Add DTLS_get_data_mtu() function

We add ssl_cipher_get_overhead() as an internal function, to avoid
having too much ciphersuite-specific knowledge in DTLS_get_data_mtu()
itself. It's going to need adjustment for TLSv1.3... but then again, so
is fairly much *all* of the SSL_CIPHER handling. This bit is in the noise.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>

1 files changed, 52 insertions, 0 deletions

diff --git a/ssl/ssl_ciph.c b/ssl/ssl_ciph.c
index adccbfc4d8..acc1840ee1 100644
--- a/ssl/ssl_ciph.c
+++ b/ssl/ssl_ciph.c
@@ -1947,3 +1947,55 @@ int SSL_CIPHER_is_aead(const SSL_CIPHER *c)
 {
     return (c->algorithm_mac & SSL_AEAD) ? 1 : 0;
 }
+
+int ssl_cipher_get_overhead(const SSL_CIPHER *c, size_t *mac_overhead,
+                            size_t *int_overhead, size_t *blocksize,
+                            size_t *ext_overhead)
+{
+    size_t mac = 0, in = 0, blk = 0, out = 0;
+
+    /* Some hard-coded numbers for the CCM/Poly1305 MAC overhead
+     * because there are no handy #defines for those. */
+    if (c->algorithm_enc & SSL_AESGCM) {
+        out = EVP_GCM_TLS_EXPLICIT_IV_LEN + EVP_GCM_TLS_TAG_LEN;
+    } else if (c->algorithm_enc & (SSL_AES128CCM | SSL_AES256CCM)) {
+        out = EVP_CCM_TLS_EXPLICIT_IV_LEN + 16;
+    } else if (c->algorithm_enc & (SSL_AES128CCM8 | SSL_AES256CCM8)) {
+        out = EVP_CCM_TLS_EXPLICIT_IV_LEN + 8;
+    } else if (c->algorithm_enc & SSL_CHACHA20POLY1305) {
+        out = 16;
+    } else if (c->algorithm_mac & SSL_AEAD) {
+        /* We're supposed to have handled all the AEAD modes above */
+        return 0;
+    } else {
+        /* Non-AEAD modes. Calculate MAC/cipher overhead separately */
+        int digest_nid = SSL_CIPHER_get_digest_nid(c);
+        const EVP_MD *e_md = EVP_get_digestbynid(digest_nid);
+
+        if (e_md == NULL)
+            return 0;
+
+        mac = EVP_MD_size(e_md);
+        if (c->algorithm_enc != SSL_eNULL) {
+            int cipher_nid = SSL_CIPHER_get_cipher_nid(c);
+            const EVP_CIPHER *e_ciph = EVP_get_cipherbynid(cipher_nid);
+
+            /* If it wasn't AEAD or SSL_eNULL, we expect it to be a
+               known CBC cipher. */
+            if (e_ciph == NULL ||
+                EVP_CIPHER_mode(e_ciph) != EVP_CIPH_CBC_MODE)
+                return 0;
+
+            in = 1; /* padding length byte */
+            out = EVP_CIPHER_iv_length(e_ciph);
+            blk = EVP_CIPHER_block_size(e_ciph);
+        }
+    }
+
+    *mac_overhead = mac;
+    *int_overhead = in;
+    *blocksize = blk;
+    *ext_overhead = out;
+
+    return 1;
+}