commit cd062d84d6ab0a4078e92c3f9a67f2276ae46fdf
author Gilles Peskine <Gilles.Peskine@arm.com> Thu Nov 02 17:16:43 2017 +0100
committer Andrzej Kurek <andrzej.kurek@mobica.com> Mon Jan 22 07:51:24 2018 -0500
tree 9ca6a07aee70a8a2ad2561825030698eb1dc3b10
parent bce41d373acb31ff329da9bc0c48bb5baf5c11bc

pk: function to calculate the signature size

Expose a function mbedtls_pk_signature_size to calculate the maximum
size of a signature made with a given key. Document that this is the
buffer size that mbedtls_pk_sign requires.

Add a corresponding field signature_size_func to the mbedtls_pk_info
structure.

library/pk.c

diff --git a/library/pk.c b/library/pk.c
index 9037646..b48f4d9 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -343,6 +343,20 @@
 }
 
 /*
+ * Maximum signature size
+ */
+size_t mbedtls_pk_signature_size( const mbedtls_pk_context *ctx )
+{
+    if( ctx == NULL || ctx->pk_info == NULL )
+        return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
+
+    if( ctx->pk_info->signature_size_func == NULL )
+        return( ( ctx->pk_info->get_bitlen( ctx->pk_ctx ) + 7 ) / 8 );
+    else
+        return( ctx->pk_info->signature_size_func( ctx->pk_ctx ) );
+}
+
+/*
  * Export debug information
  */
 int mbedtls_pk_debug( const mbedtls_pk_context *ctx, mbedtls_pk_debug_item *items )

library/pk_wrap.c

diff --git a/library/pk_wrap.c b/library/pk_wrap.c
index 55be595..0d8aee1 100644
--- a/library/pk_wrap.c
+++ b/library/pk_wrap.c
@@ -195,6 +195,7 @@
     rsa_alloc_wrap,
     rsa_free_wrap,
     rsa_debug,
+    NULL,
 };
 #endif /* MBEDTLS_RSA_C */
 
@@ -262,6 +263,12 @@
     return( ret );
 }
 
+static size_t ecdsa_signature_size( const void *ctx_arg )
+{
+    const mbedtls_ecp_keypair *ctx = ctx_arg;
+    return( MBEDTLS_ECDSA_MAX_SIG_LEN( ctx->grp.pbits ) );
+}
+
 #endif /* MBEDTLS_ECDSA_C */
 
 static int eckey_check_pair( const void *pub, const void *prv )
@@ -311,6 +318,11 @@
     eckey_alloc_wrap,
     eckey_free_wrap,
     eckey_debug,
+#if defined(MBEDTLS_ECDSA_C)
+    ecdsa_signature_size,
+#else
+    NULL,
+#endif
 };
 
 /*
@@ -336,6 +348,7 @@
     eckey_alloc_wrap,       /* Same underlying key structure */
     eckey_free_wrap,        /* Same underlying key structure */
     eckey_debug,            /* Same underlying key structure */
+    NULL,
 };
 #endif /* MBEDTLS_ECP_C */
 
@@ -400,6 +413,7 @@
     ecdsa_alloc_wrap,
     ecdsa_free_wrap,
     eckey_debug,        /* Compatible key structures */
+    ecdsa_signature_size,
 };
 #endif /* MBEDTLS_ECDSA_C */
 
@@ -519,6 +533,7 @@
     rsa_alt_alloc_wrap,
     rsa_alt_free_wrap,
     NULL,
+    NULL,
 };
 
 #endif /* MBEDTLS_PK_RSA_ALT_SUPPORT */