Introduce hardware auth keys backed by TPM

Hardware keys are created and managed exclusively in the protected
environment of the Trusted Platform Module (TPM 2.0), a separate, secure
processor isolated from the rest of the system. Keyring provides them as
an alternative to established software keys for challenge-response
authentication.

Software keys, while far more secure than naive passwords, have their
limitations. They're stored in a keyring file in an encrypted form and
can be extracted and copied. As a result, it's possible to perform
unlimited attempts to decrypt them offline. To mitigate such attacks, a
memory-hard key derivation function must be used to derive the
decryption key, and the passhprase itself must still be sufficiently
strong because a small search space will definitely be exhausted. A side
effect of this is a massive latency spike and potentially disruptive
peak memory usage.

Hardware keys, by design, are non-exportable. Total system compromise
won't lead to key exfiltration due to TPM having no such functionality,
and TPM's tamper resistance makes extraction of secrets infeasible even
with physical access to the machine (if the TPM is genuine).

PIN that is used to protect hardware keys is validated by the TPM
itself, which also locks itself out after too many failed attempts. This
lockout cannot be overridden without either issuing a lockout clear
command with authorization value or resetting the TPM outright, erasing
all keys stored in it. Because of this, low-entropy secrets (such as
six-digit PIN) can provide sufficient security.

Their only limitation is the flipside of their strength: because they're
non-exportable, you can't back them up and move them between devices.
Once created, a hardware key is bound to the machine, unlike software
keys which are usable everywhere as long as you have keyring.cbor.

Hardware keys require TPM 2.0 and an API to communicate with it.
Implementations are provided for:

- Windows via Cryptography API: Next Generation (CNG) with Microsoft
  Platform Crypto Provider.

- Unix systems with TPM2 Software Stack (TSS2).

Windows scopes keys to a Windows user, so you likely won't be able to
move them across users or installations within the same machine. The
keys will remain on the system with "SoF_Auth_" prefix and a UUID so you
can locate them in your key registry. They'll additionally have the name
you set at key creation.

Unix might require additoinal user permissions to access the TPM. For
example, adding a user to the `tss` group on Linux.

The platform input differs. Windows can take a user-friendly key name to
display, which is a good feature considering weird requirement of
Windows that key names (actual identifiers) must be unique strings, for
which I use UUIDs. Unix has no concept of key names or identifiers, but
it has to provide PIN to the TPM directly.

Windows uses its own PIN prompt from Windows Security UI that's
disconnected from the application. This is somewhat awkward because it's
modeless. Also, a ridiciulous quirk of Windows CNG API makes it so key
handle creation returns the same `NTE_INVALID_HANDLE` error no matter
what kind of error it was. In particular, it's impossible to
differentiate between the operation failing due to the TPM lockout, or
the user voluntarily closing the dialog. The user will always see
"hardware locked out" error message. Brilliant API design.

The PIN is implemented as a direct authorization value for keys, so it
might be vulnerable to the bus sniffing attack if the PIN is traveling
in clear between CPU and TPM. Though, a hypothetical adversary who's
sitting with a logic analyzer hooked up to your motherboard as you type
the PIN will realistically have easier means to log your keystrokes.

TPM is capable of remote attestation to prove its authenticity, but I
chose to avoid it to protect users' privacy (there's anonymous
attestation, but it's not always practical because it requires special
CAs) and avoid significant implementation complexity on the server that
the attestation entails, such as parsing and validating certificate
chains.

Because of the unfortunate reality, TPMs overwhelmingly have no support
for X25519 (the key exchange algorithm used in software keys). It was
added in a recent revision, but it's yet to be implemented, and only on
the newest machines. You can't upgrade the TPM hardware, so we have to
compromise. Instead, elliptic curve Diffie-Hellman over P-256 curve has
been selected, which is also the default curve used in WebAuthn
(passkey) protocol. It's ubiquitous, supported by every single TPM 2.0,
and secure if reasonably implemented.

Keys don't take up limited nonvolatile memory of the TPM. Every
reference to TPM objects necessary to perform authentication is stored
on disk, and keys and contexts are recreated on every operation and
cleared from memory afterwards.

For the client public key format, I *only* use compressed P-256 points
(1-byte parity of y coordinate followed by a full 32-byte x coordinate)
for robustness. Their designated identification byte is 0x33, and they
start with the letter M when base64url-encoded.

4 files changed, 475 insertions, 0 deletions

diff --git a/src/tpm/tpm.h b/src/tpm/tpm.h
new file mode 100644
index 0000000..7531ae2
--- /dev/null
+++ b/src/tpm/tpm.h
@@ -0,0 +1,33 @@
+// Copyright 2026 Osmium Sorcerer
+// SPDX-License-Identifier: MIT
+
+#ifndef SOF_AO2CLIENT_TPM_TPM_H
+#define SOF_AO2CLIENT_TPM_TPM_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+typedef enum {
+    STKB_SUCCESS,
+    STKB_ERROR,
+    STKB_TPM_UNAVAILABLE,
+    STKB_BAD_PIN,
+    STKB_AUTH_FAILURE,
+    STKB_LOCKOUT,
+} stkb_rc;
+
+typedef struct {
+    const uint8_t *pin;
+    size_t pin_len;
+    const wchar_t *key_name;
+} stkb_user_input;
+
+stkb_rc tpm2_init(void);
+
+stkb_rc tpm2_keygen(uint8_t *out_pub, uint8_t *out_buffer, size_t buffer_size,
+                    size_t *written, stkb_user_input input);
+
+stkb_rc tpm2_compute_ss(uint8_t *ss, const uint8_t *blob, size_t blob_len,
+                        const uint8_t *pk, stkb_user_input input);
+
+#endif /* SOF_AO2CLIENT_TPM_TPM_H */
diff --git a/src/tpm/tpm_cng.c b/src/tpm/tpm_cng.c
new file mode 100644
index 0000000..1f764c7
--- /dev/null
+++ b/src/tpm/tpm_cng.c
@@ -0,0 +1,170 @@
+// Copyright 2026 Osmium Sorcerer
+// SPDX-License-Identifier: MIT
+
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
+#include <ncrypt.h>
+#include <rpc.h>
+
+#include "tpm.h"
+
+// Global handle for the Windows API.
+static NCRYPT_PROV_HANDLE provider;
+
+// 9 characters for prefix, 36 for random UUID, and don't forget that null terminator.
+// Note: this is count of wchar_t, not size.
+static const size_t keyname_len = 46;
+
+stkb_rc tpm2_init(void)
+{
+    // "Microsoft Platform Crypto Provider" is the TPM.
+    if (NCryptOpenStorageProvider(&provider, MS_PLATFORM_CRYPTO_PROVIDER, 0) != ERROR_SUCCESS)
+        return STKB_TPM_UNAVAILABLE;
+    if (NCryptIsAlgSupported(provider, BCRYPT_ECDH_P256_ALGORITHM, 0) != ERROR_SUCCESS)
+        return STKB_TPM_UNAVAILABLE;
+    return STKB_SUCCESS;
+}
+
+
+static int generate_keyname(wchar_t *out)
+{
+    // UUID seems like the way on Windows.
+    UUID id;
+    RPC_WSTR wstr = NULL;
+
+    if (UuidCreate(&id) != RPC_S_OK)
+        return 0;
+
+    if (UuidToStringW(&id, &wstr) != RPC_S_OK)
+        return 0;
+
+    wcscpy(out, L"SoF_Auth_");
+    wcscpy(out + 9, wstr);
+
+    RpcStringFreeW(&wstr);
+
+    return 1;
+}
+
+stkb_rc tpm2_keygen(uint8_t *out_pub, uint8_t *out_blob, size_t buffer_size, size_t *offset,
+                stkb_user_input input)
+{
+    NCRYPT_KEY_HANDLE key_handle = 0;
+    // Windows heavily abstracts the TPM and prevents you from acquiring the encrypted private structure. Instead, you
+    // have to use wide null-terminated strings as canonical key identifiers and refer to them by these names later.
+    // Yes, not even opaque byte arrays despite it handling "BLOBs."
+    wchar_t keyname[keyname_len];
+    if (sizeof(keyname) > buffer_size)
+        return STKB_ERROR;
+    if (!generate_keyname(keyname))
+        return STKB_ERROR;
+
+    // One might think, when NULL is passed instead of a key name, the key will not actually be persistent. But it
+    // becomes impossible to export the key to be loaded later, even in an encrypted form (while NCryptExportKey
+    // documents NCRYPT_OPAQUETRANSPORT_BLOB, explicitly saying "Opaque BLOBs are not transferable and must be imported
+    // by using the same CSP," and NCRYPT_PROTECTED_KEY_BLOB, confusingly, also doesn't export an opaque fixed-TPM
+    // restricted key blob. So we are forced to use a unique (potentially user-facing) name as an internal specifier.
+    if (NCryptCreatePersistedKey(provider, &key_handle, BCRYPT_ECDH_P256_ALGORITHM, keyname, 0, 0) != ERROR_SUCCESS)
+        return STKB_ERROR;
+
+    // Leave PINs unused, let the platform handle the input dialog.
+    NCRYPT_UI_POLICY ui_policy = {
+        .dwVersion = 1,
+        .dwFlags = NCRYPT_UI_FORCE_HIGH_PROTECTION_FLAG,
+        .pszFriendlyName = input.key_name,
+        .pszDescription = L"SoF authentication key",
+    };
+    stkb_rc ret = STKB_ERROR;
+    if (NCryptSetProperty(key_handle, NCRYPT_UI_POLICY_PROPERTY, (PBYTE)&ui_policy, sizeof(ui_policy), 0) != ERROR_SUCCESS)
+        goto exit;
+
+    // If the TPM is locked out, of all functions, FinalizeKey fails by returning NTE_INVALID_HANDLE.
+    // The other way it can happen (memory corruption aside) is if the user cancels the key creation
+    // dialog. You can't differentiate the two.
+    if (NCryptFinalizeKey(key_handle, 0) != ERROR_SUCCESS) {
+        ret = STKB_LOCKOUT;
+        goto exit;
+    }
+
+    BCRYPT_ECCKEY_BLOB ecc_blob = { 0 };
+    BYTE pub_buffer[sizeof(ecc_blob) + 32 * 2];
+    DWORD pub_buffer_off = 0;
+    if (NCryptExportKey(key_handle, 0, BCRYPT_ECCPUBLIC_BLOB, NULL, pub_buffer, sizeof(pub_buffer), &pub_buffer_off,
+                        0) != ERROR_SUCCESS)
+        goto exit;
+    memcpy(&ecc_blob, pub_buffer, sizeof(ecc_blob));
+    // Check sanity of the exported public key.
+    if (ecc_blob.dwMagic != BCRYPT_ECDH_PUBLIC_P256_MAGIC || ecc_blob.cbKey != 32)
+        goto exit;
+    // The x and y _should_ follow the blob (which is the beginning of the buffer) contiguously, each of cbKey (32)
+    // bytes.
+    memcpy(out_pub + 1, pub_buffer + sizeof(ecc_blob), 32);
+    // Encode parity of the last byte of y (both coordinates are big-endian).
+    if (pub_buffer[sizeof(ecc_blob) + 32 + 31] & 1)
+        out_pub[0] = 0x03;
+    else
+        out_pub[0] = 0x02;
+
+    memcpy(out_blob, keyname, sizeof(keyname));
+    *offset = sizeof(keyname);
+
+    ret = STKB_SUCCESS;
+
+exit:
+    NCryptFreeObject(key_handle);
+    return ret;
+}
+
+stkb_rc tpm2_compute_ss(uint8_t *ss, const uint8_t *blob, size_t blob_len, const uint8_t *pk, stkb_user_input input)
+{
+    (void)input;
+    // Because wchar_t is 16-byte-aligned, we can't cast a byte array, we have to do this redundant copy.
+    wchar_t keyname[keyname_len];
+    if (blob_len != sizeof(keyname))
+        return STKB_ERROR;
+    memcpy(keyname, blob, sizeof(keyname));
+    stkb_rc ret = STKB_ERROR;
+    NCRYPT_KEY_HANDLE key_handle = 0;
+    NCRYPT_KEY_HANDLE pk_handle = 0;
+    NCRYPT_SECRET_HANDLE shared_point = 0;
+    if (NCryptOpenKey(provider, &key_handle, keyname, 0, 0) != ERROR_SUCCESS)
+        goto exit;
+    // The API expects us to do the same ceremony to import the key in its format instead of using it directly.
+    BCRYPT_ECCKEY_BLOB ecc_blob = {
+        .dwMagic = BCRYPT_ECDH_PUBLIC_P256_MAGIC,
+        .cbKey = 32,
+    };
+    BYTE pub_buffer[sizeof(ecc_blob) + 32 * 2];
+    memcpy(pub_buffer, &ecc_blob, sizeof(ecc_blob));
+    memcpy(pub_buffer + sizeof(ecc_blob), pk, 64);
+    if (NCryptImportKey(provider, 0, BCRYPT_ECCPUBLIC_BLOB, NULL, &pk_handle, pub_buffer, sizeof(pub_buffer), 0) != ERROR_SUCCESS)
+        goto exit;
+    // Here it can also fail due to cancelation of dialog, too many failed attempts, or a TPM lockout.
+    if (NCryptSecretAgreement(key_handle, pk_handle, &shared_point, 0) != ERROR_SUCCESS) {
+        ret = STKB_LOCKOUT;
+        goto exit;
+    }
+    // You cannot extract the x coordinate of the shared point either, you have to derive the key. Even if you want to
+    // directly use the secret. But fine, apparently RAW_SECRET is a cryptographic key derivation function, which is
+    // some unrecognized cryptographic genius. We'll use it to get our affine big-endian x coordinate, the canonical
+    // ECDH shared secret. If you specify the size to be 32, it will only output the x coordinate, after all, that's
+    // what a point is: 32 bytes of x, then 32 bytes of y, no padding, no leading bytes, no headers. Right?
+    BYTE secret[32];
+    DWORD written = 0;
+    if (NCryptDeriveKey(shared_point, BCRYPT_KDF_RAW_SECRET, NULL, secret, sizeof(secret), &written, 0) != ERROR_SUCCESS)
+        goto exit;
+    // Of course it wouldn't be that easy. Even if we guessed the behavior of this function with respect to the buffer
+    // size you pass to it, CNG API decides that it's a good idea to output point coordinates in little-endian order,
+    // opposite of how the standard defines it (and what every other implementation correctly does, including the
+    // Platform Crypto backend that CNG relies on, and CNG's own NCryptExportKey).
+    for (size_t i = 0; i < sizeof(secret); ++i)
+        ss[i] = secret[sizeof(secret) - 1 - i];
+
+    ret = STKB_SUCCESS;
+
+exit:
+    NCryptFreeObject(key_handle);
+    NCryptFreeObject(pk_handle);
+    NCryptFreeObject(shared_point);
+    return ret;
+}
diff --git a/src/tpm/tpm_stub.c b/src/tpm/tpm_stub.c
new file mode 100644
index 0000000..4e2d2c0
--- /dev/null
+++ b/src/tpm/tpm_stub.c
@@ -0,0 +1,28 @@
+#include "tpm.h"
+
+stkb_rc tpm2_init(void)
+{
+    return STKB_TPM_UNAVAILABLE;
+}
+
+stkb_rc tpm2_keygen(uint8_t *out_pub, uint8_t *out_buffer, size_t buffer_size,
+                    size_t *written, stkb_user_input input)
+{
+    (void)out_pub;
+    (void)out_buffer;
+    (void)buffer_size;
+    (void)written;
+    (void)input;
+    return STKB_TPM_UNAVAILABLE;
+}
+
+stkb_rc tpm2_compute_ss(uint8_t *ss, const uint8_t *blob, size_t blob_len,
+                        const uint8_t *pk, stkb_user_input input)
+{
+    (void)ss;
+    (void)blob;
+    (void)blob_len;
+    (void)pk;
+    (void)input;
+    return STKB_TPM_UNAVAILABLE;
+}
diff --git a/src/tpm/tpm_tss.c b/src/tpm/tpm_tss.c
new file mode 100644
index 0000000..880fa32
--- /dev/null
+++ b/src/tpm/tpm_tss.c
@@ -0,0 +1,244 @@
+// Copyright 2026 Osmium Sorcerer
+// SPDX-License-Identifier: MIT
+
+#include <string.h>
+
+#include <tss2/tss2_esys.h>
+#include <tss2/tss2_mu.h>
+
+#include "tpm.h"
+
+// Storage Key: asymmetric scheme is NULL, symmetric is defined and shall
+// use CFB mode. The key is restricted.
+static const TPM2B_PUBLIC primary_template = {
+    .publicArea = {
+        .type = TPM2_ALG_ECC,
+        .nameAlg = TPM2_ALG_SHA256,
+        .objectAttributes =
+            TPMA_OBJECT_FIXEDTPM |
+            TPMA_OBJECT_FIXEDPARENT |
+            TPMA_OBJECT_SENSITIVEDATAORIGIN |
+            TPMA_OBJECT_USERWITHAUTH |
+            TPMA_OBJECT_RESTRICTED |
+            TPMA_OBJECT_DECRYPT,
+        .parameters.eccDetail = {
+            .symmetric = {
+                .algorithm = TPM2_ALG_AES,
+                .keyBits.aes = 128,
+                .mode.aes = TPM2_ALG_CFB,
+            },
+            .scheme.scheme = TPM2_ALG_NULL,
+            .curveID = TPM2_ECC_NIST_P256,
+            .kdf.scheme = TPM2_ALG_NULL,
+        },
+    },
+};
+
+// Symmetric algorithm is NULL for a nonrestricted key. This is the template
+// for actual key exchange keys under the primary storage key, accessed with a
+// user authentication value.
+// There is TPM2_ALG_ECDH scheme, but it requires KDF.
+static const TPM2B_PUBLIC ecdh_key_template = {
+    .publicArea = {
+        .type = TPM2_ALG_ECC,
+        .nameAlg = TPM2_ALG_SHA256,
+        .objectAttributes =
+            TPMA_OBJECT_FIXEDTPM |
+            TPMA_OBJECT_FIXEDPARENT |
+            TPMA_OBJECT_SENSITIVEDATAORIGIN |
+            TPMA_OBJECT_USERWITHAUTH |
+            TPMA_OBJECT_DECRYPT,
+        .parameters.eccDetail = {
+            .symmetric.algorithm = TPM2_ALG_NULL,
+            .scheme.scheme = TPM2_ALG_NULL,
+            .curveID = TPM2_ECC_NIST_P256,
+            .kdf.scheme = TPM2_ALG_NULL,
+        },
+    },
+};
+
+static const TPM2B_SENSITIVE_CREATE empty_in_sensitive = { 0 };
+static const TPM2B_DATA             empty_outside_info = { 0 };
+static const TPML_PCR_SELECTION     empty_creation_pcr = { 0 };
+
+// Make sure TPM 2.0 is present and working correctly before using it.
+stkb_rc tpm2_init(void)
+{
+    int           ret = 0;
+    ESYS_CONTEXT *ctx;
+    if (Esys_Initialize(&ctx, NULL, NULL) != TSS2_RC_SUCCESS)
+        return STKB_TPM_UNAVAILABLE;
+    // Simple self-test. Tests only what's necessary instead of all internal
+    // functions and operations, doesn't disrupt the system.
+    ret = Esys_SelfTest(ctx, ESYS_TR_NONE, ESYS_TR_NONE, ESYS_TR_NONE, TPM2_NO);
+    Esys_Finalize(&ctx);
+    if (ret != TSS2_RC_SUCCESS)
+        return STKB_TPM_UNAVAILABLE;
+    return STKB_SUCCESS;
+}
+
+static TSS2_RC tpm2_setup_primary(ESYS_CONTEXT **ctx, ESYS_TR *primary_handle)
+{
+    TSS2_RC rc = Esys_Initialize(ctx, NULL, NULL);
+    if (rc != TSS2_RC_SUCCESS)
+        return rc;
+    rc = Esys_CreatePrimary(*ctx, ESYS_TR_RH_OWNER, ESYS_TR_PASSWORD,
+                            ESYS_TR_NONE, ESYS_TR_NONE, &empty_in_sensitive,
+                            &primary_template, &empty_outside_info,
+                            &empty_creation_pcr, primary_handle, NULL, NULL,
+                            NULL, NULL);
+    if (rc != TSS2_RC_SUCCESS)
+        Esys_Finalize(ctx);
+    return rc;
+}
+
+static stkb_rc decode_tpm_rc(TSS2_RC tpm_rc)
+{
+    // Check that the code is from the TPM itself rather than ESAPI, otherwise
+    // return a generic error as we're not interested in details.
+    if ((tpm_rc & TSS2_RC_LAYER_MASK) != TSS2_TPM_RC_LAYER)
+        return STKB_ERROR;
+
+    // Format-One return codes are composite and in particular have an added
+    // parameter value that is irrelevant to the nature of the error, clear it.
+    if (tpm_rc & TPM2_RC_FMT1)
+        tpm_rc &= ~TPM2_RC_N_MASK;
+
+    switch (tpm_rc) {
+    case TPM2_RC_BAD_AUTH:
+    case TPM2_RC_AUTH_FAIL:
+        return STKB_AUTH_FAILURE;
+    case TPM2_RC_LOCKOUT:
+        return STKB_LOCKOUT;
+    default:
+        return STKB_ERROR;
+    }
+}
+
+stkb_rc tpm2_keygen(uint8_t *out_pub, uint8_t *out_buffer, size_t buffer_size,
+                    size_t *written, stkb_user_input input)
+{
+    if (input.pin_len > TPM2_SHA256_DIGEST_SIZE)
+        return STKB_BAD_PIN;
+    TPM2B_SENSITIVE_CREATE in_sensitive = { 0 };
+    memcpy(in_sensitive.sensitive.userAuth.buffer, input.pin, input.pin_len);
+    in_sensitive.sensitive.userAuth.size = (UINT16)input.pin_len;
+    ESYS_CONTEXT *ctx;
+    ESYS_TR       primary_handle;
+    stkb_rc       ret = STKB_ERROR;
+    if (tpm2_setup_primary(&ctx, &primary_handle) != TSS2_RC_SUCCESS)
+        return ret;
+    TPM2B_PRIVATE *ecdh_priv = NULL;
+    TPM2B_PUBLIC  *ecdh_pub  = NULL;
+    TSS2_RC rc = Esys_Create(ctx, primary_handle, ESYS_TR_PASSWORD,
+                             ESYS_TR_NONE, ESYS_TR_NONE, &in_sensitive,
+                             &ecdh_key_template, &empty_outside_info,
+                             &empty_creation_pcr, &ecdh_priv, &ecdh_pub, NULL,
+                             NULL, NULL);
+    if (rc != TSS2_RC_SUCCESS) {
+        ret = decode_tpm_rc(rc);
+        goto exit;
+    }
+
+    // TPM 2.0 Library, Part 1, 44.5.3 Padding:
+    //
+    // > In ECC points returned by the TPM, the x and y values, if non-empty,
+    // > are required to be the size of their associated curve (e.g., 32 bytes
+    // > for NIST P-256).
+    if (ecdh_pub->publicArea.unique.ecc.x.size != 32 ||
+        ecdh_pub->publicArea.unique.ecc.y.size != 32)
+        goto exit;
+
+    // Compress the public key: knowing x, it's enough to only encode parity of
+    // y to fully reconstruct the point.
+    if (ecdh_pub->publicArea.unique.ecc.y.buffer[31] & 1)
+        out_pub[0] = 0x03;
+    else
+        out_pub[0] = 0x02;
+    memcpy(out_pub + 1, ecdh_pub->publicArea.unique.ecc.x.buffer, 32);
+
+    // Serialize both structures, we'll need them for Load.
+    size_t off = 0;
+    if (Tss2_MU_TPM2B_PRIVATE_Marshal(ecdh_priv, out_buffer, buffer_size, &off) !=
+        TSS2_RC_SUCCESS)
+        goto exit;
+    if (Tss2_MU_TPM2B_PUBLIC_Marshal(ecdh_pub, out_buffer, buffer_size - off,
+                                     &off) != TSS2_RC_SUCCESS)
+        goto exit;
+
+    *written = off;
+    ret     = STKB_SUCCESS;
+
+exit:
+    Esys_Free(ecdh_priv);
+    Esys_Free(ecdh_pub);
+    Esys_FlushContext(ctx, primary_handle);
+    Esys_Finalize(&ctx);
+    return ret;
+}
+
+// pk is an uncompressed 64-byte point (x || y) of the peer.
+stkb_rc tpm2_compute_ss(uint8_t *ss, const uint8_t *blob, size_t blob_len,
+                        const uint8_t *pk, stkb_user_input input)
+{
+    if (input.pin_len > TPM2_SHA256_DIGEST_SIZE)
+        return STKB_BAD_PIN;
+    TPM2B_AUTH auth_value = { 0 };
+    memcpy(auth_value.buffer, input.pin, input.pin_len);
+    auth_value.size = (UINT16)input.pin_len;
+
+    ESYS_CONTEXT *ctx;
+    ESYS_TR       primary_handle;
+    stkb_rc       ret = STKB_ERROR;
+    if (tpm2_setup_primary(&ctx, &primary_handle) != TSS2_RC_SUCCESS)
+        return ret;
+
+    ESYS_TR          ecdh_key_handle = ESYS_TR_NONE;
+    TPM2B_PRIVATE    ecdh_priv;
+    TPM2B_PUBLIC     ecdh_pub;
+    TPM2B_ECC_POINT *shared_point = NULL;
+    size_t           off          = 0;
+    if (Tss2_MU_TPM2B_PRIVATE_Unmarshal(blob, blob_len, &off, &ecdh_priv) !=
+        TSS2_RC_SUCCESS)
+        goto exit;
+    if (Tss2_MU_TPM2B_PUBLIC_Unmarshal(blob, blob_len, &off, &ecdh_pub) !=
+        TSS2_RC_SUCCESS)
+        goto exit;
+
+    TSS2_RC rc = Esys_Load(ctx, primary_handle, ESYS_TR_PASSWORD, ESYS_TR_NONE,
+                           ESYS_TR_NONE, &ecdh_priv, &ecdh_pub,
+                           &ecdh_key_handle);
+    if (rc != TSS2_RC_SUCCESS) {
+        ret = decode_tpm_rc(rc);
+        goto exit;
+    }
+
+    if (Esys_TR_SetAuth(ctx, ecdh_key_handle, &auth_value) != TSS2_RC_SUCCESS)
+        goto exit;
+
+    TPM2B_ECC_POINT in_point;
+    in_point.point.x.size = 32;
+    in_point.point.y.size = 32;
+    memcpy(in_point.point.x.buffer, pk, 32);
+    memcpy(in_point.point.y.buffer, pk + 32, 32);
+    rc = Esys_ECDH_ZGen(ctx, ecdh_key_handle, ESYS_TR_PASSWORD, ESYS_TR_NONE,
+                        ESYS_TR_NONE, &in_point, &shared_point);
+    if (rc != TSS2_RC_SUCCESS) {
+        ret = decode_tpm_rc(rc);
+        goto exit;
+    }
+
+    // Deliver the x coordinate of the shared point.
+    memcpy(ss, shared_point->point.x.buffer, 32);
+
+    ret = STKB_SUCCESS;
+
+exit:
+    Esys_Free(shared_point);
+    if (primary_handle != ESYS_TR_NONE)
+        Esys_FlushContext(ctx, primary_handle);
+    if (ecdh_key_handle != ESYS_TR_NONE)
+        Esys_FlushContext(ctx, ecdh_key_handle);
+    Esys_Finalize(&ctx);
+    return ret;
+}