fix 3DES weak keys

jcmturner · Oct 31, 2018 · 104a640 · 104a640
1 parent 62e2cb2
commit 104a640
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Showing 2 changed files with 80 additions and 3 deletions.
diff --git a/crypto/rfc3961/keyDerivation.go b/crypto/rfc3961/keyDerivation.go
@@ -1,6 +1,8 @@
 package rfc3961
 
 import (
+	"bytes"
+
 	"gopkg.in/jcmturner/gokrb5.v6/crypto/etype"
 )
 
@@ -63,10 +65,10 @@ func RandomToKey(b []byte) []byte {
 
 // DES3RandomToKey returns a key from the bytes provided according to the definition in RFC 3961 for DES3 etypes.
 func DES3RandomToKey(b []byte) []byte {
-	r := stretch56Bits(b[:7])
-	r2 := stretch56Bits(b[7:14])
+	r := fixWeakKey(stretch56Bits(b[:7]))
+	r2 := fixWeakKey(stretch56Bits(b[7:14]))
 	r = append(r, r2...)
-	r3 := stretch56Bits(b[14:21])
+	r3 := fixWeakKey(stretch56Bits(b[14:21]))
 	r = append(r, r3...)
 	return r
 }
@@ -132,3 +134,45 @@ func calcEvenParity(b byte) (uint8, uint8) {
 	}
 	return lowestbit, b
 }
+
+func fixWeakKey(b []byte) []byte {
+	if weak(b) {
+		b[7] ^= 0xF0
+	}
+	return b
+}
+
+func weak(b []byte) bool {
+	// weak keys from https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-67r1.pdf
+	weakKeys := [4][]byte{
+		{0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01},
+		{0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE},
+		{0xE0, 0xE0, 0xE0, 0xE0, 0xF1, 0xF1, 0xF1, 0xF1},
+		{0x1F, 0x1F, 0x1F, 0x1F, 0x0E, 0x0E, 0x0E, 0x0E},
+	}
+	semiWeakKeys := [12][]byte{
+		{0x01, 0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E},
+		{0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E, 0x01},
+		{0x01, 0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1},
+		{0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1, 0x01},
+		{0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE},
+		{0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01},
+		{0x1F, 0xE0, 0x1F, 0xE0, 0x0E, 0xF1, 0x0E, 0xF1},
+		{0xE0, 0x1F, 0xE0, 0x1F, 0xF1, 0x0E, 0xF1, 0x0E},
+		{0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E, 0xFE},
+		{0xFE, 0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E},
+		{0xE0, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1, 0xFE},
+		{0xFE, 0xE0, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1},
+	}
+	for _, k := range weakKeys {
+		if bytes.Equal(b, k) {
+			return true
+		}
+	}
+	for _, k := range semiWeakKeys {
+		if bytes.Equal(b, k) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/crypto/rfc3961/keyDerivation_test.go b/crypto/rfc3961/keyDerivation_test.go
@@ -0,0 +1,33 @@
+package rfc3961
+
+import "testing"
+
+func TestFixWeakKey(t *testing.T) {
+	var weakKeys = []struct {
+		key      []byte
+		lastbyte byte
+	}{
+		{[]byte{0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01, 0x01}, byte(0xF1)},
+		{[]byte{0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE, 0xFE}, byte(0x0E)},
+		{[]byte{0xE0, 0xE0, 0xE0, 0xE0, 0xF1, 0xF1, 0xF1, 0xF1}, byte(0x01)},
+		{[]byte{0x1F, 0x1F, 0x1F, 0x1F, 0x0E, 0x0E, 0x0E, 0x0E}, byte(0xFE)},
+		{[]byte{0x01, 0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E}, byte(0xFE)},
+		{[]byte{0x1F, 0x01, 0x1F, 0x01, 0x0E, 0x01, 0x0E, 0x01}, byte(0xF1)},
+		{[]byte{0x01, 0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1}, byte(0x01)},
+		{[]byte{0xE0, 0x01, 0xE0, 0x01, 0xF1, 0x01, 0xF1, 0x01}, byte(0xF1)},
+		{[]byte{0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE}, byte(0x0E)},
+		{[]byte{0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01, 0xFE, 0x01}, byte(0xF1)},
+		{[]byte{0x1F, 0xE0, 0x1F, 0xE0, 0x0E, 0xF1, 0x0E, 0xF1}, byte(0x01)},
+		{[]byte{0xE0, 0x1F, 0xE0, 0x1F, 0xF1, 0x0E, 0xF1, 0x0E}, byte(0xFE)},
+		{[]byte{0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E, 0xFE}, byte(0x0E)},
+		{[]byte{0xFE, 0x1F, 0xFE, 0x1F, 0xFE, 0x0E, 0xFE, 0x0E}, byte(0xFE)},
+		{[]byte{0xE0, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1, 0xFE}, byte(0x0E)},
+		{[]byte{0xFE, 0x2F, 0xFE, 0xE0, 0xFE, 0xF1, 0xFE, 0xF1}, byte(0xF1)}, // Non weak key
+	}
+	for i, k := range weakKeys {
+		b := fixWeakKey(k.key)
+		if b[7] != weakKeys[i].lastbyte {
+			t.Errorf("key not fixed correctly %X - %X", b, weakKeys[i].lastbyte)
+		}
+	}
+}