From 9a8bb67935c5c11d6e81211443f3e82150b7ba9b Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Thu, 2 Nov 2017 17:09:49 +0100 Subject: ECDSA: macro/function to calculate the signature size Expose a macro MBEDTLS_ECDSA_MAX_SIG_LEN and a function mbedtls_ecdsa_max_sig_len to calculate the size of an ECDSA signature based on the curve size. Document that this is the buffer size that mbedtls_ecdsa_write_signature requires. --- include/mbedtls/ecdsa.h | 43 +++++++++++++++++++++++++++++++++++++------ 1 file changed, 37 insertions(+), 6 deletions(-) diff --git a/include/mbedtls/ecdsa.h b/include/mbedtls/ecdsa.h index a277715..3666ffe 100644 --- a/include/mbedtls/ecdsa.h +++ b/include/mbedtls/ecdsa.h @@ -40,6 +40,37 @@ * (assuming ECP_MAX_BYTES is less than 126 for r and s, * and less than 124 (total len <= 255) for the sequence) */ + +/** + * \brief Maximum ECDSA signature size for a given curve bit size + * + * \param bits Curve size in bits + * \return Maximum signature size in bytes + * + * \note This macro returns a compile-time constant if its argument + * is one. It may evaluate its argument multiple times; if + * this is a problem, call the function + * mbedtls_ecdsa_max_sig_len instead. + */ +#define MBEDTLS_ECDSA_MAX_SIG_LEN( bits ) \ + ( /*T,L of SEQUENCE*/ ( ( bits ) >= 61 * 8 ? 3 : 2 ) + \ + /*T,L of r,s*/ 2 * ( ( ( bits ) >= 127 * 8 ? 3 : 2 ) + \ + /*V of r,s*/ ( ( bits ) + 8 ) / 8 ) ) + +/** + * \brief Maximum ECDSA signature size for a given curve bit size + * + * \param bits Curve size in bits + * \return Maximum signature size in bytes + * + * \note If you need a compile-time constant, call the macro + * MBEDTLS_ECDSA_MAX_SIG_LEN instead. + */ +static inline size_t mbedtls_ecdsa_max_sig_len( size_t bits ) +{ + return( MBEDTLS_ECDSA_MAX_SIG_LEN( bits ) ); +} + #if MBEDTLS_ECP_MAX_BYTES > 124 #error "MBEDTLS_ECP_MAX_BYTES bigger than expected, please fix MBEDTLS_ECDSA_MAX_LEN" #endif @@ -144,9 +175,9 @@ int mbedtls_ecdsa_verify( mbedtls_ecp_group *grp, * \param f_rng RNG function * \param p_rng RNG parameter * - * \note The "sig" buffer must be at least as large as twice the - * size of the curve used, plus 9 (eg. 73 bytes if a 256-bit - * curve is used). MBEDTLS_ECDSA_MAX_LEN is always safe. + * \note The \c sig buffer must be at least + * `MBEDTLS_ECDSA_MAX_SIG_LEN(ctx->grp.pbits)` bytes long. + * MBEDTLS_ECDSA_MAX_LEN is always safe. * * \note If the bitlength of the message hash is larger than the * bitlength of the group order, then the hash is truncated as @@ -184,9 +215,9 @@ int mbedtls_ecdsa_write_signature( mbedtls_ecdsa_context *ctx, mbedtls_md_type_t * \param slen Length of the signature written * \param md_alg MD algorithm used to hash the message * - * \note The "sig" buffer must be at least as large as twice the - * size of the curve used, plus 9 (eg. 73 bytes if a 256-bit - * curve is used). MBEDTLS_ECDSA_MAX_LEN is always safe. + * \note The \c sig buffer must be at least + * `MBEDTLS_ECDSA_MAX_SIG_LEN(ctx->grp.pbits)` bytes long. + * MBEDTLS_ECDSA_MAX_LEN is always safe. * * \note If the bitlength of the message hash is larger than the * bitlength of the group order, then the hash is truncated as -- cgit v1.1