diff --git a/keymaster/4.0/support/Keymaster.cpp b/keymaster/4.0/support/Keymaster.cpp
index fac0017..066bca4 100644
--- a/keymaster/4.0/support/Keymaster.cpp
+++ b/keymaster/4.0/support/Keymaster.cpp
@@ -16,24 +16,73 @@
 
 #include <keymasterV4_0/Keymaster.h>
 
+#include <iomanip>
+
 #include <android-base/logging.h>
 #include <android/hidl/manager/1.0/IServiceManager.h>
 #include <keymasterV4_0/Keymaster3.h>
 #include <keymasterV4_0/Keymaster4.h>
+#include <keymasterV4_0/key_param_output.h>
+#include <keymasterV4_0/keymaster_utils.h>
 
 namespace android {
 namespace hardware {
+
+template <class T>
+std::ostream& operator<<(std::ostream& os, const hidl_vec<T>& vec) {
+    os << "{ ";
+    if (vec.size()) {
+        for (size_t i = 0; i < vec.size() - 1; ++i) os << vec[i] << ", ";
+        os << vec[vec.size() - 1];
+    }
+    os << " }";
+    return os;
+}
+
+std::ostream& operator<<(std::ostream& os, const hidl_vec<uint8_t>& vec) {
+    std::ios_base::fmtflags flags(os.flags());
+    os << std::setw(2) << std::setfill('0') << std::hex;
+    for (uint8_t c : vec) os << static_cast<int>(c);
+    os.flags(flags);
+    return os;
+}
+
+template <size_t N>
+std::ostream& operator<<(std::ostream& os, const hidl_array<uint8_t, N>& vec) {
+    std::ios_base::fmtflags flags(os.flags());
+    os << std::setw(2) << std::setfill('0') << std::hex;
+    for (size_t i = 0; i < N; ++i) os << static_cast<int>(vec[i]);
+    os.flags(flags);
+    return os;
+}
+
 namespace keymaster {
 namespace V4_0 {
+
+std::ostream& operator<<(std::ostream& os, const HmacSharingParameters& params) {
+    // Note that by design, although seed and nonce are used to compute a secret, they are
+    // not secrets and it's just fine to log them.
+    os << "(seed: " << params.seed << ", nonce: " << params.nonce << ')';
+    return os;
+}
+
 namespace support {
 
 using ::android::sp;
 using ::android::hidl::manager::V1_0::IServiceManager;
 
+std::ostream& operator<<(std::ostream& os, const Keymaster& keymaster) {
+    auto& version = keymaster.halVersion();
+    os << version.keymasterName << " from " << version.authorName
+       << " SecurityLevel: " << toString(version.securityLevel)
+       << " HAL: " << keymaster.descriptor() << "/" << keymaster.instanceName();
+    return os;
+}
+
 template <typename Wrapper>
 std::vector<std::unique_ptr<Keymaster>> enumerateDevices(
     const sp<IServiceManager>& serviceManager) {
-    std::vector<std::unique_ptr<Keymaster>> result;
+    Keymaster::KeymasterSet result;
 
     bool foundDefault = false;
     auto& descriptor = Wrapper::WrappedIKeymasterDevice::descriptor;
@@ -57,7 +106,7 @@
     return result;
 }
 
-std::vector<std::unique_ptr<Keymaster>> Keymaster::enumerateAvailableDevices() {
+Keymaster::KeymasterSet Keymaster::enumerateAvailableDevices() {
     auto serviceManager = IServiceManager::getService();
     CHECK(serviceManager) << "Could not retrieve ServiceManager";
 
@@ -73,18 +122,62 @@
 
     size_t i = 1;
     LOG(INFO) << "List of Keymaster HALs found:";
-    for (auto& hal : result) {
-        auto& version = hal->halVersion();
-        LOG(INFO) << "Keymaster HAL #" << i << ": " << version.keymasterName << " from "
-                  << version.authorName << " SecurityLevel: " << toString(version.securityLevel)
-                  << " HAL : " << hal->descriptor() << " instance " << hal->instanceName();
-    }
+    for (auto& hal : result) LOG(INFO) << "Keymaster HAL #" << i++ << ": " << *hal;
 
     return result;
 }
 
+static hidl_vec<HmacSharingParameters> getHmacParameters(
+    const Keymaster::KeymasterSet& keymasters) {
+    std::vector<HmacSharingParameters> params_vec;
+    params_vec.reserve(keymasters.size());
+    for (auto& keymaster : keymasters) {
+        if (keymaster->halVersion().majorVersion < 4) continue;
+        auto rc = keymaster->getHmacSharingParameters([&](auto error, auto& params) {
+            CHECK(error == ErrorCode::OK)
+                << "Failed to get HMAC parameters from " << *keymaster << " error " << error;
+            params_vec.push_back(params);
+        });
+        CHECK(rc.isOk()) << "Failed to communicate with " << *keymaster
+                         << " error: " << rc.description();
+    }
+    std::sort(params_vec.begin(), params_vec.end());
+
+    return params_vec;
+}
+
+static void computeHmac(const Keymaster::KeymasterSet& keymasters,
+                        const hidl_vec<HmacSharingParameters>& params) {
+    if (!params.size()) return;
+
+    hidl_vec<uint8_t> sharingCheck;
+    bool firstKeymaster = true;
+    LOG(DEBUG) << "Computing HMAC with params " << params;
+    for (auto& keymaster : keymasters) {
+        if (keymaster->halVersion().majorVersion < 4) continue;
+        LOG(DEBUG) << "Computing HMAC for " << *keymaster;
+        auto rc = keymaster->computeSharedHmac(params, [&](auto error, auto& curSharingCheck) {
+            CHECK(error == ErrorCode::OK)
+                << "Failed to get HMAC parameters from " << *keymaster << " error " << error;
+            if (firstKeymaster) {
+                sharingCheck = curSharingCheck;
+                firstKeymaster = false;
+            }
+            // TODO: Validate that curSharingCheck == sharingCheck.  b/77588764
+            // CHECK(curSharingCheck == sharingCheck) << "HMAC computation failed for " <<
+            // *keymaster;
+        });
+        CHECK(rc.isOk()) << "Failed to communicate with " << *keymaster
+                         << " error: " << rc.description();
+    }
+}
+
+void Keymaster::performHmacKeyAgreement(const KeymasterSet& keymasters) {
+    computeHmac(keymasters, getHmacParameters(keymasters));
+}
+
 }  // namespace support
 }  // namespace V4_0
 }  // namespace keymaster
 }  // namespace hardware
-};  // namespace android
+}  // namespace android
diff --git a/keymaster/4.0/support/include/keymasterV4_0/Keymaster.h b/keymaster/4.0/support/include/keymasterV4_0/Keymaster.h
index f9efd51..83b1d69 100644
--- a/keymaster/4.0/support/include/keymasterV4_0/Keymaster.h
+++ b/keymaster/4.0/support/include/keymasterV4_0/Keymaster.h
@@ -37,6 +37,8 @@
  */
 class Keymaster : public IKeymasterDevice {
    public:
+    using KeymasterSet = std::vector<std::unique_ptr<Keymaster>>;
+
     Keymaster(const hidl_string& descriptor, const hidl_string& instanceName)
         : descriptor_(descriptor), instanceName_(instanceName) {}
     virtual ~Keymaster() {}
@@ -55,21 +57,33 @@
         }
     };
 
-    virtual const VersionResult& halVersion() = 0;
-    const hidl_string& descriptor() { return descriptor_; }
-    const hidl_string& instanceName() { return instanceName_; }
+    virtual const VersionResult& halVersion() const = 0;
+    const hidl_string& descriptor() const { return descriptor_; }
+    const hidl_string& instanceName() const { return instanceName_; }
 
     /**
      * Returns all available Keymaster3 and Keymaster4 instances, in order of most secure to least
      * secure (as defined by VersionResult::operator<).
      */
-    static std::vector<std::unique_ptr<Keymaster>> enumerateAvailableDevices();
+    static KeymasterSet enumerateAvailableDevices();
+
+    /**
+     * Ask provided Keymaster instances to compute a shared HMAC key using
+     * getHmacSharingParameters() and computeSharedHmac().  This computation is idempotent as long
+     * as the same set of Keymaster instances is used each time (and if all of the instances work
+     * correctly).  It must be performed once per boot, but should do no harm to be repeated.
+     *
+     * If key agreement fails, this method will crash the process (with CHECK).
+     */
+    static void performHmacKeyAgreement(const KeymasterSet& keymasters);
 
    private:
     hidl_string descriptor_;
     hidl_string instanceName_;
 };
 
+std::ostream& operator<<(std::ostream& os, const Keymaster& keymaster);
+
 }  // namespace support
 }  // namespace V4_0
 }  // namespace keymaster
diff --git a/keymaster/4.0/support/include/keymasterV4_0/Keymaster3.h b/keymaster/4.0/support/include/keymasterV4_0/Keymaster3.h
index 2bb77ca..c40be7c 100644
--- a/keymaster/4.0/support/include/keymasterV4_0/Keymaster3.h
+++ b/keymaster/4.0/support/include/keymasterV4_0/Keymaster3.h
@@ -45,8 +45,8 @@
           km3_dev_(km3_dev),
           haveVersion_(false) {}
 
-    const VersionResult& halVersion() override {
-        getVersionIfNeeded();
+    const VersionResult& halVersion() const override {
+        const_cast<Keymaster3*>(this)->getVersionIfNeeded();
         return version_;
     }
 
diff --git a/keymaster/4.0/support/include/keymasterV4_0/Keymaster4.h b/keymaster/4.0/support/include/keymasterV4_0/Keymaster4.h
index 96afb13..dfd03ef 100644
--- a/keymaster/4.0/support/include/keymasterV4_0/Keymaster4.h
+++ b/keymaster/4.0/support/include/keymasterV4_0/Keymaster4.h
@@ -37,8 +37,8 @@
           haveVersion_(false),
           dev_(km4_dev) {}
 
-    const VersionResult& halVersion() override {
-        getVersionIfNeeded();
+    const VersionResult& halVersion() const override {
+        const_cast<Keymaster4*>(this)->getVersionIfNeeded();
         return version_;
     }
 
diff --git a/keymaster/4.0/support/include/keymasterV4_0/keymaster_utils.h b/keymaster/4.0/support/include/keymasterV4_0/keymaster_utils.h
index 1c1b000..90a0f1b 100644
--- a/keymaster/4.0/support/include/keymasterV4_0/keymaster_utils.h
+++ b/keymaster/4.0/support/include/keymasterV4_0/keymaster_utils.h
@@ -23,6 +23,14 @@
 namespace hardware {
 namespace keymaster {
 namespace V4_0 {
+
+/**
+ * Define a lexicographical ordering on HmacSharingParameters.  The parameters to
+ * IKeymasterDevice::computeSharedHmac are required to be delivered in the order specified by this
+ * comparison operator.
+ */
+bool operator<(const HmacSharingParameters& a, const HmacSharingParameters& b);
+
 namespace support {
 
 inline static hidl_vec<uint8_t> blob2hidlVec(const uint8_t* data, const size_t length,
diff --git a/keymaster/4.0/support/keymaster_utils.cpp b/keymaster/4.0/support/keymaster_utils.cpp
index bc610aa..729e1c1 100644
--- a/keymaster/4.0/support/keymaster_utils.cpp
+++ b/keymaster/4.0/support/keymaster_utils.cpp
@@ -19,8 +19,24 @@
 
 namespace android {
 namespace hardware {
+
+inline static bool operator<(const hidl_vec<uint8_t>& a, const hidl_vec<uint8_t>& b) {
+    return memcmp(a.data(), b.data(), std::min(a.size(), b.size())) == -1;
+}
+
+template <size_t SIZE>
+inline static bool operator<(const hidl_array<uint8_t, SIZE>& a,
+                             const hidl_array<uint8_t, SIZE>& b) {
+    return memcmp(a.data(), b.data(), SIZE) == -1;
+}
+
 namespace keymaster {
 namespace V4_0 {
+
+bool operator<(const HmacSharingParameters& a, const HmacSharingParameters& b) {
+    return std::tie(a.seed, a.nonce) < std::tie(b.seed, b.nonce);
+}
+
 namespace support {
 
 template <typename T, typename InIter>