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Fix field element encoding for elliptic curve coefficients

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SEC 1, section 2.3.5, is explicit that the encoding of an element of the
field of definition for an elliptic curve needs to be a zero-padded octet
string whose length matches the byte size of the field's degree. So use
BN_bn2binpad() to fix this. Factor things into a simple helper to avoid
copy-pasting.

This gets rid of some of the most grotesque code in this file.

ok jsing
This commit is contained in:
tb
2024-10-14 12:50:18 +00:00
parent 5909419fb4
commit 22b55b0b9c
1 changed files with 36 additions and 45 deletions
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81
lib/libcrypto/ec/ec_asn1.c
View File

@@ -1,4 +1,4 @@
/* $OpenBSD: ec_asn1.c,v 1.70 2024/10/14 12:42:52 tb Exp $ */
/* $OpenBSD: ec_asn1.c,v 1.71 2024/10/14 12:50:18 tb Exp $ */
/*
* Written by Nils Larsch for the OpenSSL project.
*/
@@ -601,14 +601,40 @@ ec_asn1_group2fieldid(const EC_GROUP *group, X9_62_FIELDID *field)
return ret;
}
static int
ec_asn1_encode_field_element(const EC_GROUP *group, const BIGNUM *bn,
ASN1_OCTET_STRING *os)
{
unsigned char *buf;
int len;
int ret = 0;
/* Zero-pad field element per SEC 1, section 2.3.5. */
len = (EC_GROUP_get_degree(group) + 7) / 8;
/* One extra byte for historic NUL termination of ASN1_STRINGs. */
if ((buf = calloc(1, len + 1)) == NULL)
goto err;
if (BN_bn2binpad(bn, buf, len) != len)
goto err;
ASN1_STRING_set0(os, buf, len);
buf = NULL;
len = 0;
ret = 1;
err:
freezero(buf, len);
return ret;
}
static int
ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
{
BIGNUM *a = NULL, *b = NULL;
unsigned char *buffer_1 = NULL, *buffer_2 = NULL, *a_buf = NULL,
*b_buf = NULL;
size_t len_1, len_2;
unsigned char char_zero = 0;
int ret = 0;
if (!group || !curve || !curve->a || !curve->b)
@@ -619,50 +645,17 @@ ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
goto err;
}
/* get a and b */
if (!EC_GROUP_get_curve(group, NULL, a, b, NULL)) {
ECerror(ERR_R_EC_LIB);
goto err;
}
len_1 = (size_t) BN_num_bytes(a);
len_2 = (size_t) BN_num_bytes(b);
if (len_1 == 0) {
/* len_1 == 0 => a == 0 */
a_buf = &char_zero;
len_1 = 1;
} else {
if ((buffer_1 = malloc(len_1)) == NULL) {
ECerror(ERR_R_MALLOC_FAILURE);
goto err;
}
if ((len_1 = BN_bn2bin(a, buffer_1)) == 0) {
ECerror(ERR_R_BN_LIB);
goto err;
}
a_buf = buffer_1;
if (!ec_asn1_encode_field_element(group, a, curve->a)) {
ECerror(ERR_R_EC_LIB);
goto err;
}
if (len_2 == 0) {
/* len_2 == 0 => b == 0 */
b_buf = &char_zero;
len_2 = 1;
} else {
if ((buffer_2 = malloc(len_2)) == NULL) {
ECerror(ERR_R_MALLOC_FAILURE);
goto err;
}
if ((len_2 = BN_bn2bin(b, buffer_2)) == 0) {
ECerror(ERR_R_BN_LIB);
goto err;
}
b_buf = buffer_2;
}
/* set a and b */
if (!ASN1_STRING_set(curve->a, a_buf, len_1) ||
!ASN1_STRING_set(curve->b, b_buf, len_2)) {
ECerror(ERR_R_ASN1_LIB);
if (!ec_asn1_encode_field_element(group, b, curve->b)) {
ECerror(ERR_R_EC_LIB);
goto err;
}
@@ -688,8 +681,6 @@ ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
ret = 1;
err:
free(buffer_1);
free(buffer_2);
BN_free(a);
BN_free(b);