diff --git a/.github/workflows/audit.yml b/.github/workflows/audit.yml
index 786356e53..889f8d3dd 100644
--- a/.github/workflows/audit.yml
+++ b/.github/workflows/audit.yml
@@ -7,7 +7,7 @@ on:
 jobs:
   audit:
     name: Rust Audit
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
@@ -16,8 +16,7 @@ jobs:
 
       - name: Run rust-audit
         id: rust-audit
-        run: |
-          cargo audit --deny warnings
+        run: cargo audit --deny warnings
 
       - name: Notify on Slack
         uses: 8398a7/action-slack@v3
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 2f028be70..8080b611a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,17 +1,27 @@
+# You can find more information about the design decisions here:
+# https://github.com/xaynetwork/xayn_ai/pull/21
+
 name: Rust-CI
 
-on: 
+on:
   push:
 
 env:
   RUST_STABLE: 1.49.0
   RUST_NIGHTLY: nightly-2021-01-31
+  CARGO_NDK: 2.2.0
+  ANDROID_PLATFORM_VERSION: 21
+  CARGO_LIPO: 3.1.1
+  # cmd: shasum -a 256 cross
+  CROSS_CHECKSUM: 18a3599273773a7d1e333b413751f0adc1a83ec8045d060fee4f0639bd6baaf0
 
 jobs:
   registry-cache:
+    # we use the latest stable rustc + cargo version that is already installed on the image
+    # https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-README.md#rust-tools
     name: cargo-fetch
+    runs-on: ubuntu-20.04
     timeout-minutes: 5
-    runs-on: ubuntu-latest
     outputs:
       cache-key: ${{ steps.cache-key.outputs.key }}
       cache-date: ${{ steps.get-date.outputs.date }}
@@ -19,28 +29,20 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install stable toolchain
-        id: rust-toolchain
-        uses: actions-rs/toolchain@v1
-        with:
-          profile: minimal
-          toolchain: ${{ env.RUST_STABLE }}
-          default: true
-
       # We want to create a new cache after a week. Otherwise, the cache will
       # take up too much space by caching old dependencies
-      - name: Year + ISO week number
+      - name: Generate Year + ISO week number key
         id: get-date
-        run: echo "::set-output name=date::$(/bin/date -u "+%Y-%V")"
         shell: bash
+        run: echo "::set-output name=date::$(/bin/date -u "+%Y-%V")"
 
-      - name: Cache key
+      - name: Generate cargo registry cache key
         id: cache-key
-        run: echo "::set-output name=key::$(echo ${{ runner.os }}-cargo-registry-${{ steps.get-date.outputs.date }}-${{ hashFiles('**/Cargo.lock') }})"
         shell: bash
+        run: echo "::set-output name=key::$(echo ${{ runner.os }}-cargo-registry-${{ steps.get-date.outputs.date }}-${{ hashFiles('**/Cargo.lock') }})"
 
-      - name: Cache cargo registry
-        uses: actions/cache@v2
+      - name: Restore cargo registry ${{ steps.cache-key.outputs.key }} cache
+        uses: actions/cache@v2.1.4
         with:
           path: |
             ~/.cargo/registry
@@ -48,19 +50,19 @@ jobs:
           key: ${{ steps.cache-key.outputs.key }}
           restore-keys: ${{ runner.os }}-cargo-registry-${{ steps.get-date.outputs.date }}-
 
-      - name: cargo fetch
+      - name: Fetch dependencies
         run: cargo fetch
 
   format:
     name: cargo-fmt
     needs: registry-cache
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
-    runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install nightly toolchain
+      - name: Install ${{ env.RUST_NIGHTLY }} toolchain
         id: rust-toolchain
         uses: actions-rs/toolchain@v1
         with:
@@ -84,13 +86,13 @@ jobs:
   check:
     name: cargo-check
     needs: registry-cache
+    runs-on: ubuntu-20.04
     timeout-minutes: 20
-    runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install stable toolchain
+      - name: Install ${{ env.RUST_STABLE }} toolchain
         id: rust-toolchain
         uses: actions-rs/toolchain@v1
         with:
@@ -123,13 +125,13 @@ jobs:
   clippy:
     name: cargo-clippy
     needs: [registry-cache, check]
+    runs-on: ubuntu-20.04
     timeout-minutes: 20
-    runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install stable toolchain
+      - name: Install ${{ env.RUST_STABLE }} toolchain
         id: rust-toolchain
         uses: actions-rs/toolchain@v1
         with:
@@ -161,13 +163,13 @@ jobs:
   test:
     name: cargo-test
     needs: [registry-cache, check]
+    runs-on: ubuntu-20.04
     timeout-minutes: 20
-    runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install stable toolchain
+      - name: Install ${{ env.RUST_STABLE }} toolchain
         id: rust-toolchain
         uses: actions-rs/toolchain@v1
         with:
@@ -190,7 +192,6 @@ jobs:
           key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tests-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
           restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tests-${{ needs.registry-cache.outputs.cache-date }}-
 
-      # TODO: cache this as well to avoid frequent downloading
       - name: Download data
         run: sh download_data.sh
 
@@ -205,18 +206,19 @@ jobs:
   coverage:
     name: cargo-tarpaulin
     needs: [registry-cache, test]
+    runs-on: ubuntu-20.04
     timeout-minutes: 20
-    runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2
 
-      - name: Install stable toolchain
+      - name: Install ${{ env.RUST_STABLE }} toolchain
         id: rust-toolchain
         uses: actions-rs/toolchain@v1
         with:
           toolchain: ${{ env.RUST_STABLE }}
           profile: minimal
+          default: true
 
       - name: Use cached cargo registry
         uses: actions/cache@v2
@@ -233,7 +235,6 @@ jobs:
           key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tarpaulin-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
           restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-tarpaulin-${{ needs.registry-cache.outputs.cache-date }}-
 
-      # TODO: cache this as well to avoid frequent downloading
       - name: Download data
         run: sh download_data.sh
 
@@ -242,3 +243,127 @@ jobs:
         with:
           version: '0.16.0'
           args: '-v --all-features --force-clean --lib --ignore-tests --fail-under 70'
+
+  install-cargo-lipo:
+    # we use the latest stable rustc + cargo version that is already installed on the image
+    # https://github.com/actions/virtual-environments/blob/main/images/macos/macos-10.15-Readme.md#rust-tools
+    name: install-cargo-lipo
+    runs-on: macos-10.15
+    outputs:
+      cache-key: ${{ steps.cache-key.outputs.key }}
+    timeout-minutes: 15
+    steps:
+      - name: Generate cargo-lipo cache key
+        id: cache-key
+        shell: bash
+        run: echo "::set-output name=key::$(echo ${{ runner.os }}-cargo-lipo-bin-${{ env.CARGO_LIPO }})"
+
+      - name: Restore ${{ steps.cache-key.outputs.key }} cache
+        uses: actions/cache@v2.1.4
+        id: cargo-lipo-cache
+        with:
+          path: ~/.cargo/bin/cargo-lipo
+          key: ${{ steps.cache-key.outputs.key }}
+
+      - name: Install cargo-lipo ${{ env.CARGO_LIPO }}
+        if: ${{ !steps.cargo-lipo-cache.outputs.cache-hit }}
+        run: cargo install cargo-lipo --version ${{ env.CARGO_LIPO }}
+
+  test-android-libs:
+    name: test-android-libs
+    needs: [registry-cache, test]
+    runs-on: ubuntu-20.04
+    timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [aarch64-linux-android, armv7-linux-androideabi, i686-linux-android, x86_64-linux-android]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Install ${{ env.RUST_STABLE }} toolchain
+        id: rust-toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ env.RUST_STABLE }}
+          target: ${{ matrix.target }}
+          default: true
+
+      - name: Restore ${{ needs.registry-cache.outputs.cache-key }} cache
+        uses: actions/cache@v2.1.4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+          key: ${{ needs.registry-cache.outputs.cache-key }}
+
+      - name: Install Cross
+        shell: bash
+        run: |
+          wget -q -O - https://github.com/rust-embedded/cross/releases/download/v0.2.1/cross-v0.2.1-x86_64-unknown-linux-gnu.tar.gz | tar xvzf -
+          echo "${{ env.CROSS_CHECKSUM }} *cross" | shasum -c -
+          mv cross ~/.cargo/bin/
+
+      - name: Download data
+        run: sh download_data.sh
+
+      - name: Restore build artifacts
+        uses: actions/cache@v2.1.4
+        with:
+          path: ${{ github.workspace }}/target
+          key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-test-${{ matrix.target }}-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-test-${{ matrix.target }}-${{ needs.registry-cache.outputs.cache-date }}-
+
+      - name: Test Android library ${{ matrix.target }}
+        env:
+          RUSTFLAGS: "-C opt-level=3 -C debug-assertions=yes -D warnings"
+        run: cross test --target ${{ matrix.target }} --all-targets -- -Z unstable-options --report-time
+
+  build-ios-libs:
+    name: build-ios-libs
+    needs: [registry-cache, install-cargo-lipo, test]
+    runs-on: macos-10.15
+    timeout-minutes: 20
+    strategy:
+      fail-fast: false
+      matrix:
+        target: [aarch64-apple-ios, x86_64-apple-ios]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: Install ${{ env.RUST_STABLE }} toolchain with target ${{ matrix.target }}
+        id: rust-toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+          profile: minimal
+          toolchain: ${{ env.RUST_STABLE }}
+          target: ${{ matrix.target }}
+          default: true
+
+      - name: Restore ${{ needs.registry-cache.outputs.cache-key }} cache
+        uses: actions/cache@v2.1.4
+        with:
+          path: |
+            ~/.cargo/registry
+            ~/.cargo/git
+          key: ${{ needs.registry-cache.outputs.cache-key }}
+
+      - name: Restore ${{ needs.install-cargo-lipo.outputs.cache-key }} cache
+        uses: actions/cache@v2.1.4
+        with:
+          path: ~/.cargo/bin/cargo-lipo
+          key: ${{ needs.install-cargo-lipo.outputs.cache-key }}
+
+      - name: Restore build artifacts
+        uses: actions/cache@v2.1.4
+        with:
+          path: ${{ github.workspace }}/target
+          key: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-build-${{ matrix.target }}-${{ needs.registry-cache.outputs.cache-date }}-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo-${{ steps.rust-toolchain.outputs.rustc }}-build-${{ matrix.target }}-${{ needs.registry-cache.outputs.cache-date }}-
+
+      - name: Build iOS library ${{ matrix.target }}
+        working-directory: ./xayn-ai-ffi-c
+        run: cargo lipo --targets ${{ matrix.target }}
diff --git a/Cross.toml b/Cross.toml
new file mode 100644
index 000000000..1a4557b51
--- /dev/null
+++ b/Cross.toml
@@ -0,0 +1,2 @@
+[build.env]
+passthrough = ["RUSTFLAGS"]