cert_tool: Support for legacy RSA PKCS#1 v1.5

This patch enables choice of RSA version at run time to be used for
generating signatures by the cert_tool. The RSA PSS as defined in
PKCS#1 v2.1 becomes the default version and this patch enables to specify
the RSA PKCS#1 v1.5 algorithm to `cert_create` through the command line
-a option. Also, the build option `KEY_ALG` can be used to pass this
option from the build system. Please note that RSA PSS is mandated
by Trusted Board Boot requirements (TBBR) and legacy RSA support is
being added for compatibility reasons.

Fixes ARM-Software/tf-issues#499
Change-Id: Ifaa3f2f7c9b43f3d7b3effe2cde76bf6745a5d73
Co-Authored-By: Eleanor Bonnici <Eleanor.bonnici@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>

docs/user-guide.rst

@@ -407,8 +407,10 @@ Common build options
 
 -  ``KEY_ALG``: This build flag enables the user to select the algorithm to be
    used for generating the PKCS keys and subsequent signing of the certificate.
-   It accepts 2 values viz ``rsa``, ``ecdsa``. The default value of this flag
-   is ``rsa``.
+   It accepts 3 values viz ``rsa``, ``rsa_1_5``, ``ecdsa``. The ``rsa_1_5`` is
+   the legacy PKCS#1 RSA 1.5 algorithm which is not TBBR compliant and is
+   retained only for compatibility. The default value of this flag is ``rsa``
+   which is the TBBR compliant PKCS#1 RSA 2.1 scheme.
 
 -  ``LDFLAGS``: Extra user options appended to the linkers' command line in
    addition to the one set by the build system.

drivers/auth/mbedtls/mbedtls_crypto.mk

@@ -9,7 +9,7 @@ include drivers/auth/mbedtls/mbedtls_common.mk
 # The platform may define the variable 'TF_MBEDTLS_KEY_ALG' to select the key
 # algorithm to use. If the variable is not defined, select it based on algorithm
 # used for key generation `KEY_ALG`. If `KEY_ALG` is not defined or is
-# defined to `rsa`, then set the variable to `rsa`.
+# defined to `rsa`/`rsa_1_5`, then set the variable to `rsa`.
 ifeq (${TF_MBEDTLS_KEY_ALG},)
     ifeq (${KEY_ALG}, ecdsa)
         TF_MBEDTLS_KEY_ALG		:=	ecdsa

tools/cert_create/include/cert.h

@@ -48,7 +48,7 @@ struct cert_s {
 int cert_init(void);
 cert_t *cert_get_by_opt(const char *opt);
 int cert_add_ext(X509 *issuer, X509 *subject, int nid, char *value);
-int cert_new(cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk);
+int cert_new(int key_alg, cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk);
 
 /* Macro to register the certificates used in the CoT */
 #define REGISTER_COT(_certs) \

tools/cert_create/include/key.h

@@ -22,7 +22,8 @@ enum {
 
 /* Supported key algorithms */
 enum {
-	KEY_ALG_RSA,
+	KEY_ALG_RSA,		/* RSA PSS as defined by PKCS#1 v2.1 (default) */
+	KEY_ALG_RSA_1_5,	/* RSA as defined by PKCS#1 v1.5 */
 #ifndef OPENSSL_NO_EC
 	KEY_ALG_ECDSA,
 #endif /* OPENSSL_NO_EC */

tools/cert_create/src/cert.c

@@ -79,7 +79,7 @@ int cert_add_ext(X509 *issuer, X509 *subject, int nid, char *value)
 	return 1;
 }
 
-int cert_new(cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk)
+int cert_new(int key_alg, cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk)
 {
 	EVP_PKEY *pkey = keys[cert->key].key;
 	cert_t *issuer_cert = &certs[cert->issuer];
@@ -112,11 +112,18 @@ int cert_new(cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk)
 	}
 
 	EVP_MD_CTX_init(&mdCtx);
+
+	/* Sign the certificate with the issuer key */
 	if (!EVP_DigestSignInit(&mdCtx, &pKeyCtx, EVP_sha256(), NULL, ikey)) {
 		ERR_print_errors_fp(stdout);
 		goto END;
 	}
 
+	/*
+	 * Set additional parameters if algorithm is RSA PSS. This is not
+	 * required for RSA 1.5 or ECDSA.
+	 */
+	if (key_alg == KEY_ALG_RSA) {
 		if (!EVP_PKEY_CTX_set_rsa_padding(pKeyCtx, RSA_PKCS1_PSS_PADDING)) {
 			ERR_print_errors_fp(stdout);
 			goto END;
@@ -131,6 +138,7 @@ int cert_new(cert_t *cert, int days, int ca, STACK_OF(X509_EXTENSION) * sk)
 			ERR_print_errors_fp(stdout);
 			goto END;
 		}
+	}
 
 	/* x509.v3 */
 	X509_set_version(x, 2);

tools/cert_create/src/main.c

@@ -89,6 +89,7 @@ static char *strdup(const char *str)
 
 static const char *key_algs_str[] = {
 	[KEY_ALG_RSA] = "rsa",
+	[KEY_ALG_RSA_1_5] = "rsa_1_5",
 #ifndef OPENSSL_NO_EC
 	[KEY_ALG_ECDSA] = "ecdsa"
 #endif /* OPENSSL_NO_EC */
@@ -223,7 +224,8 @@ static const cmd_opt_t common_cmd_opt[] = {
 	},
 	{
 		{ "key-alg", required_argument, NULL, 'a' },
-		"Key algorithm: 'rsa' (default), 'ecdsa'"
+		"Key algorithm: 'rsa' (default) - RSAPSS scheme as per \
+PKCS#1 v2.1, 'rsa_1_5' - RSA PKCS#1 v1.5, 'ecdsa'"
 	},
 	{
 		{ "save-keys", no_argument, NULL, 'k' },
@@ -450,8 +452,8 @@ int main(int argc, char *argv[])
 			sk_X509_EXTENSION_push(sk, cert_ext);
 		}
 
-		/* Create certificate. Signed with ROT key */
-		if (cert->fn && !cert_new(cert, VAL_DAYS, 0, sk)) {
+		/* Create certificate. Signed with corresponding key */
+		if (cert->fn && !cert_new(key_alg, cert, VAL_DAYS, 0, sk)) {
 			ERROR("Cannot create %s\n", cert->cn);
 			exit(1);
 		}