From 5db079d2e5f97952be288c28a3a0690a523efdce Mon Sep 17 00:00:00 2001
From: Roland Shoemaker <rolandshoemaker@gmail.com>
Date: Sun, 29 Mar 2020 03:04:51 +0000
Subject: [PATCH] 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>
---
 src/crypto/rsa/pkcs1v15.go      |  7 +++++++
 src/crypto/rsa/pkcs1v15_test.go | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/src/crypto/rsa/pkcs1v15.go b/src/crypto/rsa/pkcs1v15.go
index 37790acb98..499242ffc5 100644
--- 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 7e62560a04..26b8c5f26f 100644
--- 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")
+	}
+}
-- 
2.50.0