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/ChangeLog b/ChangeLog
index a64de5d..6aa740c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -16,6 +16,11 @@
(rsa_rsaes_pkcs1_v15_decrypt(), rsa_rsaes_oaep_decrypt).
Found by Laurent Simon.
+Changes
+ * Clarify ECDSA documentation and improve the sample code to avoid
+ misunderstandings and potentially dangerous use of the API. Pointed out
+ by Jean-Philippe Aumasson.
+
= mbed TLS 1.3.19 branch released 2017-03-08
Security
diff --git a/include/polarssl/ecdsa.h b/include/polarssl/ecdsa.h
index 5eb83d9..47b644e 100644
--- a/include/polarssl/ecdsa.h
+++ b/include/polarssl/ecdsa.h
@@ -26,6 +26,23 @@
#include "ecp.h"
#include "md.h"
+/*
+ * RFC 4492 page 20:
+ *
+ * Ecdsa-Sig-Value ::= SEQUENCE {
+ * r INTEGER,
+ * s INTEGER
+ * }
+ *
+ * Size is at most
+ * 1 (tag) + 1 (len) + 1 (initial 0) + ECP_MAX_BYTES for each of r and s,
+ * twice that + 1 (tag) + 2 (len) for the sequence
+ * (assuming ECP_MAX_BYTES is less than 126 for r and s,
+ * and less than 124 (total len <= 255) for the sequence)
+ *
+ */
+/** Maximum size of an ECDSA signature in bytes */
+#define POLARSSL_ECDSA_MAX_LEN ( 3 + 2 * ( 3 + POLARSSL_ECP_MAX_BYTES ) )
/**
* \brief ECDSA context structure
@@ -58,6 +75,10 @@
* \param f_rng RNG function
* \param p_rng RNG parameter
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
*/
@@ -78,6 +99,10 @@
* \param blen Length of buf
* \param md_alg MD algorithm used to hash the message
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
*/
@@ -96,6 +121,10 @@
* \param r First integer of the signature
* \param s Second integer of the signature
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.4 step 3.
+ *
* \return 0 if successful,
* POLARSSL_ERR_ECP_BAD_INPUT_DATA if signature is invalid
* or a POLARSSL_ERR_ECP_XXX or POLARSSL_MPI_XXX error code
@@ -121,6 +150,10 @@
* size of the curve used, plus 7 (eg. 71 bytes if a 256-bit
* curve is used).
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.3 step 5.
+ *
* \return 0 if successful,
* or a POLARSSL_ERR_ECP, POLARSSL_ERR_MPI or
* POLARSSL_ERR_ASN1 error code
@@ -168,6 +201,10 @@
* \param sig Signature to read and verify
* \param slen Size of sig
*
+ * \note If the bitlength of the message hash is larger than the
+ * bitlength of the group order, then the hash is truncated as
+ * prescribed by SEC1 4.1.4 step 3.
+ *
* \return 0 if successful,
* POLARSSL_ERR_ECP_BAD_INPUT_DATA if signature is invalid,
* POLARSSL_ERR_ECP_SIG_LEN_MISTMATCH if the signature is
diff --git a/programs/pkey/ecdsa.c b/programs/pkey/ecdsa.c
index aa8eafb..1411e42 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -38,6 +38,7 @@
#include "polarssl/entropy.h"
#include "polarssl/ctr_drbg.h"
#include "polarssl/ecdsa.h"
+#include "polarssl/sha256.h"
#include <string.h>
#endif
@@ -102,16 +103,20 @@
ecdsa_context ctx_sign, ctx_verify;
entropy_context entropy;
ctr_drbg_context ctr_drbg;
- unsigned char hash[] = "This should be the hash of a message.";
- unsigned char sig[512];
+ sha256_context sha256_ctx;
+ unsigned char message[100];
+ unsigned char hash[32];
+ unsigned char sig[POLARSSL_ECDSA_MAX_LEN];
size_t sig_len;
const char *pers = "ecdsa";
((void) argv);
ecdsa_init( &ctx_sign );
ecdsa_init( &ctx_verify );
+ 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 +160,24 @@
dump_pubkey( " + Public key: ", &ctx_sign );
/*
- * Sign some message hash
+ * Compute message hash
*/
polarssl_printf( " . Signing message..." );
+ polarssl_printf( " . Computing message hash..." );
+ fflush( stdout );
+
+ sha256_starts( &sha256_ctx, 0 );
+ sha256_update( &sha256_ctx, message, sizeof( message ) );
+ sha256_finish( &sha256_ctx, hash );
+
+ polarssl_printf( " ok\n" );
+
+ dump_buf( " + Hash: ", hash, sizeof( hash ) );
+
+ /*
+ * Sign message hash
+ */
+ polarssl_printf( " . Signing message hash..." );
fflush( stdout );
if( ( ret = ecdsa_write_signature( &ctx_sign,
@@ -170,7 +190,6 @@
}
polarssl_printf( " ok (signature length = %u)\n", (unsigned int) sig_len );
- dump_buf( " + Hash: ", hash, sizeof hash );
dump_buf( " + Signature: ", sig, sig_len );
/*
@@ -233,6 +252,7 @@
ecdsa_free( &ctx_sign );
ctr_drbg_free( &ctr_drbg );
entropy_free( &entropy );
+ sha256_free( &sha256_ctx );
return( ret );
}