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[tls] Allow for NIST elliptic curve point formats

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The elliptic curve point representation for the x25519 curve includes
only the X value, since the curve is designed such that the Montgomery
ladder does not need to ever know or calculate a Y value.  There is no
curve point format byte: the public key data is simply the X value.
The pre-master secret is also simply the X value of the shared secret
curve point.

The point representation for the NIST curves includes both X and Y
values, and a single curve point format byte that must indicate that
the format is uncompressed.  The pre-master secret for the NIST curves
does not include both X and Y values: only the X value is used.

Extend the definition of an elliptic curve to allow the point size to
be specified separately from the key size, and extend the definition
of a TLS named curve to include an optional curve point format byte
and a pre-master secret length.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-01-21 15:29:05 +00:00
parent df7ec31766
commit c9291bc5c7
5 changed files with 40 additions and 11 deletions
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38
src/net/tls.c
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@@ -1671,6 +1671,9 @@ static int tls_send_client_key_exchange_ecdhe ( struct tls_connection *tls ) {
uint8_t public[0];
} __attribute__ (( packed )) *ecdh;
size_t param_len;
size_t pointsize;
size_t keysize;
size_t offset;
int rc;
/* Parse ServerKeyExchange record */
@@ -1706,9 +1709,13 @@ static int tls_send_client_key_exchange_ecdhe ( struct tls_connection *tls ) {
tls->server.exchange_len );
return -ENOTSUP_CURVE;
}
DBGC ( tls, "TLS %p using named curve %s\n", tls, curve->curve->name );
pointsize = curve->curve->pointsize;
keysize = curve->curve->keysize;
offset = ( curve->format ? 1 : 0 );
/* Check key length */
if ( ecdh->public_len != curve->curve->keysize ) {
if ( ecdh->public_len != ( offset + pointsize ) ) {
DBGC ( tls, "TLS %p invalid %s key\n",
tls, curve->curve->name );
DBGC_HDA ( tls, 0, tls->server.exchange,
@@ -1716,15 +1723,23 @@ static int tls_send_client_key_exchange_ecdhe ( struct tls_connection *tls ) {
return -EINVAL_KEY_EXCHANGE;
}
/* Check curve point format byte (if present) */
if ( curve->format && ( ecdh->public[0] != curve->format ) ) {
DBGC ( tls, "TLS %p invalid %s curve point format\n",
tls, curve->curve->name );
DBGC_HDA ( tls, 0, tls->server.exchange,
tls->server.exchange_len );
return -EINVAL_KEY_EXCHANGE;
}
/* Construct pre-master secret and ClientKeyExchange record */
{
size_t len = curve->curve->keysize;
uint8_t private[len];
uint8_t pre_master_secret[len];
uint8_t private[keysize];
uint8_t pre_master_secret[pointsize];
struct {
uint32_t type_length;
uint8_t public_len;
uint8_t public[len];
uint8_t public[ecdh->public_len];
} __attribute__ (( packed )) key_xchg;
/* Generate ephemeral private key */
@@ -1733,9 +1748,9 @@ static int tls_send_client_key_exchange_ecdhe ( struct tls_connection *tls ) {
return rc;
}
/* Calculate pre-master secret */
if ( ( rc = ecdhe_key ( curve->curve, ecdh->public,
private, key_xchg.public,
/* Exchange keys */
if ( ( rc = ecdhe_key ( curve->curve, ( ecdh->public + offset ),
private, ( key_xchg.public + offset ),
pre_master_secret ) ) != 0 ) {
DBGC ( tls, "TLS %p could not exchange ECDHE key: %s\n",
tls, strerror ( rc ) );
@@ -1743,14 +1758,17 @@ static int tls_send_client_key_exchange_ecdhe ( struct tls_connection *tls ) {
}
/* Generate master secret */
tls_generate_master_secret ( tls, pre_master_secret, len );
tls_generate_master_secret ( tls, pre_master_secret,
curve->pre_master_secret_len );
/* Generate Client Key Exchange record */
key_xchg.type_length =
( cpu_to_le32 ( TLS_CLIENT_KEY_EXCHANGE ) |
htonl ( sizeof ( key_xchg ) -
sizeof ( key_xchg.type_length ) ) );
key_xchg.public_len = len;
key_xchg.public_len = sizeof ( key_xchg.public );
if ( curve->format )
key_xchg.public[0] = curve->format;
/* Transmit Client Key Exchange record */
if ( ( rc = tls_send_handshake ( tls, &key_xchg,