X509: Add "code sign" as purpose for verification of certificates

Code signing certificates have other properties as for example described in
CA Browser Forum documents. This leads to "unsupported certificate purpose" errors when
verifying signed objects.
This patch adds the purpose "codesign" to the table in X.509 certificate verification and
the verification parameter "code_sign" to X509_VERIFY_PARAM.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18567)

1 files changed, 56 insertions, 0 deletions

diff --git a/crypto/x509/v3_purp.c b/crypto/x509/v3_purp.c
index fa3a8b1ebf..cac539b1e4 100644
--- a/crypto/x509/v3_purp.c
+++ b/crypto/x509/v3_purp.c
@@ -32,6 +32,8 @@ static int check_purpose_crl_sign(const X509_PURPOSE *xp, const X509 *x,
                                   int require_ca);
 static int check_purpose_timestamp_sign(const X509_PURPOSE *xp, const X509 *x,
                                         int require_ca);
+static int check_purpose_code_sign(const X509_PURPOSE *xp, const X509 *x,
+                                        int require_ca);
 static int no_check_purpose(const X509_PURPOSE *xp, const X509 *x,
                             int require_ca);
 static int check_purpose_ocsp_helper(const X509_PURPOSE *xp, const X509 *x,
@@ -61,6 +63,9 @@ static X509_PURPOSE xstandard[] = {
     {X509_PURPOSE_TIMESTAMP_SIGN, X509_TRUST_TSA, 0,
      check_purpose_timestamp_sign, "Time Stamp signing", "timestampsign",
      NULL},
+    {X509_PURPOSE_CODE_SIGN, X509_TRUST_OBJECT_SIGN, 0,
+     check_purpose_code_sign, "Code signing", "codesign",
+     NULL},
 };
 
 #define X509_PURPOSE_COUNT OSSL_NELEM(xstandard)
@@ -865,6 +870,57 @@ static int check_purpose_timestamp_sign(const X509_PURPOSE *xp, const X509 *x,
     return 1;
 }
 
+static int check_purpose_code_sign(const X509_PURPOSE *xp, const X509 *x,
+                                   int require_ca)
+{
+    int i_ext;
+
+    /* If ca is true we must return if this is a valid CA certificate. */
+    if (require_ca)
+        return check_ca(x);
+
+    /*
+     * Check the key usage and extended key usage fields:
+     *
+     * Reference: CA Browser Forum,
+     * Baseline Requirements for the Issuance and Management of 
+     * Publicly‐Trusted Code Signing Certificates, Version 3.0.0,
+     * Section 7.1.2.3: Code signing and Timestamp Certificate
+     *
+     * Checking covers Key Usage and Extended Key Usage attributes.
+     * Other properties like CRL Distribution Points and Authoriy
+     * Information Access (AIA) are not checked.
+     */
+    /* Key Usage */
+    if ((x->ex_flags & EXFLAG_KUSAGE) == 0)
+        return 0;
+    if ((x->ex_kusage & KU_DIGITAL_SIGNATURE) == 0)
+        return 0;
+    if ((x->ex_kusage & (KU_KEY_CERT_SIGN | KU_CRL_SIGN)) != 0)
+        return 0;
+
+    /* Key Usage MUST be critical */
+    i_ext = X509_get_ext_by_NID(x, NID_key_usage, -1);
+    if (i_ext < 0)
+        return 0;
+    if (i_ext >= 0) {
+        X509_EXTENSION *ext = X509_get_ext((X509 *)x, i_ext);
+        if (!X509_EXTENSION_get_critical(ext))
+            return 0;
+    }
+
+    /* Extended Key Usage */
+    if ((x->ex_flags & EXFLAG_XKUSAGE) == 0)
+        return 0;
+    if ((x->ex_xkusage & XKU_CODE_SIGN) == 0)
+        return 0;
+    if ((x->ex_xkusage & (XKU_ANYEKU | XKU_SSL_SERVER)) != 0)
+        return 0;
+
+    return 1;
+
+}
+
 static int no_check_purpose(const X509_PURPOSE *xp, const X509 *x,
                             int require_ca)
 {