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TinyCrypt PK Wrap: Remove dependency on ASN.1 writing

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The TinyCrypt PK signature wrapper uses ASN.1 writing functions
for length and tag, accounting for the only dependency of the
baremetal build on ASN.1 writing.

Since all lengths to be encoded are below 128 Bytes and are hence
ASN.1 encoded as single Bytes, the dependency on ASN.1 writing can
be removed at low complexity by writing the length and tags directly.
This commit is contained in:
Hanno Becker 2019-09-03 10:01:26 +01:00
parent 1b82685dc9
commit 96d34d57ec
2 changed files with 23 additions and 14 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

3
configs/baremetal.h
View file

@ -119,9 +119,6 @@
#define MBEDTLS_ASN1_PARSE_C
#define MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION
/* X.509 CSR writing */
#define MBEDTLS_ASN1_WRITE_C
/* RNG and PRNG */
#define MBEDTLS_NO_PLATFORM_ENTROPY
#define MBEDTLS_ENTROPY_C

34
library/pk_wrap.c
View file

@ -596,11 +596,13 @@ static int uecc_eckey_verify_wrap( void *ctx, mbedtls_md_type_t md_alg,
* p: pointer to the end of the output buffer
* start: start of the output buffer, and also of the mpi to write at the end
* n_len: length of the mpi to read from start
*
* Warning:
* The total length of the output buffer must be smaller than 128 Bytes.
*/
static int asn1_write_mpibuf( unsigned char **p, unsigned char *start,
size_t n_len )
{
int ret;
size_t len = 0;
if( (size_t)( *p - start ) < n_len )
@ -634,9 +636,11 @@ static int asn1_write_mpibuf( unsigned char **p, unsigned char *start,
len += 1;
}
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( p, start, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( p, start,
MBEDTLS_ASN1_INTEGER ) );
/* The ASN.1 length encoding is just a single Byte containing the length,
* as we assume that the total buffer length is smaller than 128 Bytes. */
*--(*p) = len;
*--(*p) = MBEDTLS_ASN1_INTEGER;
len += 2;
return( (int) len );
}
@ -648,6 +652,8 @@ static int asn1_write_mpibuf( unsigned char **p, unsigned char *start,
* [in/out] sig: the signature pre- and post-transcoding
* [in/out] sig_len: signature length pre- and post-transcoding
* [int] buf_len: the available size the in/out buffer
*
* Warning: buf_len must be smaller than 128 Bytes.
*/
static int pk_ecdsa_sig_asn1_from_uecc( unsigned char *sig, size_t *sig_len,
size_t buf_len )
@ -660,9 +666,11 @@ static int pk_ecdsa_sig_asn1_from_uecc( unsigned char *sig, size_t *sig_len,
MBEDTLS_ASN1_CHK_ADD( len, asn1_write_mpibuf( &p, sig + rs_len, rs_len ) );
MBEDTLS_ASN1_CHK_ADD( len, asn1_write_mpibuf( &p, sig, rs_len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_len( &p, sig, len ) );
MBEDTLS_ASN1_CHK_ADD( len, mbedtls_asn1_write_tag( &p, sig,
MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE ) );
/* The ASN.1 length encoding is just a single Byte containing the length,
* as we assume that the total buffer length is smaller than 128 Bytes. */
*--p = len;
*--p = MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE;
len += 2;
memmove( sig, p, len );
*sig_len = len;
@ -689,10 +697,11 @@ static int uecc_eckey_sign_wrap( void *ctx, mbedtls_md_type_t md_alg,
* Size is at most
* 1 (tag) + 1 (len) + 1 (initial 0) + NUM_ECC_BYTES for each of r and s,
* twice that + 1 (tag) + 2 (len) for the sequence
* (assuming NUM_ECC_BYTES is less than 126 for r and s,
* and less than 124 (total len <= 255) for the sequence)
*
* (The ASN.1 length encodings are all 1-Byte encodings because
* the total size is smaller than 128 Bytes).
*/
const size_t max_secp256r1_ecdsa_sig_len = 3 + 2 * ( 3 + NUM_ECC_BYTES );
#define MAX_SECP256R1_ECDSA_SIG_LEN ( 3 + 2 * ( 3 + NUM_ECC_BYTES ) )
uECC_sign( keypair->private_key, hash, hash_len, sig, uecc_curve );
*sig_len = 2 * NUM_ECC_BYTES;
@ -702,7 +711,10 @@ static int uecc_eckey_sign_wrap( void *ctx, mbedtls_md_type_t md_alg,
(void) p_rng;
(void) md_alg;
return( pk_ecdsa_sig_asn1_from_uecc( sig, sig_len, max_secp256r1_ecdsa_sig_len ) );
return( pk_ecdsa_sig_asn1_from_uecc( sig, sig_len,
MAX_SECP256R1_ECDSA_SIG_LEN ) );
#undef MAX_SECP256R1_ECDSA_SIG_LEN
}
static void *uecc_eckey_alloc_wrap( void )