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Add user-to-user (U2U) TGS exchange
Adds KerberosClient.GetTGSU2U: a user-to-user service-ticket request (RFC 4120 ENC-TKT-IN-SKEY) that presents the target user's TGT in additional-tickets and sets the ENC-TKT-IN-SKEY KDC option, so the KDC issues a ticket encrypted under the target's TGT session key rather than a long-term key. Builds on the additional-tickets encoding fixed for S4U2Proxy. The request construction is factored into buildU2UTGSReq and unit-tested without a KDC: the test confirms ENC-TKT-IN-SKEY (bit 28) is set and the target TGT is carried verbatim as an APPLICATION[1] ticket in the [11] additional-tickets field. stdlib-only; kerberos tests, go vet, and go build ./... are green.
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‎network/kerberos/v5/client.go‎

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@@ -498,6 +498,7 @@ const (
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kdcOptionCanonicalize = 15 // byte 1, 0x01 (RFC 6806)
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kdcOptionCNameInAddlTkt = 14 // byte 1, 0x02 (MS-SFU S4U2Proxy)
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kdcOptionRenewableOK = 27 // byte 3, 0x10
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kdcOptionEncTktInSKey = 28 // byte 3, 0x08 (user-to-user)
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)
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// encodeKDCOptions packs a list of bit positions into a 32-bit BitString

‎network/kerberos/v5/u2u.go‎

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package kerberos
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import (
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"fmt"
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"time"
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kerbcrypto "github.com/TheManticoreProject/Manticore/network/kerberos/v5/crypto"
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"github.com/TheManticoreProject/Manticore/network/kerberos/v5/messages"
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)
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// buildU2UTGSReq constructs the TGS-REQ for a user-to-user exchange: a normal
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// PA-TGS-REQ (AP-REQ over the client's TGT), the ENC-TKT-IN-SKEY KDC option, the
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// target user's TGT in additional-tickets, and the target user as sname. It is
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// separated from GetTGSU2U so the request shape can be tested without a KDC.
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func (c *KerberosClient) buildU2UTGSReq(targetUser, targetRealm string, targetTGTRaw []byte, nonce int) (*messages.TGSReq, error) {
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apReqBytes, err := c.buildAPReq()
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if err != nil {
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return nil, fmt.Errorf("kerberos: build AP-REQ: %w", err)
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}
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sname := messages.PrincipalName{
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NameType: messages.NameTypePrincipal,
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NameString: []string{targetUser},
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}
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realm := c.realm
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if targetRealm != "" {
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realm = targetRealm
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}
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return &messages.TGSReq{
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PVNO: messages.KerberosV5,
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MsgType: messages.MsgTypeTGSReq,
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PAData: []messages.PAData{
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{PADataType: messages.PATGSReq, PADataValue: apReqBytes},
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},
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ReqBody: messages.KDCReqBody{
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KDCOptions: encodeKDCOptions(
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kdcOptionForwardable,
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kdcOptionRenewable,
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kdcOptionCanonicalize,
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kdcOptionEncTktInSKey,
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),
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Realm: realm,
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SName: sname,
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Till: time.Now().UTC().Add(24 * time.Hour),
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Nonce: nonce,
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EType: []int{
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messages.ETypeAES256CTSHMACSHA196,
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messages.ETypeAES128CTSHMACSHA196,
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messages.ETypeRC4HMAC,
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},
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AdditTicketsRaw: [][]byte{targetTGTRaw},
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},
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}, nil
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}
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// GetTGSU2U performs a user-to-user (U2U) TGS exchange: it requests a service
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// ticket to the target user (targetUser in targetRealm, or the client's realm
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// if empty), presenting the target user's TGT (targetTGTRaw, raw APPLICATION[1]
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// bytes) in additional-tickets with the ENC-TKT-IN-SKEY option. The KDC issues a
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// ticket whose enc-part is encrypted with the session key of the target's TGT
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// (not a long-term key), which the target verifies via an AP-REQ carrying the
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// USE-SESSION-KEY option.
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//
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// GetTGT must have succeeded first (for the client's own TGT). Returns the
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// service ticket, its raw bytes, and the ticket session key.
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func (c *KerberosClient) GetTGSU2U(targetUser, targetRealm string, targetTGTRaw []byte) (messages.Ticket, []byte, []byte, error) {
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if !c.hasTGT {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: no TGT: call GetTGT first")
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}
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if targetUser == "" {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: U2U requires a target user")
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}
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if len(targetTGTRaw) == 0 {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: U2U requires the target user's TGT")
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}
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nonce := randomNonce()
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tgsReq, err := c.buildU2UTGSReq(targetUser, targetRealm, targetTGTRaw, nonce)
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if err != nil {
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return messages.Ticket{}, nil, nil, err
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}
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tgsReqBytes, err := tgsReq.Marshal()
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if err != nil {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: marshal U2U TGS-REQ: %w", err)
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}
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resp, err := kdcSend(c.kdcHost, defaultKDCPort, tgsReqBytes)
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if err != nil {
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return messages.Ticket{}, nil, nil, err
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}
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var krbErr messages.KRBError
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if _, parseErr := krbErr.Unmarshal(resp); parseErr == nil {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: U2U error %d: %s", krbErr.ErrorCode, krbErr.EText)
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}
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var tgsRep messages.TGSRep
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if _, err := tgsRep.Unmarshal(resp); err != nil {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: parse U2U TGS-REP: %w", err)
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}
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encPlain, err := kerbcrypto.Decrypt(c.sessionEType, c.sessionKey, kerbcrypto.KeyUsageTGSRepEncSessionKey, tgsRep.EncPart.Cipher)
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if err != nil {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: decrypt U2U TGS-REP enc-part: %w", err)
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}
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var encTGSRep messages.EncTGSRepPart
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if _, err := encTGSRep.Unmarshal(encPlain); err != nil {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: parse U2U EncTGSRepPart: %w", err)
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}
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if encTGSRep.Nonce != nonce {
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return messages.Ticket{}, nil, nil, fmt.Errorf("kerberos: U2U nonce mismatch: got %d, want %d", encTGSRep.Nonce, nonce)
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}
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return tgsRep.Ticket, tgsRep.TicketRaw, encTGSRep.Key.KeyValue, nil
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}

‎network/kerberos/v5/u2u_test.go‎

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package kerberos
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import (
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"bytes"
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"encoding/asn1"
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"testing"
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"github.com/TheManticoreProject/Manticore/network/kerberos/v5/messages"
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)
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// seqChild parses a SEQUENCE TLV and returns the first element with the given
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// class/tag.
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func seqChild(t *testing.T, seqTLV []byte, class, tag int) asn1.RawValue {
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t.Helper()
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var seq asn1.RawValue
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if _, err := asn1.Unmarshal(seqTLV, &seq); err != nil {
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t.Fatalf("parse SEQUENCE: %v", err)
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}
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rest := seq.Bytes
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for len(rest) > 0 {
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var e asn1.RawValue
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r, err := asn1.Unmarshal(rest, &e)
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if err != nil {
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t.Fatalf("iterate: %v", err)
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}
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if e.Class == class && e.Tag == tag {
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return e
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}
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rest = r
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}
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t.Fatalf("no element class=%d tag=%d", class, tag)
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return asn1.RawValue{}
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}
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func TestU2URequestShape(t *testing.T) {
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c := fakeTGTClient(t) // from export_test.go: a client with a populated TGT
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// The target user's TGT (any APPLICATION[1] ticket for this structural test).
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targetTkt := messages.Ticket{
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TktVno: messages.KerberosV5,
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Realm: "CORP.LOCAL",
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SName: messages.PrincipalName{NameType: messages.NameTypeSRVInst, NameString: []string{"krbtgt", "CORP.LOCAL"}},
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EncPart: messages.EncryptedData{EType: messages.ETypeAES256CTSHMACSHA196, Cipher: bytes.Repeat([]byte{0x7a}, 16)},
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}
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targetTGTRaw, err := targetTkt.Marshal()
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if err != nil {
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t.Fatal(err)
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}
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req, err := c.buildU2UTGSReq("victim", "", targetTGTRaw, 4242)
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if err != nil {
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t.Fatalf("buildU2UTGSReq: %v", err)
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}
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wire, err := req.Marshal()
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if err != nil {
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t.Fatalf("marshal: %v", err)
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}
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// APPLICATION[12] -> inner SEQUENCE -> [4] body.
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var app asn1.RawValue
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if _, err := asn1.Unmarshal(wire, &app); err != nil {
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t.Fatal(err)
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}
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if app.Class != asn1.ClassApplication || app.Tag != messages.MsgTypeTGSReq {
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t.Fatalf("outer tag: class=%d tag=%d", app.Class, app.Tag)
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}
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bodyElem := seqChild(t, app.Bytes, asn1.ClassContextSpecific, 4)
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// KDCOptions [0]: BIT STRING; ENC-TKT-IN-SKEY is bit 28 -> byte 3, 0x08.
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optElem := seqChild(t, bodyElem.Bytes, asn1.ClassContextSpecific, 0)
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var flags asn1.BitString
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if _, err := asn1.Unmarshal(optElem.Bytes, &flags); err != nil {
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t.Fatalf("parse KDCOptions: %v", err)
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}
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if flags.At(kdcOptionEncTktInSKey) != 1 {
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t.Errorf("ENC-TKT-IN-SKEY (bit 28) not set; bytes=% X", flags.Bytes)
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}
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if flags.At(kdcOptionForwardable) != 1 {
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t.Errorf("forwardable (bit 1) not set")
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}
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// additional-tickets [11] -> SEQUENCE OF -> first is APPLICATION[1], and it
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// must be the target TGT verbatim.
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addl := seqChild(t, bodyElem.Bytes, asn1.ClassContextSpecific, 11)
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tkt := seqChild(t, addl.Bytes, asn1.ClassApplication, 1)
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if !bytes.Equal(tkt.FullBytes, targetTGTRaw) {
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t.Errorf("additional ticket is not the target TGT verbatim")
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}
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}
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func TestGetTGSU2UValidatesInputs(t *testing.T) {
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c := NewClient("svc", "corp.local", "10.0.0.1").WithPassword("x")
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if _, _, _, err := c.GetTGSU2U("victim", "", []byte{1}); err == nil {
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t.Error("expected error without a TGT")
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}
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c2 := fakeTGTClient(t)
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if _, _, _, err := c2.GetTGSU2U("", "", []byte{1}); err == nil {
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t.Error("expected error with empty target user")
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}
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if _, _, _, err := c2.GetTGSU2U("victim", "", nil); err == nil {
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t.Error("expected error with no target TGT")
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}
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}

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