Skip to content

cred: support the hmac-secret-mc extension #909

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 7 commits into from
Nov 4, 2025
Merged

cred: support the hmac-secret-mc extension #909

Show file tree
Hide file tree
Changes from 1 commit
Commits
Show all changes
7 commits
Select commit Hold shift + click to select a range
f81ef57
cred: split extension inputs and outputs
LDVG Oct 27, 2025
6a75083
cred: move fido_do_ecdh() up to top level call
LDVG Oct 27, 2025
4b4d137
cbor: move definition of cbor_encode_hmac_secret_param()
LDVG Oct 27, 2025
d84a7bd
cbor: add key parameter to cbor_encode_hmac_secret_param()
LDVG Oct 30, 2025
45f6236
cred: support the hmac-secret-mc extension
LDVG Oct 27, 2025
46ca742
fuzz: exercise hmac-secret-mc
LDVG Oct 27, 2025
5a9548a
man: document hmac-secret-mc
LDVG Oct 27, 2025
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
18 changes: 17 additions & 1 deletion src/cbor.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -685,7 +685,8 @@ cbor_encode_hmac_secret_param(const char *key, const fido_dev_t *dev,
}

cbor_item_t *
cbor_encode_cred_ext(const fido_cred_extin_t *ext)
cbor_encode_cred_ext(const fido_dev_t *dev, const fido_cred_extin_t *ext,
const fido_blob_t *ecdh, const es256_pk_t *pk)
{
cbor_item_t *item = NULL;
size_t size = 0;
Expand All @@ -700,6 +701,8 @@ cbor_encode_cred_ext(const fido_cred_extin_t *ext)
size++;
if (ext->attr.mask & FIDO_EXT_MINPINLEN)
size++;
if (ext->attr.mask & FIDO_EXT_HMAC_SECRET_MC)
size++;

if (size == 0 || (item = cbor_new_definite_map(size)) == NULL)
return (NULL);
Expand Down Expand Up @@ -737,6 +740,13 @@ cbor_encode_cred_ext(const fido_cred_extin_t *ext)
return (NULL);
}
}
if (ext->attr.mask & FIDO_EXT_HMAC_SECRET_MC) {
if (cbor_encode_hmac_secret_param("hmac-secret-mc", dev, item,
ecdh, pk, &ext->hmac_salt) < 0) {
cbor_decref(&item);
return (NULL);
}
}

return (item);
}
Expand Down Expand Up @@ -1211,6 +1221,12 @@ decode_cred_extension(const cbor_item_t *key, const cbor_item_t *val, void *arg)
}
if (cbor_ctrl_value(val) == CBOR_CTRL_TRUE)
ext->attr.mask |= FIDO_EXT_HMAC_SECRET;
} else if (strcmp(type, "hmac-secret-mc") == 0) {
if (fido_blob_decode(val, &ext->hmac_secret_enc) < 0) {
fido_log_debug("%s: fido_blob_decode", __func__);
goto out;
}
ext->attr.mask |= FIDO_EXT_HMAC_SECRET_MC;
} else if (strcmp(type, "credProtect") == 0) {
if (cbor_isa_uint(val) == false ||
cbor_int_get_width(val) != CBOR_INT_8) {
Expand Down
59 changes: 57 additions & 2 deletions src/cred.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -81,7 +81,8 @@ fido_dev_make_cred_tx(fido_dev_t *dev, fido_cred_t *cred,

/* extensions */
if (cred->ext.attr.mask)
if ((argv[5] = cbor_encode_cred_ext(&cred->ext)) == NULL) {
if ((argv[5] = cbor_encode_cred_ext(dev, &cred->ext, ecdh,
pk)) == NULL) {
fido_log_debug("%s: cbor_encode_cred_ext", __func__);
r = FIDO_ERR_INTERNAL;
goto fail;
Expand Down Expand Up @@ -187,6 +188,14 @@ fido_dev_make_cred_wait(fido_dev_t *dev, fido_cred_t *cred,
return (FIDO_OK);
}

static int
decrypt_hmac_secret(const fido_dev_t *dev, fido_cred_t *cred,
const fido_blob_t *key)
{
return (aes256_cbc_dec(dev, key, &cred->authdata_ext.hmac_secret_enc,
&cred->hmac_secret));
}

int
fido_dev_make_cred(fido_dev_t *dev, fido_cred_t *cred, const char *pin)
{
Expand Down Expand Up @@ -214,14 +223,23 @@ fido_dev_make_cred(fido_dev_t *dev, fido_cred_t *cred, const char *pin)
}

if (pin != NULL || (cred->uv == FIDO_OPT_TRUE &&
fido_dev_supports_permissions(dev))) {
fido_dev_supports_permissions(dev)) ||
(cred->ext.attr.mask & FIDO_EXT_HMAC_SECRET_MC)) {
if ((r = fido_do_ecdh(dev, &pk, &ecdh, &ms)) != FIDO_OK) {
fido_log_debug("%s: fido_do_ecdh", __func__);
goto fail;
}
}

r = fido_dev_make_cred_wait(dev, cred, pk, ecdh, pin, &ms);
if (r == FIDO_OK && (cred->ext.attr.mask & FIDO_EXT_HMAC_SECRET_MC)) {
if (decrypt_hmac_secret(dev, cred, ecdh) < 0) {
fido_log_debug("%s: decrypt_hmac_secret", __func__);
r = FIDO_ERR_INTERNAL;
goto fail;
}
}

fail:
es256_pk_free(&pk);
fido_blob_free(&ecdh);
Expand Down Expand Up @@ -555,6 +573,7 @@ fido_cred_clean_authdata(fido_cred_t *cred)
fido_blob_reset(&cred->authdata_cbor);
fido_blob_reset(&cred->authdata_raw);
fido_blob_reset(&cred->attcred.id);
fido_blob_reset(&cred->authdata_ext.hmac_secret_enc);

memset(&cred->authdata_ext, 0, sizeof(cred->authdata_ext));
memset(&cred->authdata, 0, sizeof(cred->authdata));
Expand Down Expand Up @@ -589,6 +608,7 @@ fido_cred_reset_tx(fido_cred_t *cred)
fido_blob_reset(&cred->cdh);
fido_blob_reset(&cred->user.id);
fido_blob_reset(&cred->ext.blob);
fido_blob_reset(&cred->ext.hmac_salt);

free(cred->rp.id);
free(cred->rp.name);
Expand All @@ -612,6 +632,7 @@ fido_cred_reset_rx(fido_cred_t *cred)
{
fido_cred_clean_attobj(cred);
fido_blob_reset(&cred->largeblob_key);
fido_blob_reset(&cred->hmac_secret);
cred->ea.att = false;
}

Expand Down Expand Up @@ -962,6 +983,40 @@ fido_cred_set_user(fido_cred_t *cred, const unsigned char *user_id,
return (FIDO_ERR_INTERNAL);
}

int
fido_cred_set_hmac_salt(fido_cred_t *cred, const unsigned char *salt,
size_t salt_len)
{
if ((salt_len != 32 && salt_len != 64) ||
fido_blob_set(&cred->ext.hmac_salt, salt, salt_len) < 0)
return (FIDO_ERR_INVALID_ARGUMENT);

return (FIDO_OK);
}

int
fido_cred_set_hmac_secret(fido_cred_t *cred,
const unsigned char *secret, size_t secret_len)
{
if ((secret_len != 32 && secret_len != 64) ||
fido_blob_set(&cred->hmac_secret, secret, secret_len) < 0)
return (FIDO_ERR_INVALID_ARGUMENT);

return (FIDO_OK);
}

const unsigned char *
fido_cred_hmac_secret_ptr(const fido_cred_t *cred)
{
return (cred->hmac_secret.ptr);
}

size_t
fido_cred_hmac_secret_len(const fido_cred_t *cred)
{
return (cred->hmac_secret.len);
}

int
fido_cred_set_extensions(fido_cred_t *cred, int ext)
{
Expand Down
4 changes: 4 additions & 0 deletions src/export.gnu
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -147,6 +147,8 @@
fido_cred_entattest;
fido_cred_exclude;
fido_cred_flags;
fido_cred_hmac_secret_len;
fido_cred_hmac_secret_ptr;
fido_cred_largeblob_key_len;
fido_cred_largeblob_key_ptr;
fido_cred_sigcount;
Expand Down Expand Up @@ -193,6 +195,8 @@
fido_cred_set_entattest;
fido_cred_set_extensions;
fido_cred_set_fmt;
fido_cred_set_hmac_salt;
fido_cred_set_hmac_secret;
fido_cred_set_id;
fido_cred_set_options;
fido_cred_set_pin_minlen;
Expand Down
4 changes: 4 additions & 0 deletions src/export.llvm
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -145,6 +145,8 @@ _fido_cred_empty_exclude_list
_fido_cred_entattest
_fido_cred_exclude
_fido_cred_flags
_fido_cred_hmac_secret_len
_fido_cred_hmac_secret_ptr
_fido_cred_largeblob_key_len
_fido_cred_largeblob_key_ptr
_fido_cred_sigcount
Expand Down Expand Up @@ -191,6 +193,8 @@ _fido_cred_set_clientdata_hash
_fido_cred_set_entattest
_fido_cred_set_extensions
_fido_cred_set_fmt
_fido_cred_set_hmac_salt
_fido_cred_set_hmac_secret
_fido_cred_set_id
_fido_cred_set_options
_fido_cred_set_pin_minlen
Expand Down
4 changes: 4 additions & 0 deletions src/export.msvc
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -146,6 +146,8 @@ fido_cred_empty_exclude_list
fido_cred_entattest
fido_cred_exclude
fido_cred_flags
fido_cred_hmac_secret_len
fido_cred_hmac_secret_ptr
fido_cred_largeblob_key_len
fido_cred_largeblob_key_ptr
fido_cred_sigcount
Expand Down Expand Up @@ -192,6 +194,8 @@ fido_cred_set_clientdata_hash
fido_cred_set_entattest
fido_cred_set_extensions
fido_cred_set_fmt
fido_cred_set_hmac_salt
fido_cred_set_hmac_secret
fido_cred_set_id
fido_cred_set_options
fido_cred_set_pin_minlen
Expand Down
3 changes: 2 additions & 1 deletion src/extern.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,8 @@ cbor_item_t *cbor_flatten_vector(cbor_item_t **, size_t);
cbor_item_t *cbor_encode_assert_opt(fido_opt_t, fido_opt_t);
cbor_item_t *cbor_encode_change_pin_auth(const fido_dev_t *,
const fido_blob_t *, const fido_blob_t *, const fido_blob_t *);
cbor_item_t *cbor_encode_cred_ext(const fido_cred_extin_t *);
cbor_item_t *cbor_encode_cred_ext(const fido_dev_t *, const fido_cred_extin_t *,
const fido_blob_t *, const es256_pk_t *);
cbor_item_t *cbor_encode_assert_ext(fido_dev_t *,
const fido_assert_ext_t *, const fido_blob_t *, const es256_pk_t *);
cbor_item_t *cbor_encode_cred_opt(fido_opt_t, fido_opt_t);
Expand Down
4 changes: 4 additions & 0 deletions src/fido.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -123,6 +123,7 @@ const unsigned char *fido_cred_attstmt_ptr(const fido_cred_t *);
const unsigned char *fido_cred_authdata_ptr(const fido_cred_t *);
const unsigned char *fido_cred_authdata_raw_ptr(const fido_cred_t *);
const unsigned char *fido_cred_clientdata_hash_ptr(const fido_cred_t *);
const unsigned char *fido_cred_hmac_secret_ptr(const fido_cred_t *);
const unsigned char *fido_cred_id_ptr(const fido_cred_t *);
const unsigned char *fido_cred_largeblob_key_ptr(const fido_cred_t *);
const unsigned char *fido_cred_pubkey_ptr(const fido_cred_t *);
Expand Down Expand Up @@ -168,6 +169,8 @@ int fido_cred_set_clientdata_hash(fido_cred_t *, const unsigned char *, size_t);
int fido_cred_set_entattest(fido_cred_t *, int);
int fido_cred_set_extensions(fido_cred_t *, int);
int fido_cred_set_fmt(fido_cred_t *, const char *);
int fido_cred_set_hmac_salt(fido_cred_t *, const unsigned char *, size_t);
int fido_cred_set_hmac_secret(fido_cred_t *, const unsigned char *, size_t);
int fido_cred_set_id(fido_cred_t *, const unsigned char *, size_t);
int fido_cred_set_options(fido_cred_t *, bool, bool);
int fido_cred_set_pin_minlen(fido_cred_t *, size_t);
Expand Down Expand Up @@ -234,6 +237,7 @@ size_t fido_cred_attstmt_len(const fido_cred_t *);
size_t fido_cred_authdata_len(const fido_cred_t *);
size_t fido_cred_authdata_raw_len(const fido_cred_t *);
size_t fido_cred_clientdata_hash_len(const fido_cred_t *);
size_t fido_cred_hmac_secret_len(const fido_cred_t *);
size_t fido_cred_id_len(const fido_cred_t *);
size_t fido_cred_largeblob_key_len(const fido_cred_t *);
size_t fido_cred_pin_minlen(const fido_cred_t *);
Expand Down
5 changes: 3 additions & 2 deletions src/fido/param.h
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
/*
* Copyright (c) 2018-2024 Yubico AB. All rights reserved.
* Copyright (c) 2018-2025 Yubico AB. All rights reserved.
* SPDX-License-Identifier: BSD-2-Clause
*
* Redistribution and use in source and binary forms, with or without
Expand Down Expand Up @@ -131,6 +131,7 @@
#define FIDO_EXT_LARGEBLOB_KEY 0x04
#define FIDO_EXT_CRED_BLOB 0x08
#define FIDO_EXT_MINPINLEN 0x10
#define FIDO_EXT_HMAC_SECRET_MC 0x20

/* Supported credential protection policies. */
#define FIDO_CRED_PROT_UV_OPTIONAL 0x01
Expand All @@ -146,7 +147,7 @@
FIDO_EXT_CRED_BLOB)
#define FIDO_EXT_CRED_MASK (FIDO_EXT_HMAC_SECRET|FIDO_EXT_CRED_PROTECT| \
FIDO_EXT_LARGEBLOB_KEY|FIDO_EXT_CRED_BLOB| \
FIDO_EXT_MINPINLEN)
FIDO_EXT_MINPINLEN|FIDO_EXT_HMAC_SECRET_MC)
#endif /* _FIDO_INTERNAL */

/* Recognised UV modes. */
Expand Down
5 changes: 4 additions & 1 deletion src/fido/types.h
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -168,11 +168,13 @@ typedef struct fido_cred_ext {

typedef struct fido_cred_extin {
fido_cred_ext_t attr;
fido_blob_t blob; /* CTAP 2.1 credBlob */
fido_blob_t blob; /* CTAP 2.1 credBlob */
fido_blob_t hmac_salt; /* CTAP 2.2 hmac-secret-mc salt */
} fido_cred_extin_t;

typedef struct fido_cred_extout {
fido_cred_ext_t attr;
fido_blob_t hmac_secret_enc; /* CTAP 2.2 hmac-secret-mc, encrypted */
} fido_cred_extout_t;

typedef struct fido_cred_ea {
Expand All @@ -199,6 +201,7 @@ typedef struct fido_cred {
fido_attstmt_t attstmt; /* attestation statement (x509 + sig) */
fido_blob_t largeblob_key; /* decoded large blob key */
fido_cred_ea_t ea; /* enterprise attestation */
fido_blob_t hmac_secret; /* CTAP 2.2 hmac-secret-mc */
} fido_cred_t;

typedef struct fido_assert_extattr {
Expand Down