crypto/internal/fips/hkdf: correctly set the service indicator for short salts

For #69536

Change-Id: Ibe2623311c8be5fb3e7411b33e61bf66d026e14d
Reviewed-on: https://go-review.googlesource.com/c/go/+/626877
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

src/crypto/internal/fips/hkdf/hkdf.go

@@ -17,6 +17,7 @@ func Extract[H fips.Hash](h func() H, secret, salt []byte) []byte {
 		salt = make([]byte, h().Size())
 	}
 	extractor := hmac.New(h, salt)
+	hmac.MarkAsUsedInHKDF(extractor)
 	extractor.Write(secret)
 	return extractor.Sum(nil)
 }
@@ -24,6 +25,7 @@ func Extract[H fips.Hash](h func() H, secret, salt []byte) []byte {
 func Expand[H fips.Hash](h func() H, pseudorandomKey, info []byte, keyLen int) []byte {
 	out := make([]byte, 0, keyLen)
 	expander := hmac.New(h, pseudorandomKey)
+	hmac.MarkAsUsedInHKDF(expander)
 	var counter uint8
 	var buf []byte
 

src/crypto/internal/fips/hkdf/hkdf_test.go

@@ -5,6 +5,8 @@ package hkdf_test
 
 import (
 	"bytes"
+	"crypto/internal/boring"
+	"crypto/internal/fips"
 	"crypto/internal/fips/hkdf"
 	"crypto/md5"
 	"crypto/sha1"
@@ -331,6 +333,32 @@ func TestHKDFLimit(t *testing.T) {
 	hkdf.Key(hash, master, nil, info, limit+1)
 }
 
+func TestFIPSServiceIndicator(t *testing.T) {
+	if boring.Enabled {
+		t.Skip("in BoringCrypto mode HMAC is not from the Go FIPS module")
+	}
+
+	fips.ResetServiceIndicator()
+	hkdf.Key(sha256.New, []byte("YELLOW SUBMARINE"), nil, nil, 32)
+	if !fips.ServiceIndicator() {
+		t.Error("FIPS service indicator should be set")
+	}
+
+	// Key too short.
+	fips.ResetServiceIndicator()
+	hkdf.Key(sha256.New, []byte("key"), nil, nil, 32)
+	if fips.ServiceIndicator() {
+		t.Error("FIPS service indicator should not be set")
+	}
+
+	// Salt and info are short, which is ok, but translates to a short HMAC key.
+	fips.ResetServiceIndicator()
+	hkdf.Key(sha256.New, []byte("YELLOW SUBMARINE"), []byte("salt"), []byte("info"), 32)
+	if !fips.ServiceIndicator() {
+		t.Error("FIPS service indicator should be set")
+	}
+}
+
 func Benchmark16ByteMD5Single(b *testing.B) {
 	benchmarkHKDFSingle(md5.New, 16, b)
 }

src/crypto/internal/fips/hmac/hmac.go

@@ -35,9 +35,25 @@ type HMAC struct {
 	// copy of the key, but rather the marshaled state of outer/inner after
 	// opad/ipad has been fed into it.
 	marshaled bool
+
+	// forHKDF and keyLen are stored to inform the service indicator decision.
+	forHKDF bool
+	keyLen  int
 }
 
 func (h *HMAC) Sum(in []byte) []byte {
+	// Per FIPS 140-3 IG C.M, key lengths below 112 bits are only allowed for
+	// legacy use (i.e. verification only) and we don't support that. However,
+	// HKDF uses the HMAC key for the salt, which is allowed to be shorter.
+	if h.keyLen < 112/8 && !h.forHKDF {
+		fips.RecordNonApproved()
+	}
+	switch h.inner.(type) {
+	case *sha256.Digest, *sha512.Digest, *sha3.Digest:
+	default:
+		fips.RecordNonApproved()
+	}
+
 	origLen := len(in)
 	in = h.inner.Sum(in)
 
@@ -113,7 +129,7 @@ func (h *HMAC) Reset() {
 
 // New returns a new HMAC hash using the given [fips.Hash] type and key.
 func New[H fips.Hash](h func() H, key []byte) *HMAC {
-	hm := new(HMAC)
+	hm := &HMAC{keyLen: len(key)}
 	hm.outer = h()
 	hm.inner = h()
 	unique := true
@@ -129,7 +145,6 @@ func New[H fips.Hash](h func() H, key []byte) *HMAC {
 	if !unique {
 		panic("crypto/hmac: hash generation function does not produce unique values")
 	}
-	setServiceIndicator(hm.outer, key)
 	blocksize := hm.inner.BlockSize()
 	hm.ipad = make([]byte, blocksize)
 	hm.opad = make([]byte, blocksize)
@@ -151,17 +166,7 @@ func New[H fips.Hash](h func() H, key []byte) *HMAC {
 	return hm
 }
 
-func setServiceIndicator(h fips.Hash, key []byte) {
-	// Per FIPS 140-3 IG C.M, key lengths below 112 bits are only allowed for
-	// legacy use (i.e. verification only) and we don't support that.
-	if len(key) < 112/8 {
-		fips.RecordNonApproved()
-	}
-
-	switch h.(type) {
-	case *sha256.Digest, *sha512.Digest, *sha3.Digest:
-		fips.RecordApproved()
-	default:
-		fips.RecordNonApproved()
-	}
+// MarkAsUsedInHKDF records that this HMAC instance is used as part of HKDF.
+func MarkAsUsedInHKDF(h *HMAC) {
+	h.forHKDF = true
 }

src/crypto/internal/fips/indicator.go

@@ -44,6 +44,11 @@ func ServiceIndicator() bool {
 
 // RecordApproved is an internal function that records the use of an approved
 // service. It does not override RecordNonApproved calls in the same span.
+//
+// It should be called by exposed functions that perform a whole cryptographic
+// alrgorithm (e.g. by Sum, not by New, unless a cryptographic Instantiate
+// algorithm is performed) and should be called after any checks that may cause
+// the function to error out or panic.
 func RecordApproved() {
 	if getIndicator() == indicatorUnset {
 		setIndicator(indicatorTrue)