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fix: relax DH-GEX group checking #382

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Merged
kruton merged 1 commit into from
Oct 27, 2026
Merged

fix: relax DH-GEX group checking #382

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21 changes: 4 additions & 17 deletions sshlib/src/main/kotlin/org/connectbot/sshlib/crypto/DiffieHellmanGroupExchange.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -33,20 +33,6 @@ internal class DiffieHellmanGroupExchange(override val hashAlgorithm: String, va
companion object {
private val secureRandom = SecureRandom()
private const val ERROR_GROUP_NOT_SET = "DH group not set; call setGroup() first"
private const val PRIMALITY_CERTAINTY = 80
private const val GROUP_CACHE_LIMIT = 16
private val groupValidationCache = object : LinkedHashMap<BigInteger, Boolean>(GROUP_CACHE_LIMIT, 0.75f, true) {
override fun removeEldestEntry(eldest: MutableMap.MutableEntry<BigInteger, Boolean>?): Boolean = size > GROUP_CACHE_LIMIT
}

@Synchronized
private fun isSafeGroup(p: BigInteger): Boolean = groupValidationCache.getOrPut(p) {
p.isProbablePrime(PRIMALITY_CERTAINTY) &&
p.subtract(BigInteger.ONE).shiftRight(1).isProbablePrime(PRIMALITY_CERTAINTY)
}

@Synchronized
internal fun cachedGroupValidationCount(): Int = groupValidationCache.size
}

val n = maxOf(4096, min)
Expand All @@ -72,9 +58,10 @@ internal class DiffieHellmanGroupExchange(override val hashAlgorithm: String, va
if (g <= BigInteger.ONE || g >= p - BigInteger.ONE) {
throw SshException("DH group generator g is out of range: must be 1 < g < p-1")
}
if (!isSafeGroup(p)) {
throw SshException("DH group modulus is not a safe prime")
}
// Just use client-side structural checks; the server supplies a safe prime.
// Re-testing p and (p-1)/2 took 55 seconds for a 4095-bit group on Android:
// https://github.com/connectbot/connectbot/issues/2550
// The host-key signature authenticates these parameters as part of the exchange hash.
this.p = p
this.g = g
}
Expand Down
32 changes: 18 additions & 14 deletions sshlib/src/test/kotlin/org/connectbot/sshlib/crypto/DiffieHellmanGroupExchangeTest.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -74,8 +74,8 @@ class DiffieHellmanGroupExchangeTest {
@Test
fun `setGroup rejects g greater than or equal to p minus 1`() {
val gex = DiffieHellmanGroupExchange("SHA-256")
assertFailsWith<SshException> {
gex.setGroup(DhGroups.GROUP14_P, DhGroups.GROUP14_P - BigInteger.ONE)
for (g in listOf(DhGroups.GROUP14_P - BigInteger.ONE, DhGroups.GROUP14_P, DhGroups.GROUP14_P + BigInteger.ONE)) {
assertFailsWith<SshException> { gex.setGroup(DhGroups.GROUP14_P, g) }
}
}

Expand All @@ -89,13 +89,13 @@ class DiffieHellmanGroupExchangeTest {
}

@Test
fun `setGroup rejects odd composite modulus`() {
fun `setGroup leaves primality validation to the server`() {
val composite = BigInteger.ONE.shiftLeft(2047).add(BigInteger.ONE).multiply(BigInteger.valueOf(3))
val gex = DiffieHellmanGroupExchange("SHA-256")

assertFailsWith<SshException> {
gex.setGroup(composite, DhGroups.GENERATOR)
}
// OpenSSH-style checks accept in-range odd parameters without testing primality.
gex.setGroup(composite, DhGroups.GENERATOR)
assertTrue(gex.generateClientKeys().isNotEmpty())
}

@Test
Expand Down Expand Up @@ -168,16 +168,20 @@ class DiffieHellmanGroupExchangeTest {
}

@Test
fun `validated group cache remains bounded`() {
fun `setGroup rejects negative generator`() {
val gex = DiffieHellmanGroupExchange("SHA-256")
repeat(20) { index ->
val composite = BigInteger.ONE.shiftLeft(2047)
.add(BigInteger.valueOf((index * 2L) + 1L))
.multiply(BigInteger.valueOf(3))
assertFailsWith<SshException> { gex.setGroup(composite, DhGroups.GENERATOR) }
}
assertFailsWith<SshException> { gex.setGroup(DhGroups.GROUP14_P, BigInteger.ONE.negate()) }
}

assertTrue(DiffieHellmanGroupExchange.cachedGroupValidationCount() <= 16)
@Test
fun `shared secret rejects out of range server public values`() {
val p = DhGroups.GROUP14_P
for (f in listOf(BigInteger.ZERO, BigInteger.ONE, p - BigInteger.ONE, p, p + BigInteger.ONE)) {
val gex = DiffieHellmanGroupExchange("SHA-256")
gex.setGroup(p, DhGroups.GENERATOR)
gex.generateClientKeys()
assertFailsWith<SshException> { gex.computeSharedSecret(f.toByteArray()) }
}
}

@Test
Expand Down
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