Add ssh signing script

.github/workflows/tests_scripts.yml

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+# documentation: https://help.github.com/en/articles/workflow-syntax-for-github-actions
+name: Tests for scripts
+on:
+  push:
+    paths:
+      - scripts/sign_verify_file_ssh.sh
+  pull_request:
+    paths:
+      - scripts/sign_verify_file_ssh.sh
+permissions:
+  contents: read # to fetch code (actions/checkout)
+jobs:
+  build:
+    runs-on: ubuntu-24.04
+    steps:
+    - name: checkout
+      uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
+
+    - name: test sign_verify_file_ssh.sh script
+      run: |
+          # Create a PEM format ssh identity
+          ssh-keygen -t rsa -b 4096 -m PEM -f id_rsa.pem -N ""
+          # Create a file to sign
+          echo "Very important stuff" > out.txt
+          export FILE_TO_SIGN="out.txt"
+          # Sign the file
+          ./scripts/sign_verify_file_ssh.sh sign id_rsa.pem "$FILE_TO_SIGN"
+          # Create an allowed_signers file based on the public key
+          echo -n "allowed_identity " > allowed_signers
+          cat id_rsa.pem.pub >> allowed_signers
+          # Verify the signature
+          ./scripts/sign_verify_file_ssh.sh verify allowed_signers "$FILE_TO_SIGN"
+          # Make a new signature that does not appear in the allowed signers file
+          ssh-keygen -t rsa -b 4096 -m PEM -f id_rsa.alt.pem -N ""
+          # Replace the allowed signers file
+          echo -n "disallowed_identity " > allowed_signers
+          cat id_rsa.alt.pem.pub >> allowed_signers
+          # Make sure signature checking fails in this case
+          ./scripts/sign_verify_file_ssh.sh verify allowed_signers "$FILE_TO_SIGN" && exit 1 || echo "Expected failure for unknown identity"

scripts/sign_verify_file_ssh.sh

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+#!/bin/bash
+#
+# SSH Signature Signing and Verification Script
+# - Sign a file using an SSH private key.
+# - Verify a signed file using an allowed signers file.
+#
+# Generates a signature file named `<file>.sig` in the same directory.
+#
+# Author: Alan O'Cais
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+# Usage message
+usage() {
+    cat <<EOF
+Usage:
+  $0 sign <private_key> <file>
+  $0 verify <allowed_signers_file> <file> [signature_file]
+
+Options:
+  sign:
+    - <private_key>: Path to SSH private key (use KEY_PASSPHRASE env for passphrase)
+    - <file>: File to sign
+
+  verify:
+    - <allowed_signers_file>: Path to the allowed signers file
+    - <file>: File to verify
+    - [signature_file]: Optional, defaults to '<file>.sig'
+
+Example allowed signers format:
+  identity_1 <public-key>
+EOF
+    exit 9
+}
+
+# Error codes
+FILE_PROBLEM=1
+CONVERSION_FAILURE=2
+VALIDATION_FAILED=3
+
+# Ensure minimum arguments
+[ "$#" -lt 3 ] && usage
+
+MODE="$1"
+FILE_TO_SIGN="$3"
+
+# Ensure the target file exists
+if [ ! -f "$FILE_TO_SIGN" ]; then
+    echo "Error: File '$FILE_TO_SIGN' not found."
+    exit $FILE_PROBLEM
+fi
+
+# Use a very conservatuve umask throughout this script since we are dealing with sensitive things
+umask 077 || { echo "Error: Failed to set 0177 umask."; exit $FILE_PROBLEM; }
+
+# Create a restricted temporary directory and ensure cleanup on exit
+TEMP_DIR=$(mktemp -d) || { echo "Error: Failed to create temporary directory."; exit $FILE_PROBLEM; }
+trap 'rm -rf "$TEMP_DIR"' EXIT
+
+# Converts the SSH private key to OpenSSH format and generates a public key
+convert_private_key() {
+    local input_key="$1"
+    local output_key="$2"
+
+    echo "Converting SSH key to OpenSSH format..."
+    cp "$input_key" "$output_key" || { echo "Error: Failed to copy $input_key to $output_key"; exit $FILE_PROBLEM; }
+
+    # This saves the key in the default OpenSSH format (which is required for signing)
+    ssh-keygen -p -f "$output_key" -P "${KEY_PASSPHRASE:-}" -N "${KEY_PASSPHRASE:-}" || {
+        echo "Error: Failed to convert key to OpenSSH format."
+        exit $CONVERSION_FAILURE
+    }
+
+    # Extract the public key from the private key
+    ssh-keygen -y -f "$input_key" -P "${KEY_PASSPHRASE:-}" > "${output_key}.pub" || {
+        echo "Error: Failed to extract public key."
+        exit $CONVERSION_FAILURE
+    }
+}
+
+# Sign mode
+if [ "$MODE" == "sign" ]; then
+    PRIVATE_KEY="$2"
+    TEMP_KEY="$TEMP_DIR/converted_key"
+    SIG_FILE="${FILE_TO_SIGN}.sig"
+
+    # Check for key and existing signature
+    [ ! -f "$PRIVATE_KEY" ] && { echo "Error: Private key not found."; exit $FILE_PROBLEM; }
+    [ -f "$SIG_FILE" ] && { echo "Error: Signature already exists. Remove to re-sign."; exit $FILE_PROBLEM; }
+
+    convert_private_key "$PRIVATE_KEY" "$TEMP_KEY"
+
+    echo "Signing the file..."
+    ssh-keygen -Y sign -f "$TEMP_KEY" -P "${KEY_PASSPHRASE:-}" -n file "$FILE_TO_SIGN"
+
+    [ ! -f "$SIG_FILE" ] && { echo "Error: Signing failed."; exit $FILE_PROBLEM; }
+    echo "Signature created: $SIG_FILE"
+
+    cat <<EOF
+
+For verification, your allowed signers file could contain:
+identity_1 $(cat "${TEMP_KEY}.pub")
+EOF
+
+    echo "Validating the signature..."
+    ssh-keygen -Y check-novalidate -n file -f "${TEMP_KEY}.pub" -s "$SIG_FILE" < "$FILE_TO_SIGN" || {
+        echo "Error: Signature validation failed."
+        exit $VALIDATION_FAILED
+    }
+
+# Verify mode
+elif [ "$MODE" == "verify" ]; then
+    ALLOWED_SIGNERS_FILE="$2"
+    SIG_FILE="${4:-${FILE_TO_SIGN}.sig}"
+
+    # Ensure required files exist
+    for file in "$ALLOWED_SIGNERS_FILE" "$SIG_FILE"; do
+        [ ! -f "$file" ] && { echo "Error: File '$file' not found."; exit $FILE_PROBLEM; }
+    done
+
+    echo "Verifying the signature against allowed signers..."
+
+    # Iterate through each principal in the allowed signers file
+    while IFS= read -r line || [[ -n "$line" ]]; do
+        [[ -z "$line" || "$line" == \#* ]] && continue
+
+        # Extract and process each principal
+        principals=$(echo "$line" | cut -d' ' -f1)
+        IFS=',' read -ra principal_list <<< "$principals"
+
+        for principal in "${principal_list[@]}"; do
+            echo "Checking principal: $principal"
+            if ssh-keygen -Y verify -f "$ALLOWED_SIGNERS_FILE" -n file -I "$principal" -s "$SIG_FILE" < "$FILE_TO_SIGN"; then
+                echo "Signature is valid for principal: $principal"
+                exit 0
+            fi
+        done
+    done < "$ALLOWED_SIGNERS_FILE"
+
+    echo "Error: No valid signature found."
+    exit $VALIDATION_FAILED
+
+else
+    usage
+fi