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48 lines
1.6 KiB
48 lines
1.6 KiB
3 years ago
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package crypto
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import (
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"math/big"
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"github.com/go-faster/errors"
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)
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// CheckDHParams checks that g_a, g_b and g params meet key exchange conditions.
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//
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// https://core.telegram.org/mtproto/auth_key#dh-key-exchange-complete
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func CheckDHParams(dhPrime, g, gA, gB *big.Int) error {
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one := big.NewInt(1)
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dhPrimeMinusOne := big.NewInt(0).Sub(dhPrime, one)
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if !InRange(g, one, dhPrimeMinusOne) {
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return errors.New("kex: bad g, g must be 1 < g < dh_prime - 1")
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}
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if !InRange(gA, one, dhPrimeMinusOne) {
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return errors.New("kex: bad g_a, g_a must be 1 < g_a < dh_prime - 1")
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}
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if !InRange(gB, one, dhPrimeMinusOne) {
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return errors.New("kex: bad g_b, g_b must be 1 < g_b < dh_prime - 1")
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}
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// IMPORTANT: Apart from the conditions on the Diffie-Hellman prime
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// dh_prime and generator g, both sides are to check that g, g_a and
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// g_b are greater than 1 and less than dh_prime - 1. We recommend
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// checking that g_a and g_b are between 2^{2048-64} and
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// dh_prime - 2^{2048-64} as well.
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// 2^{2048-64}
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safetyRangeMin := big.NewInt(0).Exp(big.NewInt(2), big.NewInt(RSAKeyBits-64), nil)
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safetyRangeMax := big.NewInt(0).Sub(dhPrime, safetyRangeMin)
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if !InRange(gA, safetyRangeMin, safetyRangeMax) {
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return errors.New("kex: bad g_a, g_a must be 2^{2048-64} < g_a < dh_prime - 2^{2048-64}")
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}
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if !InRange(gB, safetyRangeMin, safetyRangeMax) {
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return errors.New("kex: bad g_b, g_b must be 2^{2048-64} < g_b < dh_prime - 2^{2048-64}")
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}
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return nil
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}
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// InRange checks whether x is in (min, max) range, i.e. min < x < max.
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func InRange(x, min, max *big.Int) bool {
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return x.Cmp(min) > 0 && x.Cmp(max) < 0
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}
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