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Added a function that decompresses EC public keys #521

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Added a function that decompresses EC public keys #521

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c614219
added mbedtls_ecp_decompress_pubkey function
Jun 27, 2016
77cad20
Added mbedtls_ecp_decompress_pubkey declaration
Jun 27, 2016
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15 changes: 15 additions & 0 deletions include/mbedtls/ecp.h
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Original file line number Diff line number Diff line change
Expand Up @@ -653,6 +653,21 @@ int mbedtls_ecp_gen_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
*/
int mbedtls_ecp_check_pub_priv( const mbedtls_ecp_keypair *pub, const mbedtls_ecp_keypair *prv );

/**
* \brief Decompresses an EC Public Keypair
*
* \param grp ECP group
* \param input Compressed EC Public Key (Created using mbedtls_ecp_point_write_binary with format = MBEDTLS_ECP_PF_COMPRESSED)
* \param ilen Input length
* \param output Uncompressed EC Public Key
* \param olen Uncompressed EC Public Key output length
* \param osize Size of the output buffer
*
* \return 0 on success, or a specific error code.
*/
int mbedtls_ecp_decompress_pubkey( const mbedtls_ecp_group *grp, const unsigned char *input, size_t ilen,
unsigned char *output, size_t *olen, size_t osize );

#if defined(MBEDTLS_SELF_TEST)
/**
* \brief Checkup routine
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58 changes: 58 additions & 0 deletions library/ecp.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1958,6 +1958,64 @@ int mbedtls_ecp_check_pub_priv( const mbedtls_ecp_keypair *pub, const mbedtls_ec
return( ret );
}

/*
* Decompresses an EC Public Key
*/
int mbedtls_ecp_decompress_pubkey( const mbedtls_ecp_group *grp, const unsigned char *input, size_t ilen, unsigned char *output, size_t *olen, size_t osize ){
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This line length and the following indentation don't match our coding style.


int ret;
mbedtls_mpi x, x3, ax, z, zexp, y;
size_t plen;

plen = mbedtls_mpi_size( &grp->P );
*olen = 2 * plen + 1;

if( ilen != plen + 1 || input[0] == 0x04 )
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

if( osize < 1 )
return( 0 );

if( osize < *olen )
return( MBEDTLS_ERR_ECP_BUFFER_TOO_SMALL );

mbedtls_mpi_init( &x );
mbedtls_mpi_init( &x3 );
mbedtls_mpi_init( &ax );
mbedtls_mpi_init( &z );
mbedtls_mpi_init( &zexp );
mbedtls_mpi_init( &y );


MBEDTLS_MPI_CHK( mbedtls_mpi_read_binary( &x, input+1, ilen-1 ) ); // X point of the pubkey
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &x3, &x, &x ) ); // X^2
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &x3, &x3, &x ) ); // X^3
MBEDTLS_MPI_CHK( mbedtls_mpi_mul_mpi( &ax, &grp->A, &x ) ); // AX
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &z, &x3, &ax ) ); // X^3 + AX
MBEDTLS_MPI_CHK( mbedtls_mpi_add_mpi( &z, &z, &grp->B ) ); // X^3 + AX + B
MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &zexp, &grp->P) ); // Z exponent
MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( &zexp, &zexp, 1 ) ); // Z exponent + 1
MBEDTLS_MPI_CHK( mbedtls_mpi_div_int( &zexp, 0, &zexp, 4 ) ); // Z exponent / 4
MBEDTLS_MPI_CHK( mbedtls_mpi_exp_mod( &y, &z, &zexp, &grp->P, 0 ) ); // Z^Zexp % P
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This way of extracting square roots mod P only works for some Ps (those that are congruent to 3 mod 4), but will not work for all curves supported by Mbed TLS.

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@mpg what curves use Ps that are congruent to 3 mod 4? I would like to add that to my readme here: https://github.com/mwarning/mbedtls_ecp_compression

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It's easy to check:

#include "mbedtls/ecp.h"
#include <stdio.h>

int main(void)
{
    const mbedtls_ecp_curve_info *info;
    mbedtls_ecp_group grp;
    mbedtls_mpi_uint r;

    for( info = mbedtls_ecp_curve_list(); info->name != NULL; info++ )
    {
        mbedtls_ecp_group_init( &grp );
        mbedtls_ecp_group_load( &grp, info->grp_id );
        mbedtls_mpi_mod_int( &r, &grp.P, 4 );
        printf( "%s: p = %u mod 4\n", info->name, (unsigned) r );
        mbedtls_ecp_group_free( &grp );
    }
}

then

% make lib && gcc -Iinclude p-mod-4.c library/libmbedcrypto.a -o p-mod-4 && ./p-mod-4                                                                  git|development|
secp521r1: p = 3 mod 4
brainpoolP512r1: p = 3 mod 4
secp384r1: p = 3 mod 4
brainpoolP384r1: p = 3 mod 4
secp256r1: p = 3 mod 4
secp256k1: p = 3 mod 4
brainpoolP256r1: p = 3 mod 4
secp224r1: p = 1 mod 4
secp224k1: p = 1 mod 4
secp192r1: p = 3 mod 4
secp192k1: p = 3 mod 4

MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( &y, 0, input[0] == 0x03 ? 1 : 0 ) ); // Setting the correct sign

output[0] = 0x04; // Uncompressed format
memcpy( output+1, input+1, ilen-1 ); //0x04 + X
MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &y, (output+1+ilen-1), plen ) ); // 0x04 + X + Y

cleanup:
mbedtls_mpi_free( &x );
mbedtls_mpi_free( &x3 );
mbedtls_mpi_free( &ax );
mbedtls_mpi_free( &z );
mbedtls_mpi_free( &zexp );
mbedtls_mpi_free( &y );


return( ret );
}


#if defined(MBEDTLS_SELF_TEST)

/*
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