From 5d96a3dcde7e89bdaf4e0cc15721af433c40799f Mon Sep 17 00:00:00 2001 From: Janos Follath Date: Fri, 10 Mar 2017 11:31:41 +0000 Subject: [PATCH] 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. --- ChangeLog | 5 +++++ include/polarssl/ecdsa.h | 37 +++++++++++++++++++++++++++++++++++++ programs/pkey/ecdsa.c | 30 +++++++++++++++++++++++++----- 3 files changed, 67 insertions(+), 5 deletions(-) diff --git a/ChangeLog b/ChangeLog index a64de5dd1..6aa740c67 100644 --- a/ChangeLog +++ b/ChangeLog @@ -16,6 +16,11 @@ Bugfix (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 5eb83d9d2..47b644ef6 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 @@ extern "C" { * \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 @@ int ecdsa_sign( ecp_group *grp, mpi *r, mpi *s, * \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 @@ int ecdsa_sign_det( ecp_group *grp, mpi *r, mpi *s, * \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 @@ int ecdsa_verify( ecp_group *grp, * 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 @@ int ecdsa_write_signature_det( ecdsa_context *ctx, * \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 aa8eafb97..1411e42b0 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 #endif @@ -102,16 +103,20 @@ int main( int argc, char *argv[] ) 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 @@ int main( int argc, char *argv[] ) 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 @@ int main( int argc, char *argv[] ) } 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 @@ exit: ecdsa_free( &ctx_sign ); ctr_drbg_free( &ctr_drbg ); entropy_free( &entropy ); + sha256_free( &sha256_ctx ); return( ret ); }