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: 6040f7990633630a0ad157cb17e016bb7db98a27
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>

diff --git a/src/crypto/rsa/pkcs1v15.go b/src/crypto/rsa/pkcs1v15.go
index 37790acb9860a236a0331dd6a5ab8c93a000a5fc..499242ffc5b574196f51eb273925097c7270ec5f 100644 (file)
--- a/src/crypto/rsa/pkcs1v15.go
+++ b/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)

diff --git a/src/crypto/rsa/pkcs1v15_test.go b/src/crypto/rsa/pkcs1v15_test.go
index 7e62560a04fe109386fa250558a1349b6f3a8bbc..26b8c5f26fefd249ab2ac03eab1383e2436653ee 100644 (file)
--- a/src/crypto/rsa/pkcs1v15_test.go
+++ b/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")
+       }
+}