libgcrypt: Add helper to extract MPIs into ssh_strings

* include/libssh/libgcrypt.h (ssh_sexp_extract_mpi): New prototype.
* src/libgcrypt.c (ssh_sexp_extract_mpi): New function.

Signed-off-by: Justus Winter <justus@g10code.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/libgcrypt.h

@@ -71,6 +71,16 @@ char *ssh_gcry_bn2dec(bignum bn);
 #define bignum_bn2bin(num,datalen,data) gcry_mpi_print(GCRYMPI_FMT_USG,data,datalen,NULL,num)
 #define bignum_cmp(num1,num2) gcry_mpi_cmp(num1,num2)
 
+/* Helper functions for data conversions.  */
+
+/* Extract an MPI from the given s-expression SEXP named NAME which is
+   encoded using INFORMAT and store it in a newly allocated ssh_string
+   encoded using OUTFORMAT.  */
+ssh_string ssh_sexp_extract_mpi(const gcry_sexp_t sexp,
+                                const char *name,
+                                enum gcry_mpi_format informat,
+                                enum gcry_mpi_format outformat);
+
 #endif /* HAVE_LIBGCRYPT */
 
 struct ssh_cipher_struct *ssh_get_ciphertab(void);

src/libgcrypt.c

@@ -2,6 +2,7 @@
  * This file is part of the SSH Library
  *
  * Copyright (c) 2009 by Aris Adamantiadis
+ * Copyright (C) 2016 g10 Code GmbH
  *
  * The SSH Library is free software; you can redistribute it and/or modify
  * it under the terms of the GNU Lesser General Public License as published by
@@ -27,6 +28,7 @@
 #include "libssh/session.h"
 #include "libssh/crypto.h"
 #include "libssh/wrapper.h"
+#include "libssh/string.h"
 
 #ifdef HAVE_LIBGCRYPT
 #include <gcrypt.h>
@@ -598,4 +600,54 @@ struct ssh_cipher_struct *ssh_get_ciphertab(void)
   return ssh_ciphertab;
 }
 
+/*
+ * Extract an MPI from the given s-expression SEXP named NAME which is
+ * encoded using INFORMAT and store it in a newly allocated ssh_string
+ * encoded using OUTFORMAT.
+ */
+ssh_string ssh_sexp_extract_mpi(const gcry_sexp_t sexp,
+                                const char *name,
+                                enum gcry_mpi_format informat,
+                                enum gcry_mpi_format outformat)
+{
+    gpg_error_t err;
+    ssh_string result = NULL;
+    gcry_sexp_t fragment = NULL;
+    gcry_mpi_t mpi = NULL;
+    size_t size;
+
+    fragment = gcry_sexp_find_token(sexp, name, 0);
+    if (fragment == NULL) {
+        goto fail;
+    }
+
+    mpi = gcry_sexp_nth_mpi(fragment, 1, informat);
+    if (mpi == NULL) {
+        goto fail;
+    }
+
+    err = gcry_mpi_print(outformat, NULL, 0, &size, mpi);
+    if (err != 0) {
+        goto fail;
+    }
+
+    result = ssh_string_new(size);
+    if (result == NULL) {
+        goto fail;
+    }
+
+    err = gcry_mpi_print(outformat, ssh_string_data(result), size, NULL, mpi);
+    if (err != 0) {
+        ssh_string_burn(result);
+        ssh_string_free(result);
+        result = NULL;
+        goto fail;
+    }
+
+fail:
+    gcry_sexp_release(fragment);
+    gcry_mpi_release(mpi);
+    return result;
+}
+
 #endif