rfc2865: add CHAP password verification helpers

rfc2865/chap.go

@@ -0,0 +1,57 @@
+package rfc2865
+
+import (
+	"crypto/md5"
+	"crypto/subtle"
+
+	"layeh.com/radius"
+)
+
+// CHAPMatch reports whether the given CHAP-Password value corresponds to
+// password and challenge.
+//
+// chapPassword must be the raw 17-octet CHAP-Password attribute value: a
+// 1-octet CHAP identifier followed by the 16-octet MD5 response, as described
+// in RFC 1994 and RFC 2865, Section 2.2. challenge is the CHAP challenge,
+// i.e. the CHAP-Challenge attribute value, or the request Authenticator when
+// the request does not carry a CHAP-Challenge attribute.
+//
+// The comparison of the expected and received responses is done in constant
+// time.
+func CHAPMatch(password, chapPassword, challenge []byte) bool {
+	if len(chapPassword) != 17 {
+		return false
+	}
+
+	h := md5.New()
+	h.Write(chapPassword[:1]) // CHAP identifier
+	h.Write(password)
+	h.Write(challenge)
+
+	var sum [md5.Size]byte
+	return subtle.ConstantTimeCompare(h.Sum(sum[:0]), chapPassword[1:]) == 1
+}
+
+// CHAPVerify reports whether password is the correct password for the CHAP
+// authentication carried in p.
+//
+// It combines the CHAP-Password attribute with the CHAP-Challenge attribute. If
+// p does not contain a CHAP-Challenge attribute, the packet's Authenticator is
+// used as the challenge, as described in RFC 2865, Section 2.2.
+//
+// CHAPVerify returns false if p does not contain a validly sized CHAP-Password
+// attribute. It only handles standard CHAP (MD5); MS-CHAP and MS-CHAPv2 use
+// different attributes and algorithms.
+func CHAPVerify(p *radius.Packet, password []byte) bool {
+	chapPassword := CHAPPassword_Get(p)
+	if len(chapPassword) == 0 {
+		return false
+	}
+
+	challenge := CHAPChallenge_Get(p)
+	if len(challenge) == 0 {
+		challenge = p.Authenticator[:]
+	}
+
+	return CHAPMatch(password, chapPassword, challenge)
+}

rfc2865/chap_test.go

@@ -0,0 +1,85 @@
+package rfc2865
+
+import (
+	"crypto/md5"
+	"testing"
+
+	"layeh.com/radius"
+)
+
+func makeCHAPPassword(ident byte, password, challenge []byte) []byte {
+	h := md5.New()
+	h.Write([]byte{ident})
+	h.Write(password)
+	h.Write(challenge)
+
+	out := make([]byte, 17)
+	out[0] = ident
+	copy(out[1:], h.Sum(nil))
+	return out
+}
+
+func TestCHAPMatch(t *testing.T) {
+	password := []byte("hello")
+	challenge := []byte("0123456789abcdef")
+	chapPassword := makeCHAPPassword(0x42, password, challenge)
+
+	if !CHAPMatch(password, chapPassword, challenge) {
+		t.Fatal("CHAPMatch returned false for a valid password")
+	}
+	if CHAPMatch([]byte("wrong"), chapPassword, challenge) {
+		t.Fatal("CHAPMatch returned true for an invalid password")
+	}
+	if CHAPMatch(password, chapPassword, []byte("different-chal")) {
+		t.Fatal("CHAPMatch returned true for a mismatched challenge")
+	}
+	if CHAPMatch(password, chapPassword[:16], challenge) {
+		t.Fatal("CHAPMatch returned true for a malformed CHAP-Password")
+	}
+}
+
+func TestCHAPVerify_withCHAPChallenge(t *testing.T) {
+	secret := []byte("secret")
+	password := []byte("testing123")
+	challenge := []byte("a-random-challenge")
+
+	p := radius.New(radius.CodeAccessRequest, secret)
+	if err := CHAPChallenge_Add(p, challenge); err != nil {
+		t.Fatal(err)
+	}
+	if err := CHAPPassword_Add(p, makeCHAPPassword(0x01, password, challenge)); err != nil {
+		t.Fatal(err)
+	}
+
+	if !CHAPVerify(p, password) {
+		t.Fatal("CHAPVerify returned false for a valid password")
+	}
+	if CHAPVerify(p, []byte("nope")) {
+		t.Fatal("CHAPVerify returned true for an invalid password")
+	}
+}
+
+func TestCHAPVerify_authenticatorAsChallenge(t *testing.T) {
+	secret := []byte("secret")
+	password := []byte("testing123")
+
+	p := radius.New(radius.CodeAccessRequest, secret)
+	// No CHAP-Challenge attribute: the Authenticator is the challenge.
+	if err := CHAPPassword_Add(p, makeCHAPPassword(0x07, password, p.Authenticator[:])); err != nil {
+		t.Fatal(err)
+	}
+
+	if !CHAPVerify(p, password) {
+		t.Fatal("CHAPVerify returned false for a valid password")
+	}
+	if CHAPVerify(p, []byte("nope")) {
+		t.Fatal("CHAPVerify returned true for an invalid password")
+	}
+}
+
+func TestCHAPVerify_noCHAPPassword(t *testing.T) {
+	p := radius.New(radius.CodeAccessRequest, []byte("secret"))
+	if CHAPVerify(p, []byte("anything")) {
+		t.Fatal("CHAPVerify returned true for a packet without a CHAP-Password")
+	}
+}