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ECDSA: macro/function to calculate the signature size

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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.
This commit is contained in:
Gilles Peskine 2017-11-02 17:09:49 +01:00 committed by Andrzej Kurek
parent 373deea06d
commit 9a8bb67935
1 changed files with 37 additions and 6 deletions
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43
include/mbedtls/ecdsa.h
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@ -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