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DHM: add notes about leading zeros

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Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-04-13 22:10:24 +02:00
parent ebe9b6a51d
commit 03299dcf5b
2 changed files with 6 additions and 2 deletions
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6
library/dhm.c
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@ -223,7 +223,8 @@ int mbedtls_dhm_make_params( mbedtls_dhm_context *ctx, int x_size,
goto cleanup; goto cleanup;
/* /*
* export P, G, GX * Export P, G, GX. RFC 5246 §4.4 states that "leading zero octets are
* not required". We omit leading zeros for compactness.
*/ */
#define DHM_MPI_EXPORT( X, n ) \ #define DHM_MPI_EXPORT( X, n ) \
do { \ do { \
@ -436,8 +437,9 @@ int mbedtls_dhm_calc_secret( mbedtls_dhm_context *ctx,
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->K, &ctx->K, &ctx->P ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &ctx->K, &ctx->K, &ctx->P ) );
} }
/* Output the secret without any leading zero byte. This is mandatory
* for TLS per RFC 5246 §8.1.2. */
*olen = mbedtls_mpi_size( &ctx->K ); *olen = mbedtls_mpi_size( &ctx->K );
MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &ctx->K, output, *olen ) ); MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( &ctx->K, output, *olen ) );
cleanup: cleanup:

2
tests/suites/test_suite_dhm.function
View File

@ -16,6 +16,8 @@ static int check_dhm_param_output( const mbedtls_mpi *expected,
TEST_ASSERT( size >= *offset + 2 ); TEST_ASSERT( size >= *offset + 2 );
n = ( buffer[*offset] << 8 ) | buffer[*offset + 1]; n = ( buffer[*offset] << 8 ) | buffer[*offset + 1];
*offset += 2; *offset += 2;
/* The DHM param output from Mbed TLS has leading zeros stripped, as
* permitted but not required by RFC 5246 \S4.4. */
TEST_EQUAL( n, mbedtls_mpi_size( expected ) ); TEST_EQUAL( n, mbedtls_mpi_size( expected ) );
TEST_ASSERT( size >= *offset + n ); TEST_ASSERT( size >= *offset + n );
TEST_EQUAL( 0, mbedtls_mpi_read_binary( &actual, buffer + *offset, n ) ); TEST_EQUAL( 0, mbedtls_mpi_read_binary( &actual, buffer + *offset, n ) );