diff --git a/.github/workflows/cargo_check.yml b/.github/workflows/cargo_check.yml
index 5681368..185b028 100644
--- a/.github/workflows/cargo_check.yml
+++ b/.github/workflows/cargo_check.yml
@@ -82,3 +82,9 @@ jobs:
         with:
           command: check
           args: --target thumbv7em-none-eabi --release --examples
+
+      - name: Check bootloader
+        uses: actions-rs/cargo@v1
+        with:
+          command: check
+          args: --manifest-path bootloader/Cargo.toml --target thumbv7em-none-eabi --release
diff --git a/.github/workflows/cargo_fmt.yml b/.github/workflows/cargo_fmt.yml
index b17f60c..e327f17 100644
--- a/.github/workflows/cargo_fmt.yml
+++ b/.github/workflows/cargo_fmt.yml
@@ -71,3 +71,9 @@ jobs:
         with:
           command: fmt
           args: --manifest-path tools/heapviz/Cargo.toml --all -- --check
+
+      - name: Cargo format bootloader
+        uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: --manifest-path bootloader/Cargo.toml --all -- --check
diff --git a/bootloader/.cargo/config b/bootloader/.cargo/config
new file mode 100644
index 0000000..524503b
--- /dev/null
+++ b/bootloader/.cargo/config
@@ -0,0 +1,2 @@
+[target.thumbv7em-none-eabi]
+linker = "arm-none-eabi-gcc"
diff --git a/bootloader/Cargo.toml b/bootloader/Cargo.toml
new file mode 100644
index 0000000..bebe244
--- /dev/null
+++ b/bootloader/Cargo.toml
@@ -0,0 +1,28 @@
+[package]
+name = "bootloader"
+version = "0.1.0"
+authors = [
+  "Fabian Kaczmarczyck <kaczmarczyck@google.com>",
+]
+build = "build.rs"
+license = "Apache-2.0"
+edition = "2018"
+
+[dependencies]
+byteorder = { version = "1", default-features = false }
+cortex-m = "^0.6.0"
+cortex-m-rt = "*"
+cortex-m-rt-macros = "*"
+panic-abort = "0.3.2"
+rtt-target = { version = "*", features = ["cortex-m"] }
+
+[profile.dev]
+panic = "abort"
+lto = true
+opt-level = 3
+
+[profile.release]
+panic = "abort"
+lto = true
+# Level "z" may decrease the binary size more of necessary.
+opt-level = 3
diff --git a/bootloader/README.md b/bootloader/README.md
new file mode 100644
index 0000000..ae4f510
--- /dev/null
+++ b/bootloader/README.md
@@ -0,0 +1,36 @@
+# OpenSK Bootloader
+
+This bootloader supports upgradability for OpenSK. Its functionality is to
+
+-   check images on A/B partitions,
+-   boot the most recent valid partition.
+
+## How to use
+
+The bootloader is built and deployed by OpenSK's `deploy.py`. If your board
+defines a metadata address, it is detected as an upgradable board and this
+bootloader is flashed to memory address 0.
+
+## How to debug
+
+The bootloader prints debug message over RTT when compiled in debug mode. Using
+`nrfjprog` for flashing and inspecting memory is recommended for debugging.
+
+```shell
+RUSTFLAGS="-C link-arg=-Wl,-Tlink.x -C link-arg=-nostartfiles" \
+    cargo build --target thumbv7em-none-eabi
+llvm-objcopy -O ihex target/thumbv7em-none-eabi/debug/bootloader \
+    target/thumbv7em-none-eabi/debug/bootloader.hex
+nrfjprog --program target/thumbv7em-none-eabi/debug/bootloader.hex \
+    --sectorerase -f nrf52 --reset
+```
+
+To read the debug messages, open two terminals for:
+
+```shell
+JLinkRTTLogger -device NRF52840_XXAA -if swd -speed 1000 -RTTchannel 0
+JLinkRTTClient
+```
+
+The first command also logs the output to a file. The second shows all output in
+real time.
diff --git a/bootloader/build.rs b/bootloader/build.rs
new file mode 100644
index 0000000..128eba6
--- /dev/null
+++ b/bootloader/build.rs
@@ -0,0 +1,17 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+fn main() {
+    println!("cargo:rerun-if-changed=memory.x");
+}
diff --git a/bootloader/memory.x b/bootloader/memory.x
new file mode 100644
index 0000000..82f87af
--- /dev/null
+++ b/bootloader/memory.x
@@ -0,0 +1,21 @@
+/* Copyright 2021 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+MEMORY
+{
+  FLASH (rx) : ORIGIN = 0x00000000, LENGTH = 16K
+  RAM (rwx) : ORIGIN = 0x20020000, LENGTH = 128K
+}
+
diff --git a/bootloader/rust-toolchain b/bootloader/rust-toolchain
new file mode 100644
index 0000000..645c2cb
--- /dev/null
+++ b/bootloader/rust-toolchain
@@ -0,0 +1,4 @@
+[toolchain]
+channel = "nightly-2021-03-25"
+components = ["clippy", "rustfmt"]
+targets = ["thumbv7em-none-eabi"]
diff --git a/bootloader/src/main.rs b/bootloader/src/main.rs
new file mode 100644
index 0000000..51bd03d
--- /dev/null
+++ b/bootloader/src/main.rs
@@ -0,0 +1,149 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#![no_main]
+#![no_std]
+
+extern crate cortex_m;
+extern crate cortex_m_rt as rt;
+
+use byteorder::{ByteOrder, LittleEndian};
+use core::convert::TryInto;
+use core::ptr;
+use cortex_m::asm;
+use panic_abort as _;
+use rt::entry;
+#[cfg(debug_assertions)]
+use rtt_target::{rprintln, rtt_init_print};
+
+/// Size of a flash page in bytes.
+const PAGE_SIZE: usize = 0x1000;
+
+/// A flash page.
+type Page = [u8; PAGE_SIZE];
+
+/// Reads a page of memory.
+unsafe fn read_page(address: usize) -> Page {
+    debug_assert!(address % PAGE_SIZE == 0);
+    let address_pointer = address as *const Page;
+    ptr::read(address_pointer)
+}
+
+/// Parsed metadata for a firmware partition.
+struct Metadata {
+    checksum: [u8; 32],
+    timestamp: u32,
+    address: u32,
+}
+
+impl Metadata {
+    pub const DATA_LEN: usize = 40;
+}
+
+/// Reads the metadata from a flash page.
+impl From<Page> for Metadata {
+    fn from(page: Page) -> Self {
+        Metadata {
+            checksum: page[0..32].try_into().unwrap(),
+            timestamp: LittleEndian::read_u32(&page[32..36]),
+            address: LittleEndian::read_u32(&page[36..Metadata::DATA_LEN]),
+        }
+    }
+}
+
+/// Location of a firmware partition's data.
+struct BootPartition {
+    firmware_address: usize,
+    metadata_address: usize,
+}
+
+impl BootPartition {
+    const _FIRMWARE_LENGTH: usize = 0x00040000;
+
+    /// Reads the metadata, returns the timestamp if all checks pass.
+    pub fn read_timestamp(&self) -> Result<u32, ()> {
+        let metadata_page = unsafe { read_page(self.metadata_address) };
+        let hash_value = self.compute_upgrade_hash(&metadata_page);
+        let metadata = Metadata::from(metadata_page);
+        if self.firmware_address != metadata.address as usize {
+            #[cfg(debug_assertions)]
+            rprintln!(
+                "Firmware address mismatch: expected 0x{:08X}, metadata 0x{:08X}",
+                self.firmware_address,
+                metadata.address as usize
+            );
+            return Err(());
+        }
+        if hash_value != metadata.checksum {
+            #[cfg(debug_assertions)]
+            rprintln!("Hash mismatch");
+            return Err(());
+        }
+        Ok(metadata.timestamp)
+    }
+
+    /// Placeholder for the SHA256 implementation.
+    ///
+    /// TODO implemented in next PR
+    /// Without it, the bootloader will never boot anything.
+    fn compute_upgrade_hash(&self, _metadata_page: &[u8]) -> [u8; 32] {
+        [0; 32]
+    }
+
+    /// Jump to the firmware.
+    pub fn boot(&self) -> ! {
+        let address = self.firmware_address;
+
+        #[cfg(debug_assertions)]
+        rprintln!("Boot jump to {:08X}", address);
+        let address_pointer = address as *const u32;
+        // https://docs.rs/cortex-m/0.7.2/cortex_m/asm/fn.bootload.html
+        unsafe { asm::bootload(address_pointer) };
+    }
+}
+
+#[entry]
+fn main() -> ! {
+    #[cfg(debug_assertions)]
+    rtt_init_print!();
+    #[cfg(debug_assertions)]
+    rprintln!("Starting bootloader");
+    let partition_a = BootPartition {
+        firmware_address: 0x20000,
+        metadata_address: 0x4000,
+    };
+    let partition_b = BootPartition {
+        firmware_address: 0x60000,
+        metadata_address: 0x5000,
+    };
+    #[cfg(debug_assertions)]
+    rprintln!("Reading partition A");
+    let timestamp_a = partition_a.read_timestamp();
+    #[cfg(debug_assertions)]
+    rprintln!("Reading partition B");
+    let timestamp_b = partition_b.read_timestamp();
+
+    match (timestamp_a, timestamp_b) {
+        (Ok(t1), Ok(t2)) => {
+            if t1 >= t2 {
+                partition_a.boot()
+            } else {
+                partition_b.boot()
+            }
+        }
+        (Ok(_), Err(_)) => partition_a.boot(),
+        (Err(_), Ok(_)) => partition_b.boot(),
+        (Err(_), Err(_)) => panic!(),
+    }
+}
diff --git a/deploy.py b/deploy.py
index de264ec..cd22078 100755
--- a/deploy.py
+++ b/deploy.py
@@ -384,6 +384,39 @@ class OpenSKInstaller:
     self._check_invariants()
     self._build_app_or_example(is_example=False)
 
+  def build_bootloader(self):
+    """Builds the upgrade bootloader."""
+    props = SUPPORTED_BOARDS[self.args.board]
+    info("Building bootloader")
+    rust_flags = [
+        f"--remap-path-prefix={os.getcwd()}=",
+        "-C",
+        "link-arg=-Wl,-Tlink.x",
+        "-C",
+        "link-arg=-nostartfiles",
+    ]
+    env = os.environ.copy()
+    env["RUSTFLAGS"] = " ".join(rust_flags)
+    cargo_command = ["cargo", "build", "--release", f"--target={props.arch}"]
+    self.checked_command(cargo_command, cwd="bootloader", env=env)
+    binary_path = os.path.join("target", props.arch, "release", "bootloader")
+    objcopy_command = [
+        "llvm-objcopy", "-O", "binary", binary_path, f"{binary_path}.bin"
+    ]
+    self.checked_command(objcopy_command, cwd="bootloader")
+
+  def flash_bootloader(self):
+    """Flashes the upgrade bootloader."""
+    props = SUPPORTED_BOARDS[self.args.board]
+    info("Flashing bootloader")
+    bin_file = os.path.join("bootloader", "target", props.arch, "release",
+                            "bootloader.bin")
+    if not os.path.exists(bin_file):
+      fatal(f"File not found: {bin_file}")
+    with open(bin_file, "rb") as bootloader_bin:
+      bootloader = bootloader_bin.read()
+    self.write_binary(bootloader, 0)
+
   def _build_app_or_example(self, is_example: bool):
     """Builds the application specified through args.
 
@@ -732,9 +765,13 @@ class OpenSKInstaller:
       return 0
 
     # Compile what needs to be compiled
+    board_props = SUPPORTED_BOARDS[self.args.board]
     if self.args.tockos:
       self.build_tockos()
 
+    if board_props.metadata_address is not None:
+      self.build_bootloader()
+
     if self.args.application == "ctap2":
       self.generate_crypto_materials(self.args.regenerate_keys)
       self.build_opensk()
@@ -748,7 +785,6 @@ class OpenSKInstaller:
       self.clear_storage()
 
     # Flashing
-    board_props = SUPPORTED_BOARDS[self.args.board]
     if self.args.programmer in ("jlink", "openocd"):
       # We rely on Tockloader to do the job
       if self.args.clear_apps:
@@ -756,6 +792,9 @@ class OpenSKInstaller:
       if self.args.tockos:
         # Install Tock OS
         self.install_tock_os()
+      if board_props.metadata_address is not None:
+        # Install the bootloader
+        self.flash_bootloader()
       # Install padding and application if needed
       if self.args.application:
         self.install_padding()
diff --git a/docs/boards/nrf52840dk.md b/docs/boards/nrf52840dk.md
index 5be25b3..6976442 100644
--- a/docs/boards/nrf52840dk.md
+++ b/docs/boards/nrf52840dk.md
@@ -48,3 +48,25 @@ There are 3 switches that need to be in the correct position:
 *   Power (bottom left): On
 *   nRF power source (center left): VDD
 *   SW6 (top right): DEFAULT
+
+### Upgradability
+
+There are variants of the board that introduce A/B partitions for upgrading the
+firmware. You can bootstrap an upgradable board using one of the two commands:
+
+```shell
+./deploy.py --board=nrf52840dk_opensk_a --opensk
+./deploy.py --board=nrf52840dk_opensk_b --opensk
+```
+
+Afterwards, you can upgrade the other partition with
+
+```shell
+./tools/perform_upgrade.sh nrf52840dk_opensk_b
+./tools/perform_upgrade.sh nrf52840dk_opensk_a
+```
+
+respectively. You can only upgrade the partition that is not currently running,
+so always alternate your calls to `perform_upgrade.sh`. Otherwise, this script
+works like `deploy.py`. You can call it even after you locked down your device,
+to deploy changes to your development board.
diff --git a/docs/install.md b/docs/install.md
index c7183bb..3a661bd 100644
--- a/docs/install.md
+++ b/docs/install.md
@@ -28,6 +28,7 @@ following:
 *   `nrfutil` (can be installed using `pip3 install nrfutil`) if you want to flash
     a device with DFU
 *   `uuid-runtime` if you are missing the `uuidgen` command.
+*   `llvm` if you want to use the upgradability feature.
 
 The proprietary software to use the default programmer can be found on the
 [Segger website](https://www.segger.com/downloads/jlink). Please follow their
@@ -149,3 +150,23 @@ If your board is already flashed with Tock OS, you may skip installing it:
 
 For more options, we invite you to read the help of our `deploy.py` script by
 running `./deploy.py --help`.
+
+### Upgradability
+
+We experiment with a new CTAP command to allow upgrading your device without
+access to its debugging port. For that purpose, the flash storage is split into
+4 parts:
+
+*   the bootloader to decide with partition to boot
+*   firmware partition A
+*   firmware partition B
+*   the persistent storage for credentials
+
+The storage is backward compatible to non-upgradable boards. Deploying an
+upgradable board automatically installs the bootloader. Please keep in mind that
+you have to safely store your private signing key for upgrades if you want to
+use this feature. For more information on the cryptographic material, see
+[Customization](customization.md).
+
+So far, upgradability is only supported for the development board. See the
+instructions on the [board specific page](boards/nrf52840dk.md).
diff --git a/run_desktop_tests.sh b/run_desktop_tests.sh
index 28fa740..3c0e9f4 100755
--- a/run_desktop_tests.sh
+++ b/run_desktop_tests.sh
@@ -29,6 +29,9 @@ cd ../..
 cd tools/heapviz
 cargo fmt --all -- --check
 cd ../..
+cd bootloader
+cargo fmt --all -- --check
+cd ..
 
 echo "Running Clippy lints..."
 cargo clippy --all-targets --features std -- -A clippy::new_without_default -D warnings
@@ -55,6 +58,11 @@ echo "Checking that examples build properly..."
 cargo check --release --target=thumbv7em-none-eabi --examples
 cargo check --release --target=thumbv7em-none-eabi --examples --features with_nfc
 
+echo "Checking that bootloader builds properly..."
+cd bootloader
+cargo check --release --target=thumbv7em-none-eabi
+cd ..
+
 echo "Checking that fuzz targets build properly..."
 cargo fuzz build
 cd libraries/cbor