fix a MITM attack in the protocol

diff --git a/d0_blind_id.c b/d0_blind_id.c
index d8d90f12a6a40949111c749d4227ea5f3821a143..9c7a5137fb91ea02c0868aa166a7cb65805c3ad7 100644 (file)
--- a/d0_blind_id.c
+++ b/d0_blind_id.c
@@ -68,7 +68,8 @@ struct d0_blind_id_s
        d0_bignum_t *rsa_blind_signature_camouflage; // random number blind signature
 
        d0_bignum_t *r; // random number for schnorr ID
-       d0_bignum_t *other_4_to_r; // for DH key exchange
+       d0_bignum_t *t; // for DH key exchange
+       d0_bignum_t *other_4_to_t; // for DH key exchange
        d0_bignum_t *challenge; // challenge
 
        char msghash[SCHNORR_HASHSIZE]; // init hash
@@ -305,7 +306,8 @@ void d0_blind_id_clear(d0_blind_id_t *ctx)
        if(ctx->rsa_blind_signature_camouflage) d0_bignum_free(ctx->rsa_blind_signature_camouflage);
        if(ctx->r) d0_bignum_free(ctx->r);
        if(ctx->challenge) d0_bignum_free(ctx->challenge);
-       if(ctx->other_4_to_r) d0_bignum_free(ctx->other_4_to_r);
+       if(ctx->t) d0_bignum_free(ctx->t);
+       if(ctx->other_4_to_t) d0_bignum_free(ctx->other_4_to_t);
        memset(ctx, 0, sizeof(*ctx));
 }
 
@@ -322,7 +324,8 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_copy(d0_blind_id_t *ctx, const d0_blind_id_t
        if(src->rsa_blind_signature_camouflage) CHECK_ASSIGN(ctx->rsa_blind_signature_camouflage, d0_bignum_mov(NULL, src->rsa_blind_signature_camouflage));
        if(src->r) CHECK_ASSIGN(ctx->r, d0_bignum_mov(NULL, src->r));
        if(src->challenge) CHECK_ASSIGN(ctx->challenge, d0_bignum_mov(NULL, src->challenge));
-       if(src->other_4_to_r) CHECK_ASSIGN(ctx->other_4_to_r, d0_bignum_mov(NULL, src->other_4_to_r));
+       if(src->t) CHECK_ASSIGN(ctx->t, d0_bignum_mov(NULL, src->t));
+       if(src->other_4_to_t) CHECK_ASSIGN(ctx->other_4_to_t, d0_bignum_mov(NULL, src->other_4_to_t));
        memcpy(ctx->msg, src->msg, sizeof(ctx->msg));
        ctx->msglen = src->msglen;
        memcpy(ctx->msghash, src->msghash, sizeof(ctx->msghash));
@@ -775,7 +778,7 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_challenge(d0_bl
                USING(schnorr_G);
        }
        USING(rsa_e); USING(rsa_n);
-       REPLACING(challenge); REPLACING(msg); REPLACING(msglen); REPLACING(msghash); REPLACING(r);
+       REPLACING(challenge); REPLACING(msg); REPLACING(msglen); REPLACING(msghash); REPLACING(r); REPLACING(t);
 
        in = d0_iobuf_open_read(inbuf, inbuflen);
        out = d0_iobuf_open_write(outbuf, *outbuflen);
@@ -823,13 +826,12 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_challenge(d0_bl
 
        // send challenge
        CHECK_ASSIGN(ctx->challenge, d0_bignum_rand_bit_atmost(ctx->challenge, SCHNORR_BITS));
-
        CHECK(d0_iobuf_write_bignum(out, ctx->challenge));
 
-       // Diffie Hellmann
+       // Diffie Hellmann send
        CHECK(d0_dl_get_order(temp0, ctx->schnorr_G));
-       CHECK_ASSIGN(ctx->r, d0_bignum_rand_range(ctx->r, zero, temp0));
-       CHECK(d0_bignum_mod_pow(temp0, four, ctx->r, ctx->schnorr_G));
+       CHECK_ASSIGN(ctx->t, d0_bignum_rand_range(ctx->t, zero, temp0));
+       CHECK(d0_bignum_mod_pow(temp0, four, ctx->t, ctx->schnorr_G));
        CHECK(d0_iobuf_write_bignum(out, temp0));
 
        if(status)
@@ -852,7 +854,7 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_response(d0_bli
        d0_iobuf_t *out = NULL;
 
        // temps: 0 order, 1 prod, 2 y, 3 challenge
-       REPLACING(other_4_to_r);
+       REPLACING(other_4_to_t); REPLACING(t);
        USING(schnorr_G); USING(schnorr_s); USING(r);
 
        in = d0_iobuf_open_read(inbuf, inbuflen);
@@ -862,11 +864,6 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_response(d0_bli
        CHECK(d0_bignum_cmp(temp3, zero) >= 0);
        CHECK(d0_bignum_size(temp3) <= SCHNORR_BITS);
 
-       // Diffie Hellmann
-       CHECK_ASSIGN(ctx->other_4_to_r, d0_iobuf_read_bignum(in, ctx->other_4_to_r));
-       CHECK(d0_bignum_cmp(ctx->other_4_to_r, zero) > 0);
-       CHECK(d0_bignum_cmp(ctx->other_4_to_r, ctx->schnorr_G) < 0);
-
        // send response for schnorr ID scheme
        // i.challenge. r + ctx->schnorr_s * temp3
        CHECK(d0_dl_get_order(temp0, ctx->schnorr_G));
@@ -874,6 +871,24 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_response(d0_bli
        CHECK(d0_bignum_mod_add(temp2, temp1, ctx->r, temp0));
        CHECK(d0_iobuf_write_bignum(out, temp2));
 
+       // Diffie Hellmann recv
+       CHECK_ASSIGN(ctx->other_4_to_t, d0_iobuf_read_bignum(in, ctx->other_4_to_t));
+       CHECK(d0_bignum_cmp(ctx->other_4_to_t, zero) > 0);
+       CHECK(d0_bignum_cmp(ctx->other_4_to_t, ctx->schnorr_G) < 0);
+       // Diffie Hellmann send
+       CHECK(d0_dl_get_order(temp0, ctx->schnorr_G));
+       CHECK_ASSIGN(ctx->t, d0_bignum_rand_range(ctx->t, zero, temp0));
+       // modify DH key value! (add |G|-r)
+       CHECK(d0_bignum_add(temp1, ctx->t, temp0));
+       CHECK(d0_bignum_sub(temp2, temp1, ctx->r));
+       // can be undone by multiplying with 4^r in the end
+       // ensures the party of the DH key exchange is the same party as the one of
+       // the auth protocol (MITM who changes DH key exchange must break auth protocol)
+       // trick is that MITM has no knowledge about g^r at this point, as he only
+       // knows it in hashed form
+       CHECK(d0_bignum_mod_pow(temp0, four, temp2, ctx->schnorr_G));
+       CHECK(d0_iobuf_write_bignum(out, temp0));
+
        d0_iobuf_close(in, NULL);
        return d0_iobuf_close(out, outbuflen);
 
@@ -895,7 +910,7 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_verify(d0_blind
 
        // temps: 0 y 1 order
        USING(challenge); USING(schnorr_G);
-       REPLACING(other_4_to_r);
+       REPLACING(other_4_to_t);
 
        in = d0_iobuf_open_read(inbuf, inbuflen);
 
@@ -905,18 +920,24 @@ WARN_UNUSED_RESULT BOOL d0_blind_id_authenticate_with_private_id_verify(d0_blind
        CHECK(d0_bignum_cmp(temp0, temp1) < 0);
 
        // verify schnorr ID scheme
-       // we need 4^temp0 (g^s)^-challenge
+       // we need 4^r = 4^temp0 (g^s)^-challenge
        CHECK(d0_bignum_neg(temp1, ctx->challenge));
        CHECK(d0_bignum_mod_pow(temp2, ctx->schnorr_4_to_s, temp1, ctx->schnorr_G));
        CHECK(d0_bignum_mod_pow(temp1, four, temp0, ctx->schnorr_G));
-       CHECK_ASSIGN(ctx->other_4_to_r, d0_bignum_mod_mul(ctx->other_4_to_r, temp1, temp2, ctx->schnorr_G));
-       // hash must be equal to msghash
+       CHECK_ASSIGN(temp3, d0_bignum_mod_mul(temp3, temp1, temp2, ctx->schnorr_G));
+
+       // Diffie Hellmann recv
+       CHECK_ASSIGN(ctx->other_4_to_t, d0_iobuf_read_bignum(in, ctx->other_4_to_t));
+       CHECK(d0_bignum_cmp(ctx->other_4_to_t, zero) > 0);
+       CHECK(d0_bignum_cmp(ctx->other_4_to_t, ctx->schnorr_G) < 0);
+       // recover DH key value!
+       CHECK(d0_bignum_mod_mul(ctx->other_4_to_t, ctx->other_4_to_t, temp3, ctx->schnorr_G));
 
        // hash it, hash it, everybody hash it
        conv = d0_iobuf_open_write(convbuf, sizeof(convbuf));
-       CHECK(d0_iobuf_write_bignum(conv, ctx->other_4_to_r));
+       CHECK(d0_iobuf_write_bignum(conv, temp3));
        CHECK(d0_iobuf_write_packet(conv, ctx->msg, ctx->msglen));
-       CHECK(d0_iobuf_write_bignum(conv, ctx->other_4_to_r));
+       CHECK(d0_iobuf_write_bignum(conv, temp3));
        d0_iobuf_close(conv, &sz);
        conv = NULL;
        if(memcmp(sha(convbuf, sz), ctx->msghash, SCHNORR_HASHSIZE))
@@ -975,10 +996,10 @@ fail:
 
 BOOL d0_blind_id_sessionkey_public_id(const d0_blind_id_t *ctx, char *outbuf, size_t *outbuflen)
 {
-       USING(r); USING(other_4_to_r); USING(schnorr_G);
+       USING(t); USING(other_4_to_t); USING(schnorr_G);
 
        // temps: temp0 result
-       CHECK(d0_bignum_mod_pow(temp0, ctx->other_4_to_r, ctx->r, ctx->schnorr_G));
+       CHECK(d0_bignum_mod_pow(temp0, ctx->other_4_to_t, ctx->t, ctx->schnorr_G));
        return d0_longhash_destructive(temp0, outbuf, *outbuflen);
 
 fail:

diff --git a/d0_blind_id.txt b/d0_blind_id.txt
index 5b7db0765fb45d7c75eeeb93e656aaad7335d739..2fa73acf50a4987ec57d21755169ed03ce6da3b9 100644 (file)
--- a/d0_blind_id.txt
+++ b/d0_blind_id.txt
@@ -68,19 +68,21 @@ Authentication protocol:
        - Server verifies H = h'(I(S))
        - Server receives x, m
        - Server generates c in [0, 2^k0[ at random
-       - Server generates R in [0, |G|[ at random
-       - Server sends c and g^R
+       - Server generates T in [0, |G|[ at random
+       - Server sends c and g^T
        "response":
-       - Client receives c and g^R
+       - Client receives c and g^T
        - Client verifies that the received values are in the allowed ranges
        - Client sends y = r + s * c mod |G|
-       - Client calculates K = (g^R)^r
+       - Client generates t in [0, |G|[ at random
+       - Client g^(t-r)
+       - Client calculates K = (g^T)^t
        "verify":
-       - Server receives y
+       - Server receives y and g^(t-r)
        - Server calculates z = g^y S^-c
        - Server calculates x' = h("z || m || z")
        - Server verifies x == x'
-       - Server calculates K = z^R
+       - Server calculates K = (g^(t-r) * z)^T
 
 Protocol variant: g and G can be also part of the public ID. In this case, g
 and G are sent as part of this protocol additionally to S, H.

diff --git a/main.c b/main.c
index ef192054e972735ca28dd48fae45d24cc180e66b..2e8e24e1ef344508a0e3b5d56345dacb937d7ce8 100644 (file)
--- a/main.c
+++ b/main.c
@@ -52,6 +52,7 @@ static void errx(int status, const char *format, ...)
        va_list ap;
        va_start(ap, format);
        vfprintf(stderr, format, ap);
+       fputs("\n", stderr);
        exit(status);
 }