I've only seen this once, but it's weird and I want to keep track of it.

CC a couple people who touched DBus recently.

I think it's this CHECK:

  switch (target_key_type) {
    case KEY_TYPE_SALTED_SHA256_TOP_HALF: {
    <snip>
    default:
      // The resulting key will be sent to cryptohomed. It should always be
      // hashed. If hashing fails, crash instead of sending a plain-text key.
      CHECK(false);
      return;

Test: desktopui_MashLogin.
Suite: bvt-perbuild.
Chrome Version: None.
Build: parrot-release/R63-9976.0.0.

Reason:
None.
build artifacts:
https://storage.cloud.google.com/?arg=chromeos-image-archive/parrot-release/R63-9976.0.0.
results log: http://ubercautotest.corp.google.com/tko/retrieve_logs.cgi?job=/results/144942634-chromeos-test/chromeos6-row2-rack4-host5/debug/.
status log: http://ubercautotest.corp.google.com/tko/retrieve_logs.cgi?job=/results/144942634-chromeos-test/chromeos6-row2-rack4-host5/status.log.
job link: http://cautotest-prod.corp.google.com/afe/#tab_id=view_job&object_id=144942634.

 0  chrome!chromeos::Key::Transform(chromeos::Key::KeyType, std::string const&) [key.cc : 81 + 0x1]
    rax = 0x00007fd9cefdc238   rdx = 0x00001beeeb70cb08
    rcx = 0x0000000000000000   rbx = 0x00001beeec8b5a28
    rsi = 0x0000000000000001   rdi = 0x00001beeec8b5a28
    rbp = 0x00007fff0009a4c0   rsp = 0x00007fff0009a440
     r8 = 0x00001beeeb70cb08    r9 = 0x00007fd9ce34b780
    r10 = 0x33392863632e7265   r11 = 0x0000000000000000
    r12 = 0x00001beeed430808   r13 = 0x00007fff0009a520
    r14 = 0x00001beeeb70cb08   r15 = 0x0000000000000001
    rip = 0x00007fd9d36c1ea3
    Found by: given as instruction pointer in context
 1  chrome!chromeos::(anonymous namespace)::OnGetSystemSalt(base::WeakPtr<chromeos::AuthAttemptState> const&, scoped_refptr<chromeos::CryptohomeAuthenticator>, bool, bool, std::string const&) [cryptohome_authenticator.cc : 293 + 0x10]
    rbx = 0x00001beeeb70cb08   rbp = 0x00007fff0009a510
    rsp = 0x00007fff0009a4d0   r12 = 0x00001beeed430808
    r13 = 0x00007fff0009a520   r14 = 0x0000000000000000
    r15 = 0x0000000000000000   rip = 0x00007fd9d36beb25
    Found by: call frame info
 2  chrome!base::internal::Invoker<base::internal::BindState<void (*)(base::WeakPtr<chromeos::AuthAttemptState> const&, scoped_refptr<chromeos::CryptohomeAuthenticator>, bool, bool, std::string const&), base::WeakPtr<chromeos::AuthAttemptState>, scoped_refptr<chromeos::CryptohomeAuthenticator>, bool, bool>, void (std::string const&)>::Run(base::internal::BindStateBase*, std::string const&) [bind_internal.h : 149 + 0xd]
    rbx = 0x00007fff0009a560   rbp = 0x00007fff0009a530
    rsp = 0x00007fff0009a520   r12 = 0x00001beeebc20ea0
    r13 = 0x00001beeeb70cb08   r14 = 0x00001beeeb701e10
    r15 = 0x00007fff0009a750   rip = 0x00007fd9d36bebf2
    Found by: call frame info
 3  chrome!chromeos::SystemSaltGetter::DidGetSystemSalt(base::RepeatingCallback<void (std::string const&)> const&, chromeos::DBusMethodCallStatus, std::vector<unsigned char, std::allocator<unsigned char> > const&) [callback.h : 92 + 0x6]
    rbx = 0x00007fff0009a560   rbp = 0x00007fff0009a710
    rsp = 0x00007fff0009a540   r12 = 0x00001beeebc20ea0
    r13 = 0x00001beeeb70cb08   r14 = 0x00001beeeb701e10
    r15 = 0x00007fff0009a750   rip = 0x00007fd9d368b985
    Found by: call frame info
 4  chrome!chromeos::(anonymous namespace)::CryptohomeClientImpl::OnGetSystemSalt(base::RepeatingCallback<void (chromeos::DBusMethodCallStatus, std::vector<unsigned char, std::allocator<unsigned char> > const&)> const&, dbus::Response*) [callback.h : 92 + 0x3]
    rbx = 0x00007fff0009a810   rbp = 0x00007fff0009a7d0
    rsp = 0x00007fff0009a720   r12 = 0x00001beeebc20ea0
    r13 = 0x00007fff0009a8d8   r14 = 0x00001beeeb701e10
    r15 = 0x00001beeeb9bd0d0   rip = 0x00007fd9d36a020e
    Found by: call frame info
 5  chrome!dbus::ObjectProxy::OnCallMethod(std::string const&, std::string const&, base::OnceCallback<void (dbus::Response*)>, dbus::Response*, dbus::ErrorResponse*) [callback.h : 64 + 0x3]
    rbx = 0x00007fff0009a810   rbp = 0x00007fff0009a8b0
    rsp = 0x00007fff0009a7e0   r12 = 0x00001beeebc20ea0
    r13 = 0x00007fff0009a8d8   r14 = 0x00001beeeb701e10
    r15 = 0x00001beeeb7aa2b0   rip = 0x00007fd9d37bc084
    Found by: call frame info