Clarify the use of ECDSA API
In the ecdsa.c sample application we don't use hashing, we use ecdsa
directly on a buffer containing plain text. Although the text explains
that it should be the message hash it still can be confusing.
Any misunderstandings here are potentially very dangerous, because ECDSA
truncates the message hash if necessary and this can lead to trivial
signature forgeries if the API is misused and the message is passed
directly to the function without hashing.
This commit adds a hash computation step to the ecdsa.c sample
application and clarification to the doxygen documentation of the
ECDSA functions involved.
diff --git a/programs/pkey/ecdsa.c b/programs/pkey/ecdsa.c
index 069d312..c3ce56a 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -37,6 +37,7 @@
#include "mbedtls/entropy.h"
#include "mbedtls/ctr_drbg.h"
#include "mbedtls/ecdsa.h"
+#include "mbedtls/sha256.h"
#include <string.h>
#endif
@@ -101,8 +102,10 @@
mbedtls_ecdsa_context ctx_sign, ctx_verify;
mbedtls_entropy_context entropy;
mbedtls_ctr_drbg_context ctr_drbg;
- unsigned char hash[] = "This should be the hash of a message.";
- unsigned char sig[512];
+ mbedtls_sha256_context sha256_ctx;
+ unsigned char message[100];
+ unsigned char hash[32];
+ unsigned char sig[MBEDTLS_ECDSA_MAX_LEN];
size_t sig_len;
const char *pers = "ecdsa";
((void) argv);
@@ -110,8 +113,10 @@
mbedtls_ecdsa_init( &ctx_sign );
mbedtls_ecdsa_init( &ctx_verify );
mbedtls_ctr_drbg_init( &ctr_drbg );
+ mbedtls_sha256_init( &sha256_ctx );
- memset(sig, 0, sizeof( sig ) );
+ memset( sig, 0, sizeof( sig ) );
+ memset( message, 0x25, sizeof( message ) );
ret = 1;
if( argc != 1 )
@@ -155,9 +160,23 @@
dump_pubkey( " + Public key: ", &ctx_sign );
/*
- * Sign some message hash
+ * Compute message hash
*/
- mbedtls_printf( " . Signing message..." );
+ mbedtls_printf( " . Computing message hash..." );
+ fflush( stdout );
+
+ mbedtls_sha256_starts( &sha256_ctx, 0 );
+ mbedtls_sha256_update( &sha256_ctx, message, sizeof( message ) );
+ mbedtls_sha256_finish( &sha256_ctx, hash );
+
+ mbedtls_printf( " ok\n" );
+
+ dump_buf( " + Hash: ", hash, sizeof( hash ) );
+
+ /*
+ * Sign message hash
+ */
+ mbedtls_printf( " . Signing message hash..." );
fflush( stdout );
if( ( ret = mbedtls_ecdsa_write_signature( &ctx_sign, MBEDTLS_MD_SHA256,
@@ -170,7 +189,6 @@
}
mbedtls_printf( " ok (signature length = %u)\n", (unsigned int) sig_len );
- dump_buf( " + Hash: ", hash, sizeof hash );
dump_buf( " + Signature: ", sig, sig_len );
/*
@@ -224,6 +242,7 @@
mbedtls_ecdsa_free( &ctx_sign );
mbedtls_ctr_drbg_free( &ctr_drbg );
mbedtls_entropy_free( &entropy );
+ mbedtls_sha256_free( &sha256_ctx );
return( ret );
}