Introduce a signer interface intented to make TLS privsep simpler

to implement.

Add a tls_config_set_sign_cb() function that allows to register
a callback for the signing operation on a tls_config. When used,
the context installs fake pivate keys internally, and the callback
receives the hash of the public key.

Add a tls_signer_*() set of functions to manage tls_signer objects.
A tls_signer is an opaque structure on which keys are added.
It is used to compute signatures with private keys identified by
their associated public key hash.

Discussed with and ok jsing@ tb@

1 files changed, 19 insertions, 1 deletions

diff --git a/src/lib/libtls/tls.c b/src/lib/libtls/tls.c
index 608f0a3acd..fd525aa428 100644
--- a/src/lib/libtls/tls.c
+++ b/src/lib/libtls/tls.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls.c,v 1.92 2021/10/21 14:31:21 tb Exp $ */
+/* $OpenBSD: tls.c,v 1.93 2022/01/25 21:51:24 eric Exp $ */
 /*
  * Copyright (c) 2014 Joel Sing <jsing@openbsd.org>
  *
@@ -387,6 +387,8 @@ tls_keypair_to_pkey(struct tls *ctx, struct tls_keypair *keypair, EVP_PKEY **pke
 static int
 tls_keypair_setup_pkey(struct tls *ctx, struct tls_keypair *keypair, EVP_PKEY *pkey)
 {
+        RSA_METHOD *rsa_method;
+        ECDSA_METHOD *ecdsa_method;
         RSA *rsa = NULL;
         EC_KEY *eckey = NULL;
         int ret = -1;
@@ -407,6 +409,14 @@ tls_keypair_setup_pkey(struct tls *ctx, struct tls_keypair *keypair, EVP_PKEY *p
                         tls_set_errorx(ctx, "RSA key setup failure");
                         goto err;
                 }
+                if (ctx->config->sign_cb == NULL)
+                        break;
+                if ((rsa_method = tls_signer_rsa_method()) == NULL ||
+                    RSA_set_ex_data(rsa, 1, ctx->config) == 0 ||
+                    RSA_set_method(rsa, rsa_method) == 0) {
+                        tls_set_errorx(ctx, "failed to setup RSA key");
+                        goto err;
+                }
                 break;
         case EVP_PKEY_EC:
                 if ((eckey = EVP_PKEY_get1_EC_KEY(pkey)) == NULL ||
@@ -414,6 +424,14 @@ tls_keypair_setup_pkey(struct tls *ctx, struct tls_keypair *keypair, EVP_PKEY *p
                         tls_set_errorx(ctx, "EC key setup failure");
                         goto err;
                 }
+                if (ctx->config->sign_cb == NULL)
+                        break;
+                if ((ecdsa_method = tls_signer_ecdsa_method()) == NULL ||
+                    ECDSA_set_ex_data(eckey, 1, ctx->config) == 0 ||
+                    ECDSA_set_method(eckey, ecdsa_method) == 0) {
+                        tls_set_errorx(ctx, "failed to setup EC key");
+                        goto err;
+                }
                 break;
         default:
                 tls_set_errorx(ctx, "incorrect key type");