set16
        dup addr-version_out set16
        0x0303 >= { can-tls12 }

        \ If the client sent TLS_EMPTY_RENEGOTIATION_INFO_SCSV, then
        \ we should mark the client as "supporting secure renegotiation".
        reneg-scsv if 2 addr-reneg set8 then

        \ If, at that point, the 'reneg' value is still 0, then the client
        \ did not send the extension or the SCSV, so we have to assume
        \ that secure renegotiation is not supported by that client.
        addr-reneg get8 ifnot 1 addr-reneg set8 then

        \ Check compression.
        ok-compression ifnot 40 fail-alert then

        \ Filter hash function support by what the server also supports.
        \ If no common hash function remains with RSA and/or ECDSA, then
        \ the corresponding ECDHE suites are not possible.
        supported-hash-functions drop 257 * 0xFFFF0000 or
        addr-hashes get32 and dup addr-hashes set32
        \ In 'can-ecdhe', bit 12 is set if ECDHE_RSA is possible, bit 13 is
        \ set if ECDHE_ECDSA is possible.
        dup 0xFF and 0<> neg
        swap 8 >> 0<> 2 and or 12 << { can-ecdhe }

        \ Filter supported curves. If there is no common curve between
        \ client and us, then ECDHE suites cannot be used. Note that we
        \ may still allow ECDH, depending on the EC key handler.
        addr-curves get32 supported-curves and dup addr-curves set32
        ifnot 0 >can-ecdhe then

        \ If resuming a session, then the next steps are not necessary;
        \ we won't invoke the policy handler.
        resume if -1 ret then

        \ We are not resuming, so a new session ID should be generated.
        \ We don't check that the new ID is distinct from the one sent
        \ by the client because probability of such an event is 2^(-256),
        \ i.e. much (much) lower than that of an undetected transmission
        \ error or hardware miscomputation, and with similar consequences
        \ (handshake simply fails).
        addr-session_id 32 mkrand
        32 addr-session_id_len set8

        \ Translate common cipher suites, then squeeze out holes: there
        \ may be holes because of the way we fill the list when the
        \ server preference order is enforced, and also in case some
        \ suites are filtered out. In particular:
        \ -- ECDHE suites are removed if there is no common hash function
        \    (for the relevant signature algorithm) or no common curve.
        \ -- TLS-1.2-only suites are removed if the negociated version is
        \    TLS-1.1 or lower.
        addr-client_suites dup >css-off
        begin dup css-max < while
                dup get16 dup cipher-suite-to-elements
                dup 12 >> dup 1 = swap 2 = or if
                        dup can-ecdhe and ifnot
                                2drop 0 dup
                        then
                then
                can-tls12 ifnot
                        \ Suites compatible with TLS-1.0 and TLS-1.1 are
                        \ exactly the ones that use HMAC/SHA-1.
                        dup 0xF0 and 0x20 <> if
                                2drop 0 dup
                        then
                then
                dup if
                        css-off 2+ set16 css-off set16
                        css-off 4 + >css-off
                else
                        2drop
                then
                4 +
        repeat
        drop
        css-off addr-client_suites - 2 >>
        dup ifnot
                \ No common cipher suite: handshake failure.
                40 fail-alert
        then
        addr-client_suites_num set8

        \ Check ALPN.
        addr-selected_protocol get16 0xFFFF = if
                3 flag? if 120 fail-alert then
                0 addr-selected_protocol set16
        then

        \ Call policy handler to obtain the cipher suite and other
        \ parameters.
        call-policy-handler ifnot 40 fail-alert then

        \ We are not resuming a session.
        0 ;

\ Write ServerHello.
: write-ServerHello ( initial -- )
        { initial }
        \ Compute ServerHello length.
        2 write8 70

        \ Compute length of Secure Renegotiation extension.
        addr-reneg get8 2 = if
                initial if 5 else 29 then
        else
                0
        then
        { ext-reneg-len }

        \ Compute length of Max Fragment Length extension.
        addr-peer_log_max_frag_len get8 if 5 else 0 then
        { ext-max-frag-len }

        \ Compute length of ALPN extension. This also copy the
        \ selected protocol name into the pad.
        addr-selected_protocol get16 dup if 1- copy-protocol-name 7 + then
        { ext-ALPN-len }

        \ Adjust ServerHello length to account for the extensions.
        ext-reneg-len ext-max-frag-len + ext-ALPN-len + dup if 2 + then +
        write24

        \ Protocol version
        addr-version get16 write16

        \ Server random
        addr-server_random 4 bzero
        addr-server_random 4 + 28 mkrand
        addr-server_random 32 write-blob

        \ Session ID
        \ TODO: if we have no session cache at all, we might send here
        \ an empty session ID. This would save a bit of network
        \ bandwidth.
        32 write8
        addr-session_id 32 write-blob

        \ Cipher suite
        addr-cipher_suite get16 write16

        \ Compression method
        0 write8

        \ Extensions
        ext-reneg-len ext-max-frag-len + ext-ALPN-len + dup if
                write16
                ext-reneg-len dup if
                        0xFF01 write16
                        4 - dup write16
                        1- addr-saved_finished swap write-blob-head8
                else
                        drop
                then
                ext-max-frag-len if
                        0x0001 write16
                        1 write16 addr-peer_log_max_frag_len get8 8 - write8
                then
                ext-ALPN-len dup if
                        \ Note: the selected protocol name was previously
                        \ copied into the pad.
                        0x0010 write16
                        4 - dup write16
                        2- dup write16
                        1- addr-pad swap write-blob-head8
                else
                        drop
                then
        else
                drop
        then ;

\ Do the first part of ECDHE. Returned value is the computed signature
\ length, or a negative error code on error.
cc: do-ecdhe-part1 ( curve -- len ) {
        int curve = T0_POPi();
        T0_PUSHi(do_ecdhe_part1(CTX, curve));
}

\ Get index of first bit set to 1 (in low to high order).
: lowest-1 ( bits -- n )
        dup ifnot drop -1 ret then
        0 begin dup2 >> 1 and 0= while 1+ repeat
        swap drop ;

\ Write the Server Key Exchange message (if applicable).
: write-ServerKeyExchange ( -- )
        addr-cipher_suite get16 use-ecdhe? ifnot ret then

        \ We must select an appropriate curve among the curves that
        \ are supported both by us and the peer. Right now, we apply
        \ a fixed preference order: Curve25519, P-256, P-384, P-521,
        \ then the common curve with the lowest ID.
        \ (TODO: add some option to make that behaviour configurable.)
        \
        \ This loop always terminates because previous processing made
        \ sure that ECDHE suites are not selectable if there is no common
        \ curve.
        addr-curves get32
        dup 0x20000000 and if
                drop 29
        else
                dup 0x38000000 and dup if swap then
                drop lowest-1
        then
        { curve-id }

        \ Compute the signed curve point to send.
        curve-id do-ecdhe-part1 dup 0< if neg fail then { sig-len }

        \ If using TLS-1.2+, then the hash function and signature
        \ algorithm are explicitly encoded in the message.
        addr-version get16 0x0303 >= { tls1.2+ }

        12 write8
        sig-len addr-ecdhe_point_len get8 + tls1.2+ 2 and + 6 + write24

        \ Curve parameters: named curve with 16-bit ID.
        3 write8 curve-id write16

        \ Public point.
        addr-ecdhe_point addr-ecdhe_point_len get8 write-blob-head8

        \ If TLS-1.2+, write hash and signature identifiers.
        tls1.2+ if
                \ sign_hash_id contains either a hash identifier,
                \ or the complete 16-bit value to write.
                addr-sign_hash_id get16
                dup 0xFF00 < if
                        write16
                else
                        0xFF and write8
                        \ 'use-rsa-ecdhe?' returns -1 for RSA, 0 for
                        \ ECDSA. The byte on the wire shall be 1 for RSA,
                        \ 3 for ECDSA.
                        addr-cipher_suite get16 use-rsa-ecdhe? 1 << 3 + write8
                then
        then

        \ Signature.
        sig-len write16
        addr-pad sig-len write-blob ;

\ Get length of the list of anchor names to send to the client. The length
\ includes the per-name 2-byte header, but _not_ the 2-byte header for
\ the list itself. If no client certificate is requested, then this
\ returns 0.
cc: ta-names-total-length ( -- len ) {
        size_t u, len;

        len = 0;
        if (CTX->ta_names != NULL) {
                for (u = 0; u < CTX->num_tas; u ++) {
                        len += CTX->ta_names[u].len + 2;
                }
        } else if (CTX->tas != NULL) {
                for (u = 0; u < CTX->num_tas; u ++) {
                        len += CTX->tas[u].dn.len + 2;
                }
        }
        T0_PUSH(len);
}

\ Compute length and optionally write the contents of the list of
\ supported client authentication methods.
: write-list-auth ( do_write -- len )
        0
        addr-cipher_suite get16 use-ecdh? if
                2+ over if 65 write8 66 write8 then
        then
        supports-rsa-sign? if 1+ over if 1 write8 then then
        supports-ecdsa? if 1+ over if 64 write8 then then
        swap drop ;

: write-signhash-inner2 ( dow algo hashes len id -- dow algo hashes len )
        { id }
        over 1 id << and ifnot ret then
        2+
        3 pick if id write8 2 pick write8 then ;

: write-signhash-inner1 ( dow algo hashes -- dow len )
        0
        4 write-signhash-inner2
        5 write-signhash-inner2
        6 write-signhash-inner2
        3 write-signhash-inner2
        2 write-signhash-inner2
        -rot 2drop ;

\ Compute length and optionally write the contents of the list of
\ supported sign+hash algorithms.
: write-list-signhash ( do_write -- len )
        0 { len }
        \ If supporting neither RSA nor ECDSA in the engine, then we
        \ will do only static ECDH, and thus we claim support for
        \ everything (for the X.509 validator).
        supports-rsa-sign? supports-ecdsa? or ifnot
                1 0x7C write-signhash-inner1 >len
                3 0x7C write-signhash-inner1 len +
                swap drop ret
        then
        supports-rsa-sign? if
                1 supported-hash-functions drop
                write-signhash-inner1 >len
        then
        supports-ecdsa? if
                3 supported-hash-functions drop
                write-signhash-inner1 len + >len
        then
        drop len ;

\ Initialise index for sending the list of anchor DN.
cc: begin-ta-name-list ( -- ) {
        CTX->cur_dn_index = 0;
}

\ Switch to next DN in the list. Returned value is the DN length, or -1
\ if the end of the list was reached.
cc: begin-ta-name ( -- len ) {
        const br_x500_name *dn;
        if (CTX->cur_dn_index >= CTX->num_tas) {
                T0_PUSHi(-1);
        } else {
                if (CTX->ta_names == NULL) {
                        dn = &CTX->tas[CTX->cur_dn_index].dn;
                } else {
                        dn = &CTX->ta_names[CTX->cur_dn_index];
                }
                CTX->cur_dn_index ++;
                CTX->cur_dn = dn->data;
                CTX->cur_dn_len = dn->len;
                T0_PUSH(CTX->cur_dn_len);
        }
}

\ Copy a chunk of the current DN into the pad. Returned value is the
\ chunk length; this is 0 when the end of the current DN is reached.
cc: copy-dn-chunk ( -- len ) {
        size_t clen;

        clen = CTX->cur_dn_len;
        if (clen > sizeof ENG->pad) {
                clen = sizeof ENG->pad;
        }
        memcpy(ENG->pad, CTX->cur_dn, clen);
        CTX->cur_dn += clen;
        CTX->cur_dn_len -= clen;
        T0_PUSH(clen);
}

\ Write a CertificateRequest message.
: write-CertificateRequest ( -- )
        \ The list of client authentication types includes:
        \    rsa_sign (1)
        \    ecdsa_sign (64)
        \    rsa_fixed_ecdh (65)
        \    ecdsa_fixed_ecdh (66)
        \ rsa_sign and ecdsa_sign require, respectively, RSA and ECDSA
        \ support. Static ECDH requires that the cipher suite is ECDH.
        \ When we ask for static ECDH, we always send both rsa_fixed_ecdh
        \ and ecdsa_fixed_ecdh because what matters there is what the
        \ X.509 engine may support, and we do not control that.
        \
        \ With TLS 1.2, we must also send a list of supported signature
        \ and hash algorithms. That list is supposed to qualify both
        \ the engine itself, and the X.509 validator, which are separate
        \ in BearSSL. There again, we use the engine capabilities in that
        \ list, and resort to a generic all-support list if only
        \ static ECDH is accepted.
        \
        \ (In practice, client implementations tend to have at most one
        \ or two certificates, and send the chain regardless of what
        \ algorithms are used in it.)

        0 write-list-auth
        addr-version get16 0x0303 >= if
                2+ 0 write-list-signhash +
        then
        ta-names-total-length + 3 +

        \ Message header
        13 write8 write24

        \ List of authentication methods
        0 write-list-auth write8 1 write-list-auth drop

        \ For TLS 1.2+, list of sign+hash
        addr-version get16 0x0303 >= if
                0 write-list-signhash write16 1 write-list-signhash drop
        then

        \ Trust anchor names
        ta-names-total-length write16
        begin-ta-name-list
        begin
                begin-ta-name
                dup 0< if drop ret then write16
                begin copy-dn-chunk dup while
                        addr-pad swap write-blob
                repeat
                drop
        again ;

\ Write the Server Hello Done message.
: write-ServerHelloDone ( -- )
        14 write8 0 write24 ;

\ Perform RSA decryption of the client-sent pre-master secret. The value
\ is in the pad, and its length is provided as parameter.
cc: do-rsa-decrypt ( len prf_id -- ) {
        int prf_id = T0_POPi();
        size_t len = T0_POP();
        do_rsa_decrypt(CTX, prf_id, ENG->pad, len);
}

\ Perform ECDH (not ECDHE). The point from the client is in the pad, and
\ its length is provided as parameter.
cc: do-ecdh ( len prf_id -- ) {
        int prf_id = T0_POPi();
        size_t len = T0_POP();
        do_ecdh(CTX, prf_id, ENG->pad, len);
}

\ Do the second part of ECDHE.
cc: do-ecdhe-part2 ( len prf_id -- ) {
        int prf_id = T0_POPi();
        size_t len = T0_POP();
        do_ecdhe_part2(CTX, prf_id, ENG->pad, len);
}

\ Perform static ECDH. The point from the client is the public key
\ extracted from its certificate.
cc: do-static-ecdh ( prf_id -- ) {
        do_static_ecdh(CTX, T0_POP());
}

\ Read a ClientKeyExchange header.
: read-ClientKeyExchange-header ( -- len )
        read-handshake-header 16 = ifnot ERR_UNEXPECTED fail then ;

\ Read the Client Key Exchange contents (non-empty case).
: read-ClientKeyExchange-contents ( lim -- )
        \ What we should get depends on the cipher suite.
        addr-cipher_suite get16 use-rsa-keyx? if
                \ RSA key exchange: we expect a RSA-encrypted value.
                read16
                dup 512 > if ERR_LIMIT_EXCEEDED fail then
                dup { enc-rsa-len }
                addr-pad swap read-blob
                enc-rsa-len addr-cipher_suite get16 prf-id do-rsa-decrypt
        then
        addr-cipher_suite get16 dup use-ecdhe? swap use-ecdh? { ecdhe ecdh }
        ecdh ecdhe or if
                \ ECDH or ECDHE key exchange: we expect an EC point.
                read8 dup { ec-point-len }
                addr-pad swap read-blob
                ec-point-len addr-cipher_suite get16 prf-id
                ecdhe if do-ecdhe-part2 else do-ecdh then
        then
        close-elt ;

\ Read the Client Key Exchange (normal case).
: read-ClientKeyExchange ( -- )
        read-ClientKeyExchange-header
        read-ClientKeyExchange-contents ;

\ Obtain all possible hash values for handshake messages so far. This
\ is done because we need the hash value for the CertificateVerify
\ _before_ knowing which hash function will actually be used, as this
\ information is obtained from decoding the message header itself.
\ All hash values are stored in the pad (208 bytes in total).
cc: compute-hash-CV ( -- ) {
        int i;

        for (i = 1; i <= 6; i ++) {
                br_multihash_out(&ENG->mhash, i,
                        ENG->pad + HASH_PAD_OFF[i - 1]);
        }
}

\ Copy the proper hash value from the pad into the dedicated buffer.
\ Returned value is true (-1) on success, false (0) on error (error
\ being an unimplemented hash function). The id has already been verified
\ to be either 0 (for MD5+SHA-1) or one of the SHA-* functions.
cc: copy-hash-CV ( hash_id -- bool ) {
        int id = T0_POP();
        size_t off, len;

        if (id == 0) {
                off = 0;
                len = 36;
        } else {
                if (br_multihash_getimpl(&ENG->mhash, id) == 0) {
                        T0_PUSH(0);
                        T0_RET();
                }
                off = HASH_PAD_OFF[id - 1];
                len = HASH_PAD_OFF[id] - off;
        }
        memcpy(CTX->hash_CV, ENG->pad + off, len);
        CTX->hash_CV_len = len;
        CTX->hash_CV_id = id;
        T0_PUSHi(-1);
}

\ Verify signature in CertificateVerify. Output is 0 on success, or a
\ non-zero error code.
cc: verify-CV-sig ( sig-len -- err ) {
        int err;

        err = verify_CV_sig(CTX, T0_POP());
        T0_PUSHi(err);
}

\ Process static ECDH.
: process-static-ECDH ( ktu -- )
        \ Static ECDH is allowed only if the cipher suite uses ECDH, and
        \ the client's public key has type EC and allows key exchange.
        \ BR_KEYTYPE_KEYX is 0x10, and BR_KEYTYPE_EC is 2.
        0x1F and 0x12 = ifnot ERR_WRONG_KEY_USAGE fail then
        addr-cipher_suite get16
        dup use-ecdh? ifnot ERR_UNEXPECTED fail then
        prf-id
        do-static-ecdh ;

\ Read CertificateVerify header.
: read-CertificateVerify-header ( -- lim )
        compute-hash-CV
        read-handshake-header 15 = ifnot ERR_UNEXPECTED fail then ;

\ Read CertificateVerify. The client key type + usage is expected on the
\ stack.
: read-CertificateVerify ( ktu -- )
        \ Check that the key allows for signatures.
        dup 0x20 and ifnot ERR_WRONG_KEY_USAGE fail then
        0x0F and { key-type }

        \ Get header.
        read-CertificateVerify-header

        \ With TLS 1.2+, there is an explicit hash + signature indication,
        \ which must be compatible with the key type.
        addr-version get16 0x0303 >= if
                \ Get hash function, then signature algorithm. The
                \ signature algorithm is 1 (RSA) or 3 (ECDSA) while our
                \ symbolic constants for key types are 1 (RSA) or 2 (EC).
                read16
                dup 0xFF and 1+ 1 >> key-type = ifnot
                        ERR_BAD_SIGNATURE fail
                then
                8 >>

                \ We support only SHA-1, SHA-224, SHA-256, SHA-384
                \ and SHA-512. We explicitly reject MD5.
                dup 2 < over 6 > or if ERR_INVALID_ALGORITHM fail then
        else
                \ With TLS 1.0 and 1.1, hash is MD5+SHA-1 (0) for RSA,
                \ SHA-1 (2) for ECDSA.
                key-type 0x01 = if 0 else 2 then
        then
        copy-hash-CV ifnot ERR_INVALID_ALGORITHM fail then

        \ Read signature.
        read16 dup { sig-len }
        dup 512 > if ERR_LIMIT_EXCEEDED fail then
        addr-pad swap read-blob
        sig-len verify-CV-sig
        dup if fail then drop

        close-elt ;

\ Send a HelloRequest.
: send-HelloRequest ( -- )
        flush-record
        begin can-output? not while wait-co drop repeat
        22 addr-record_type_out set8
        0 write8 0 write24 flush-record
        23 addr-record_type_out set8 ;

\ Make a handshake.
: do-handshake ( initial -- )
        0 addr-application_data set8
        22 addr-record_type_out set8
        0 addr-selected_protocol set16
        multihash-init
        read-ClientHello
        more-incoming-bytes? if ERR_UNEXPECTED fail then
        if
                \ Session resumption
                write-ServerHello
                0 write-CCS-Finished
                0 read-CCS-Finished
        else
                \ Not a session resumption
                write-ServerHello
                write-Certificate drop
                write-ServerKeyExchange
                ta-names-total-length if
                        write-CertificateRequest
                then
                write-ServerHelloDone
                flush-record

                \ If we sent a CertificateRequest then we expect a
                \ Certificate message.
                ta-names-total-length if
                        \ Read client certificate.
                        0 read-Certificate

                        choice
                                dup 0< uf
                                        \ Client certificate validation failed.
                                        2 flag? ifnot neg fail then
                                        drop
                                        read-ClientKeyExchange
                                        read-CertificateVerify-header
                                        dup skip-blob drop
                                enduf
                                dup 0= uf
                                        \ Client sent no certificate at all.
                                        drop
                                        2 flag? ifnot
                                                ERR_NO_CLIENT_AUTH fail
                                        then
                                        read-ClientKeyExchange
                                enduf

                                \ Client certificate was validated.
                                read-ClientKeyExchange-header
                                dup ifnot
                                        \ Empty ClientKeyExchange.
                                        drop
                                        process-static-ECDH
                                else
                                        read-ClientKeyExchange-contents
                                        read-CertificateVerify
                                then
                        endchoice
                else
                        \ No client certificate request, we just expect
                        \ a non-empty ClientKeyExchange.
                        read-ClientKeyExchange
                then
                0 read-CCS-Finished
                0 write-CCS-Finished
                save-session
        then
        1 addr-application_data set8
        23 addr-record_type_out set8 ;

\ Entry point.
: main ( -- ! )
        \ Perform initial handshake.
        -1 do-handshake

        begin
                \ Wait for further invocation. At that point, we should
                \ get either an explicit call for renegotiation, or
                \ an incoming ClientHello handshake message.
                wait-co
                dup 0x07 and case
                        0x00 of
                                0x10 and if
                                        \ The best we can do is ask for a
                                        \ renegotiation, then wait for it
                                        \ to happen.
                                        send-HelloRequest
                                then
                        endof
                        0x01 of
                                \ Reject renegotiations if the peer does not
                                \ support secure renegotiation, or if the
                                \ "no renegotiation" flag is set.
                                drop
                                addr-reneg get8 1 = 1 flag? or if
                                        flush-record
                                        begin can-output? not while
                                                wait-co drop
                                        repeat
                                        100 send-warning
                                else
                                        0 do-handshake
                                then
                        endof
                        ERR_UNEXPECTED fail
                endcase
        again
        ;