[PATCH] lib: rsa: Set conventional salt length RSA-PSS parameter

[Loic Poulain]

RFC 3447 says that Typical salt length are either 0 or the length
of the output of the digest algorithm, RFC 4055 also recommends
hash value length as the salt length. Moreover, By convention,
most of the signing infrastructures/libraries use the length of
the digest algorithm (such as google cloud kms:
                      https://cloud.google.com/kms/docs/algorithms).

If the salt-length parameter is not set, openssl default to the
maximum allowed value, which is a openssl 'specificity', so this
works well for local signing, but restricts compatibility with
other engines (e.g pkcs11/libkmsp11):

```
returning 0x71 from C_SignInit due to status INVALID_ARGUMENT:
    at rsassa_pss.cc:53: expected salt length for key XX is 32,
    but 478 was supplied in the parameters
Could not obtain signature: error:41000070:PKCS#11 module::Mechanism invalid
```

To improve compatibility, we set the default RSA-PSS salt-length
value to the conventional one. A further improvement could consist
in making it configurable as signature FIT node attribute.

rfc3447: https://datatracker.ietf.org/doc/html/rfc3447
rfc4055: https://datatracker.ietf.org/doc/html/rfc4055

Signed-off-by: Loic Poulain <loic.poulain at linaro.org>
---
 lib/rsa/rsa-sign.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lib/rsa/rsa-sign.c b/lib/rsa/rsa-sign.c
index 2304030e32..fa9e143b4c 100644
--- a/lib/rsa/rsa-sign.c
+++ b/lib/rsa/rsa-sign.c
@@ -428,6 +428,15 @@ static int rsa_sign_with_key(EVP_PKEY *pkey, struct padding_algo *padding_algo,
 			ret = rsa_err("Signer padding setup failed");
 			goto err_sign;
 		}
+
+		/* Per RFC 3447 (and convention) the Typical salt length is the
+		 * length of the output of the digest algorithm.
+		 */
+		if (EVP_PKEY_CTX_set_rsa_pss_saltlen(ckey,
+						     checksum_algo->checksum_len) <= 0) {
+			ret = rsa_err("Signer salt length setup failed");
+			goto err_sign;
+		}
 	}
 
 	for (i = 0; i < region_count; i++) {
-- 
2.34.1