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Add DhHandshake::check_config method.

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GitOrigin-RevId: 1f9bbd0539454c2e6acbfe3be514614985a2d713
This commit is contained in:
levlam 2018-08-09 22:56:24 +03:00
parent 6c45d2c479
commit 466ebd2908
2 changed files with 58 additions and 57 deletions
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111
td/mtproto/crypto.cpp
View File

@ -134,17 +134,21 @@ void RSA::decrypt(Slice from, MutableSlice to) const {
std::memcpy(to.data(), result.c_str(), 256);
}
/*** DH ***/
Status DhHandshake::dh_check(Slice prime_str, const BigNum &prime, int32 g_int, const BigNum &g_a, const BigNum &g_b,
BigNumContext &ctx, DhCallback *callback) {
// 2. g generates a cyclic subgroup of prime order (p - 1) / 2, i.e. is a quadratic residue mod p.
// Since g is always equal to 2, 3, 4, 5, 6 or 7, this is easily done using quadratic reciprocity law,
// yielding a simple condition on
// * p mod 4g - namely, p mod 8 = 7 for g = 2; p mod 3 = 2 for g = 3;
// * no extra condition for g = 4;
// * p mod 5 = 1 or 4 for g = 5;
// * p mod 24 = 19 or 23 for g = 6;
// * p mod 7 = 3, 5 or 6 for g = 7.
Status DhHandshake::check_config(Slice prime_str, const BigNum &prime, int32 g_int, BigNumContext &ctx,
DhCallback *callback) {
// check that 2^2047 <= p < 2^2048
if (prime.get_num_bits() != 2048) {
return Status::Error("p is not 2048-bit number");
}
// g generates a cyclic subgroup of prime order (p - 1) / 2, i.e. is a quadratic residue mod p.
// Since g is always equal to 2, 3, 4, 5, 6 or 7, this is easily done using quadratic reciprocity law,
// yielding a simple condition on
// * p mod 4g - namely, p mod 8 = 7 for g = 2; p mod 3 = 2 for g = 3;
// * no extra condition for g = 4;
// * p mod 5 = 1 or 4 for g = 5;
// * p mod 24 = 19 or 23 for g = 6;
// * p mod 7 = 3, 5 or 6 for g = 7.
bool mod_ok;
uint32 mod_r;
@ -174,40 +178,7 @@ Status DhHandshake::dh_check(Slice prime_str, const BigNum &prime, int32 g_int,
return Status::Error("Bad prime mod 4g");
}
// IMPORTANT: Apart from the conditions on the Diffie-Hellman prime dh_prime and generator g, both sides are
// to check that g, g_a and g_b are greater than 1 and less than dh_prime - 1.
// We recommend checking that g_a and g_b are between 2^{2048-64} and dh_prime - 2^{2048-64} as well.
// check that 2^2047 <= p < 2^2048
if (prime.get_num_bits() != 2048) {
return Status::Error("p is not 2048-bit number");
}
BigNum left;
left.set_value(0);
left.set_bit(2048 - 64);
BigNum right;
BigNum::sub(right, prime, left);
if (BigNum::compare(left, g_a) > 0 || BigNum::compare(g_a, right) > 0 || BigNum::compare(left, g_b) > 0 ||
BigNum::compare(g_b, right) > 0) {
std::string x(2048, '0');
std::string y(2048, '0');
for (int i = 0; i < 2048; i++) {
if (g_a.is_bit_set(i)) {
x[i] = '1';
}
if (g_b.is_bit_set(i)) {
y[i] = '1';
}
}
LOG(ERROR) << x;
LOG(ERROR) << y;
return Status::Error("g^a or g^b is not between 2^{2048-64} and dh_prime - 2^{2048-64}");
}
// check whether p = dh_prime is a safe 2048-bit prime (meaning that both p and (p - 1) / 2 are prime)
// check whether p is a safe prime (meaning that both p and (p - 1) / 2 are prime)
int is_good_prime = -1;
if (callback) {
is_good_prime = callback->is_good_prime(prime_str);
@ -234,21 +205,41 @@ Status DhHandshake::dh_check(Slice prime_str, const BigNum &prime, int32 g_int,
if (callback) {
callback->add_good_prime(prime_str);
}
return Status::OK();
}
// TODO(perf):
// Checks:
// After g and p have been checked by the client, it makes sense to cache the result,
// so as not to repeat lengthy computations in future.
Status DhHandshake::dh_check(Slice prime_str, const BigNum &prime, int32 g_int, const BigNum &g_a, const BigNum &g_b,
BigNumContext &ctx, DhCallback *callback) {
TRY_STATUS(check_config(prime_str, prime, g_int, ctx, callback));
// If the verification takes too long time (which is the case for older mobile devices),
// one might initially run only 15 Miller-Rabin iterations for verifying primeness of p and (p - 1)/2
// with error probability not exceeding one billionth, and do more iterations later in the background.
// IMPORTANT: Apart from the conditions on the Diffie-Hellman prime dh_prime and generator g, both sides are
// to check that g, g_a and g_b are greater than 1 and less than dh_prime - 1.
// We recommend checking that g_a and g_b are between 2^{2048-64} and dh_prime - 2^{2048-64} as well.
// Another optimization is to embed into the client application code a small table with some known "good"
// couples (g,p) (or just known safe primes p, since the condition on g is easily verified during execution),
// checked during code generation phase, so as to avoid doing such verification during runtime altogether.
// Server changes these values rarely, thus one usually has to put the current value of server's dh_prime
// into such a table. For example, current value of dh_prime equals (in big-endian byte order) ...
CHECK(prime.get_num_bits() == 2048);
BigNum left;
left.set_value(0);
left.set_bit(2048 - 64);
BigNum right;
BigNum::sub(right, prime, left);
if (BigNum::compare(left, g_a) > 0 || BigNum::compare(g_a, right) > 0 || BigNum::compare(left, g_b) > 0 ||
BigNum::compare(g_b, right) > 0) {
std::string x(2048, '0');
std::string y(2048, '0');
for (int i = 0; i < 2048; i++) {
if (g_a.is_bit_set(i)) {
x[i] = '1';
}
if (g_b.is_bit_set(i)) {
y[i] = '1';
}
}
LOG(ERROR) << x;
LOG(ERROR) << y;
return Status::Error("g^a or g^b is not between 2^{2048-64} and dh_prime - 2^{2048-64}");
}
return Status::OK();
}
@ -276,6 +267,12 @@ void DhHandshake::set_config(int32 g_int, Slice prime_str) {
BigNum::mod_exp(g_b_, g_, b_, prime_, ctx_);
}
Status DhHandshake::check_config(int32 g_int, Slice prime_str, DhCallback *callback) {
BigNumContext ctx;
auto prime = BigNum::from_binary(prime_str);
return check_config(prime_str, prime, g_int, ctx, callback);
}
void DhHandshake::set_g_a_hash(Slice g_a_hash) {
has_g_a_hash_ = true;
ok_g_a_hash_ = false;

4
td/mtproto/crypto.h
View File

@ -60,6 +60,8 @@ class DhHandshake {
public:
void set_config(int32 g_int, Slice prime_str);
static Status check_config(int32 g_int, Slice prime_str, DhCallback *callback);
bool has_config() const {
return has_config_;
}
@ -133,6 +135,8 @@ class DhHandshake {
}
private:
static Status check_config(Slice prime_str, const BigNum &prime, int32 g_int, BigNumContext &ctx,
DhCallback *callback) TD_WARN_UNUSED_RESULT;
static Status dh_check(Slice prime_str, const BigNum &prime, int32 g_int, const BigNum &g_a, const BigNum &g_b,
BigNumContext &ctx, DhCallback *callback) TD_WARN_UNUSED_RESULT;