Fix RSA-OAEP to allow zero-length plaintext per RFC 8017

RsaPublicEncryptEx() rejected inLen==0 unconditionally with
BAD_FUNC_ARG. RFC 8017 Section 7.1.1 (RSAES-OAEP-ENCRYPT) permits
zero-length messages: the only length constraint is
mLen <= k - 2*hLen - 2, which mLen=0 always satisfies.

RsaPrivateDecryptEx() converted a zero-length decryption result to
RSA_BUFFER_E (unless WOLFSSL_RSA_DECRYPT_TO_0_LEN was defined).
RFC 8017 Section 7.1.2 (RSAES-OAEP-DECRYPT) produces the original
message M which may be empty. The fix uses constant-time masking to
allow ret==0 when pad_type is WC_RSA_OAEP_PAD, preserving the
existing timing-safe behavior for other padding types.

Both OpenSSL and BoringSSL accept empty OAEP plaintexts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MarkAtwood

wolfcrypt/src/rsa.c

@@ -3318,7 +3318,15 @@ static int RsaPublicEncryptEx(const byte* in, word32 inLen, byte* out,
     RsaPadding padding;
 #endif
 
-    if (in == NULL || inLen == 0 || out == NULL || key == NULL) {
+    if (out == NULL || key == NULL) {
+        return BAD_FUNC_ARG;
+    }
+
+    /* For OAEP padding (RFC 8017, Section 7.1.1), zero-length messages are
+     * permitted: the spec requires mLen <= k - 2*hLen - 2, and mLen = 0
+     * satisfies this for all supported key sizes. For other padding types,
+     * a zero-length input is invalid. */
+    if (in == NULL || (inLen == 0 && pad_type != WC_RSA_OAEP_PAD)) {
         return BAD_FUNC_ARG;
     }
 
@@ -3706,8 +3714,13 @@ static int RsaPrivateDecryptEx(const byte* in, word32 inLen, byte* out,
             ret = ctMaskSelInt(ctMaskLTE(ret, (int)outLen), ret,
                                WC_NO_ERR_TRACE(RSA_BUFFER_E));
     #ifndef WOLFSSL_RSA_DECRYPT_TO_0_LEN
-            ret = ctMaskSelInt(ctMaskNotEq(ret, 0), ret,
-                               WC_NO_ERR_TRACE(RSA_BUFFER_E));
+            /* RFC 8017 Section 7.1.2: OAEP decryption may produce a valid
+             * zero-length message. Only reject ret==0 for non-OAEP types. */
+            {
+                int zeroOk = ctMaskEq(pad_type, WC_RSA_OAEP_PAD);
+                ret = ctMaskSelInt(ctMaskNotEq(ret, 0) | zeroOk, ret,
+                                   WC_NO_ERR_TRACE(RSA_BUFFER_E));
+            }
     #endif
 #else
             if (outLen < (word32)ret)