crypto/rsa: reject invalid length PKCS#1v1.5 signatures

Per RFC 8017, reject signatures which are not the same length as the RSA
modulus. This matches the behavior of SignPKCS1v15 which properly left pads
the signatures it generates to the size of the modulus.

Fixes #21896

Change-Id: I2c42a0b24cf7fff158ece604b6f0c521a856d932
GitHub-Last-Rev: 6040f79906
GitHub-Pull-Request: golang/go#38140
Reviewed-on: https://go-review.googlesource.com/c/go/+/226203
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/rsa/pkcs1v15.go

@@ -277,6 +277,13 @@ func VerifyPKCS1v15(pub *PublicKey, hash crypto.Hash, hashed []byte, sig []byte)
 		return ErrVerification
 	}
 
+	// RFC 8017 Section 8.2.2: If the length of the signature S is not k
+	// octets (where k is the length in octets of the RSA modulus n), output
+	// "invalid signature" and stop.
+	if k != len(sig) {
+		return ErrVerification
+	}
+
 	c := new(big.Int).SetBytes(sig)
 	m := encrypt(new(big.Int), pub, c)
 	em := leftPad(m.Bytes(), k)

src/crypto/rsa/pkcs1v15_test.go

@@ -9,6 +9,7 @@ import (
 	"crypto"
 	"crypto/rand"
 	"crypto/sha1"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/hex"
 	"io"
@@ -296,3 +297,20 @@ var rsaPrivateKey = &PrivateKey{
 		fromBase10("94560208308847015747498523884063394671606671904944666360068158221458669711639"),
 	},
 }
+
+func TestShortPKCS1v15Signature(t *testing.T) {
+	pub := &PublicKey{
+		E: 65537,
+		N: fromBase10("8272693557323587081220342447407965471608219912416565371060697606400726784709760494166080686904546560026343451112103559482851304715739629410219358933351333"),
+	}
+	sig, err := hex.DecodeString("193a310d0dcf64094c6e3a00c8219b80ded70535473acff72c08e1222974bb24a93a535b1dc4c59fc0e65775df7ba2007dd20e9193f4c4025a18a7070aee93")
+	if err != nil {
+		t.Fatalf("failed to decode signature: %s", err)
+	}
+
+	h := sha256.Sum256([]byte("hello"))
+	err = VerifyPKCS1v15(pub, crypto.SHA256, h[:], sig)
+	if err == nil {
+		t.Fatal("VerifyPKCS1v15 accepted a truncated signature")
+	}
+}