SSH Daemon Project - XT-Emporium

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#include "includes.h"
#include "algo.h"
#include "buffer.h"
#include "session.h"
#include "dbrandom.h"
#include "crypto_desc.h"
#include "curve25519.h"
#include "kex.h"

/* PQ hybrids also use curve25519 internally */
#if DROPBEAR_CURVE25519_DEP

struct kex_curve25519_param *gen_kexcurve25519_param() {
    /* Per http://cr.yp.to/ecdh.html */
    struct kex_curve25519_param *param = m_malloc(sizeof(*param));
    const unsigned char basepoint[32] = {9};

    genrandom(param->priv, CURVE25519_LEN);
    dropbear_curve25519_scalarmult(param->pub, param->priv, basepoint);

    return param;
}

void free_kexcurve25519_param(struct kex_curve25519_param *param) {
    m_burn(param->priv, CURVE25519_LEN);
    m_free(param);
}

/* out must be CURVE25519_LEN */
void kexcurve25519_derive(const struct kex_curve25519_param *param, const buffer *buf_pub_them,
    unsigned char *out) {
    char zeroes[CURVE25519_LEN] = {0};
    if (buf_pub_them->len != CURVE25519_LEN)
    {
        dropbear_exit("Bad curve25519");
    }

    dropbear_curve25519_scalarmult(out, param->priv, buf_pub_them->data);

    if (constant_time_memcmp(zeroes, out, CURVE25519_LEN) == 0) {
        dropbear_exit("Bad curve25519");
    }
}

#endif /* DROPBEAR_CURVE25519_DEP */

#if DROPBEAR_CURVE25519

static void put_ssh_mpint_bytes(buffer *buf, const unsigned char *bytes,
	unsigned int len)
{
	unsigned int first = 0;
	unsigned int outlen;

	while (first < len && bytes[first] == 0) {
		first++;
	}
	if (first == len) {
		buf_putint(buf, 0);
		return;
	}

	outlen = len - first;
	if (bytes[first] & 0x80) {
		buf_putint(buf, outlen + 1);
		buf_putbyte(buf, 0);
	} else {
		buf_putint(buf, outlen);
	}
	buf_putbytes(buf, bytes + first, outlen);
}

/* Only required for x25519 directly */
void kexcurve25519_comb_key(const struct kex_curve25519_param *param, const buffer *buf_pub_them,
    sign_key *hostkey) {
    unsigned char out[CURVE25519_LEN];
    const unsigned char* Q_C = NULL;
    const unsigned char* Q_S = NULL;

    kexcurve25519_derive(param, buf_pub_them, out);

    ses.dh_K_bytes = buf_new(CURVE25519_LEN + 5);
    put_ssh_mpint_bytes(ses.dh_K_bytes, out, CURVE25519_LEN);
    m_burn(out, sizeof(out));

    /* Create the remainder of the hash buffer, to generate the exchange hash.
       See RFC5656 section 4 page 7 */
    if (IS_DROPBEAR_CLIENT) {
        Q_C = param->pub;
        Q_S = buf_pub_them->data;
    } else {
        Q_S = param->pub;
        Q_C = buf_pub_them->data;
    }

    /* K_S, the host key */
    buf_put_pub_key(ses.kexhashbuf, hostkey, ses.newkeys->algo_hostkey);
    /* Q_C, client's ephemeral public key octet string */
    buf_putstring(ses.kexhashbuf, (const char*)Q_C, CURVE25519_LEN);
    /* Q_S, server's ephemeral public key octet string */
    buf_putstring(ses.kexhashbuf, (const char*)Q_S, CURVE25519_LEN);
    /* K, the shared secret */
    buf_putbytes(ses.kexhashbuf, ses.dh_K_bytes->data, ses.dh_K_bytes->len);

    /* calculate the hash H to sign */
    finish_kexhashbuf();
}

#endif /* DROPBEAR_CURVE25519 */
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