lints: warn for RSA-PSS sigalg in cabf lint, not err. (#342)

The `e_signature_algorithm_not_supported` lint enforces Section 6.1.5 of
the baseline requirements by checking certificate signature algorithms
against a fixed set. Previously this set did not include the RSA-PSS
signature algorithms and would mistakenly flag certificates signed with
a RSA-PSS algorithm with an error result.

The BRs do not forbid using RSA-PSS signature algorithms (provided the
associated digest algorithm is one of the three approved in 6.1.5). The
Mozilla root program requirements do forbid RSA-PSS in v2.7+ but that
should be checked in a separate Mozilla scoped lint.

This commit adjusts the `e_signature_algorithm_not_supported` lint to
return `lint.Warn` for RSA-PSS with SHA256, SHA384 or SHA512.

See #326 for more background.

lints/cabf_br/lint_signature_algorithm_not_supported.go

@@ -20,6 +20,38 @@ import (
 	"github.com/zmap/zlint/util"
 )
 
+var (
+	// Any of the following x509.SignatureAlgorithms are acceptable per §6.1.5 of
+	// the BRs.
+	passSigAlgs = map[x509.SignatureAlgorithm]bool{
+		x509.SHA256WithRSA:   true,
+		x509.SHA384WithRSA:   true,
+		x509.SHA512WithRSA:   true,
+		x509.DSAWithSHA256:   true,
+		x509.ECDSAWithSHA256: true,
+		x509.ECDSAWithSHA384: true,
+		x509.ECDSAWithSHA512: true,
+		// NOTE: BRs section §6.1.5 does not include SHA1 digest algorithms in the
+		// current version. We allow these here for historic reasons and check for
+		// SHA1 usage after the deprecation date in the separate
+		// `e_sub_cert_or_sub_ca_using_sha1` lint.
+		x509.SHA1WithRSA:   true,
+		x509.DSAWithSHA1:   true,
+		x509.ECDSAWithSHA1: true,
+	}
+	// The BRs do not forbid the use of RSA-PSS as a signature scheme in
+	// certificates but it is not broadly supported by user-agents. Since
+	// the BRs do not forbid the practice we return a warning result.
+	// NOTE: The Mozilla root program policy *does* forbid their use since v2.7.
+	// This should be covered by a lint scoped to the Mozilla source instead of in
+	// this CABF lint.
+	warnSigAlgs = map[x509.SignatureAlgorithm]bool{
+		x509.SHA256WithRSAPSS: true,
+		x509.SHA384WithRSAPSS: true,
+		x509.SHA512WithRSAPSS: true,
+	}
+)
+
 type signatureAlgorithmNotSupported struct{}
 
 func (l *signatureAlgorithmNotSupported) Initialize() error {
@@ -31,11 +63,15 @@ func (l *signatureAlgorithmNotSupported) CheckApplies(c *x509.Certificate) bool
 }
 
 func (l *signatureAlgorithmNotSupported) Execute(c *x509.Certificate) *lint.LintResult {
-
-	if c.SignatureAlgorithm == x509.SHA1WithRSA || c.SignatureAlgorithm == x509.SHA256WithRSA || c.SignatureAlgorithm == x509.SHA384WithRSA || c.SignatureAlgorithm == x509.SHA512WithRSA || c.SignatureAlgorithm == x509.DSAWithSHA1 || c.SignatureAlgorithm == x509.DSAWithSHA256 || c.SignatureAlgorithm == x509.ECDSAWithSHA1 || c.SignatureAlgorithm == x509.ECDSAWithSHA256 || c.SignatureAlgorithm == x509.ECDSAWithSHA384 || c.SignatureAlgorithm == x509.ECDSAWithSHA512 {
-		return &lint.LintResult{Status: lint.Pass}
-	} else {
-		return &lint.LintResult{Status: lint.Error}
+	sigAlg := c.SignatureAlgorithm
+	status := lint.Error
+	if passSigAlgs[sigAlg] {
+		status = lint.Pass
+	} else if warnSigAlgs[sigAlg] {
+		status = lint.Warn
+	}
+	return &lint.LintResult{
+		Status: status,
 	}
 }
 

lints/cabf_br/lint_signature_algorithm_not_supported_test.go

@@ -38,3 +38,12 @@ func TestSignatureAlgorithmSHA1Supported(t *testing.T) {
 		t.Errorf("%s: expected %s, got %s", inputPath, expected, out.Status)
 	}
 }
+
+func TestSignatureAlgorithmRSAPSSWarn(t *testing.T) {
+	inputPath := "../../testlint/testCerts/sha256WithRSAPSSSignatureAlgorithm.pem"
+	expected := lint.Warn
+	out := lint.Lints["e_signature_algorithm_not_supported"].Execute(util.ReadCertificate(inputPath))
+	if out.Status != expected {
+		t.Errorf("%s: expected %s, got %s", inputPath, expected, out.Status)
+	}
+}

testlint/testCerts/sha256WithRSAPSSSignatureAlgorithm.pem

@@ -0,0 +1,37 @@
+Adopted from:
+https://github.com/golang/go/blob/d5967a710094b4e901175948727bbda7a197565c/src/crypto/x509/x509_test.go#L1021-L1055
+-----BEGIN CERTIFICATE-----
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++f9b5TXKXVyjlUL7uHxowWrT2AtTchDH22wTEtqLEF9Z3Q==
+-----END CERTIFICATE-----