Implement sshsig functions

Signed-off-by: Nicolas Graves <ngraves@ngraves.fr>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Eshan Kelkar <eshankelkar@galorithm.com>

include/libssh/libssh.h

@@ -888,6 +888,27 @@ LIBSSH_API void *ssh_buffer_get(ssh_buffer buffer);
 LIBSSH_API uint32_t ssh_buffer_get_len(ssh_buffer buffer);
 LIBSSH_API int ssh_session_set_disconnect_message(ssh_session session, const char *message);
 
+/* SSHSIG hashes data independently from the key used, so we use a new enum
+   to avoid confusion. See
+   https://gitlab.com/jas/ietf-sshsig-format/-/blob/cc70a225cbd695d5a6f20aaebdb4b92b0818e43a/ietf-sshsig-format.md#L137
+ */
+enum sshsig_digest_e {
+    SSHSIG_DIGEST_SHA2_256 = 0,
+    SSHSIG_DIGEST_SHA2_512 = 1,
+};
+
+LIBSSH_API int sshsig_sign(const void *data,
+                           size_t data_length,
+                           ssh_key privkey,
+                           const char *sig_namespace,
+                           enum sshsig_digest_e hash_alg,
+                           char **signature);
+LIBSSH_API int sshsig_verify(const void *data,
+                             size_t data_length,
+                             const char *signature,
+                             const char *sig_namespace,
+                             ssh_key *sign_key);
+
 #ifndef LIBSSH_LEGACY_0_4
 #include "libssh/legacy.h"
 #endif

include/libssh/pki.h

@@ -51,6 +51,15 @@
 #define SSH_KEY_FLAG_PRIVATE 0x0002
 #define SSH_KEY_FLAG_PKCS11_URI 0x0004
 
+/* Constants matching the Lightweight Secure Shell Signature Format */
+/* https://datatracker.ietf.org/doc/draft-josefsson-sshsig-format */
+#define SSHSIG_VERSION            0x01
+#define SSHSIG_MAGIC_PREAMBLE     "SSHSIG"
+#define SSHSIG_MAGIC_PREAMBLE_LEN (sizeof(SSHSIG_MAGIC_PREAMBLE) - 1)
+#define SSHSIG_BEGIN_SIGNATURE    "-----BEGIN SSH SIGNATURE-----"
+#define SSHSIG_END_SIGNATURE      "-----END SSH SIGNATURE-----"
+#define SSHSIG_LINE_LENGTH        76
+
 struct ssh_key_struct {
     enum ssh_keytypes_e type;
     int flags;