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piv: implement ECDH key agreement (tag 0x85)

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Add tag 0x85 parsing and ECDH shared secret computation for P-256 and
P-384 curves using mbedtls_ecdh APIs. The peer's uncompressed public key
is read from tag 0x85, the card's private key is loaded, and the shared
secret is returned in tag 0x82 within a 0x7C dynamic authentication
template, per NIST SP 800-73-4.

Unsupported algorithms (e.g. X25519, RSA) return SW_INCORRECT_P1P2."
This commit is contained in:
Sammy ETUR
2026-03-03 02:53:58 +01:00
parent c31ec29963
commit 450fd96381
1 changed files with 61 additions and 1 deletions
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src/openpgp/piv.c
+61 -1
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@@ -31,6 +31,7 @@
#include "asn1.h"
#include "mbedtls/aes.h"
#include "mbedtls/des.h"
#include "mbedtls/ecdh.h"
#include "mbedtls/x509_crt.h"
#include "openpgp.h"
@@ -606,10 +607,11 @@ static int cmd_authenticate() {
if (!asn1_find_tag(&ctxi, 0x7C, &a7c) || asn1_len(&ctxi) == 0) {
return SW_WRONG_DATA();
}
asn1_ctx_t a80 = { 0 }, a81 = { 0 }, a82 = { 0 };
asn1_ctx_t a80 = { 0 }, a81 = { 0 }, a82 = { 0 }, a85 = { 0 };
asn1_find_tag(&a7c, 0x80, &a80);
asn1_find_tag(&a7c, 0x81, &a81);
asn1_find_tag(&a7c, 0x82, &a82);
asn1_find_tag(&a7c, 0x85, &a85);
if (a80.data) {
if (a80.len == 0) {
memcpy(challenge, random_bytes_get(sizeof(challenge)), sizeof(challenge));
@@ -834,6 +836,64 @@ static int cmd_authenticate() {
}
}
}
if (a85.data) {
if (algo == PIV_ALGO_ECCP256 || algo == PIV_ALGO_ECCP384) {
file_t *ef_key = search_by_fid(key_ref, NULL, SPECIFY_EF);
if (!file_has_data(ef_key)) {
return SW_MEMORY_FAILURE();
}
mbedtls_ecdsa_context ecdsa_ctx;
mbedtls_ecdsa_init(&ecdsa_ctx);
int r = load_private_key_ecdsa(&ecdsa_ctx, ef_key, false);
if (r != PICOKEY_OK) {
mbedtls_ecdsa_free(&ecdsa_ctx);
return SW_EXEC_ERROR();
}
mbedtls_ecdh_context ecdh_ctx;
mbedtls_ecdh_init(&ecdh_ctx);
mbedtls_ecp_group_id gid = ecdsa_ctx.grp.id;
r = mbedtls_ecdh_setup(&ecdh_ctx, gid);
if (r != 0) {
mbedtls_ecdsa_free(&ecdsa_ctx);
mbedtls_ecdh_free(&ecdh_ctx);
return SW_EXEC_ERROR();
}
r = mbedtls_mpi_copy(&ecdh_ctx.ctx.mbed_ecdh.d, &ecdsa_ctx.d);
mbedtls_ecdsa_free(&ecdsa_ctx);
if (r != 0) {
mbedtls_ecdh_free(&ecdh_ctx);
return SW_EXEC_ERROR();
}
// mbedtls_ecdh_read_public expects TLS format: [length][point_data]
// PIV tag 0x85 contains raw uncompressed point: [04][X][Y]
uint8_t pub_buf[1 + MBEDTLS_ECP_MAX_PT_LEN];
pub_buf[0] = (uint8_t)a85.len;
memcpy(pub_buf + 1, a85.data, a85.len);
r = mbedtls_ecdh_read_public(&ecdh_ctx, pub_buf, a85.len + 1);
if (r != 0) {
mbedtls_ecdh_free(&ecdh_ctx);
return SW_DATA_INVALID();
}
size_t olen = 0;
uint8_t shared_secret[MBEDTLS_ECP_MAX_BYTES];
r = mbedtls_ecdh_calc_secret(&ecdh_ctx, &olen, shared_secret,
sizeof(shared_secret), random_gen, NULL);
mbedtls_ecdh_free(&ecdh_ctx);
if (r != 0) {
return SW_EXEC_ERROR();
}
res_APDU[res_APDU_size++] = 0x7C;
res_APDU[res_APDU_size++] = olen + 2;
res_APDU[res_APDU_size++] = 0x82;
res_APDU[res_APDU_size++] = olen;
memcpy(res_APDU + res_APDU_size, shared_secret, olen);
res_APDU_size += olen;
mbedtls_platform_zeroize(shared_secret, sizeof(shared_secret));
}
else {
return SW_INCORRECT_P1P2();
}
}
if (meta[1] == PINPOLICY_ALWAYS) {
has_pwpiv = false;
}