replace-secret: Init

Add a small utility script which securely replaces secrets in
files. Doing this with `sed`, `replace-literal` or similar utilities
leaks the secrets through the spawned process' `/proc/<pid>/cmdline` file.

pkgs/build-support/replace-secret/replace-secret.nix

@@ -0,0 +1,35 @@
+{ stdenv, lib, python3 }:
+
+stdenv.mkDerivation {
+  name = "replace-secret";
+  buildInputs = [ python3 ];
+  phases = [ "installPhase" "checkPhase" ];
+  installPhase = ''
+    install -D ${./replace-secret.py} $out/bin/replace-secret
+    patchShebangs $out
+  '';
+  doCheck = true;
+  checkPhase = ''
+    install -m 0600 ${./test/input_file} long_test
+    $out/bin/replace-secret "replace this" ${./test/passwd} long_test
+    $out/bin/replace-secret "and this" ${./test/rsa} long_test
+    diff ${./test/expected_long_output} long_test
+
+    install -m 0600 ${./test/input_file} short_test
+    $out/bin/replace-secret "replace this" <(echo "a") short_test
+    $out/bin/replace-secret "and this" <(echo "b") short_test
+    diff ${./test/expected_short_output} short_test
+  '';
+  meta = with lib; {
+    platforms = platforms.all;
+    maintainers = with maintainers; [ talyz ];
+    license = licenses.mit;
+    description = "Replace a string in one file with a secret from a second file";
+    longDescription = ''
+      Replace a string in one file with a secret from a second file.
+
+      Since the secret is read from a file, it won't be leaked through
+      '/proc/<pid>/cmdline', unlike when 'sed' or 'replace' is used.
+    '';
+  };
+}

pkgs/build-support/replace-secret/replace-secret.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python
+
+import argparse
+from argparse import RawDescriptionHelpFormatter
+
+description = """
+Replace a string in one file with a secret from a second file.
+
+Since the secret is read from a file, it won't be leaked through
+'/proc/<pid>/cmdline', unlike when 'sed' or 'replace' is used.
+"""
+
+parser = argparse.ArgumentParser(
+    description=description,
+    formatter_class=RawDescriptionHelpFormatter
+)
+parser.add_argument("string_to_replace", help="the string to replace")
+parser.add_argument("secret_file", help="the file containing the secret")
+parser.add_argument("file", help="the file to perform the replacement on")
+args = parser.parse_args()
+
+with open(args.secret_file) as sf, open(args.file, 'r+') as f:
+    old = f.read()
+    secret = sf.read().strip("\n")
+    new_content = old.replace(args.string_to_replace, secret)
+    f.seek(0)
+    f.write(new_content)
+    f.truncate()

pkgs/build-support/replace-secret/test/expected_long_output

@@ -0,0 +1,30 @@
+beginning
+middle $6$UcbJUl5g$HRMfKNKsLTfVbcQb.P5o0bmZUfHDYkWseMSuZ8F5jSIGZZcI3Jnit23f8ZeZOGi4KL86HVM9RYqrpYySOu/fl0 not this
+-----BEGIN RSA PRIVATE KEY-----
+MIIEowIBAAKCAQEAzrru6v5tfwQl6L+rOUjtLo8kbhMUlCLXP7TYngSGrkzPMWe+
+0gB04UAmiPZXfBmvj5fPqYiFjIaEDHE/SD41vJB/RJKKtId2gCAIHhBLkbr+4+60
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+v1bjOJb/AA/yxvT8kLUtHQKBgGIA9iwqXJv/EfRNQytDdS0HQ4vHGtJZMr3YRVoa
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+cNtLYGD3vgwLmg6tTur4U60XN+tYDzWGteez8J9GwTMfKJmuS9af
+-----END RSA PRIVATE KEY-----
+end

pkgs/build-support/replace-secret/test/expected_short_output

@@ -0,0 +1,4 @@
+beginning
+middle a not this
+b
+end

pkgs/build-support/replace-secret/test/input_file

@@ -0,0 +1,4 @@
+beginning
+middle replace this not this
+and this
+end

pkgs/build-support/replace-secret/test/passwd

@@ -0,0 +1 @@
+$6$UcbJUl5g$HRMfKNKsLTfVbcQb.P5o0bmZUfHDYkWseMSuZ8F5jSIGZZcI3Jnit23f8ZeZOGi4KL86HVM9RYqrpYySOu/fl0

pkgs/build-support/replace-secret/test/rsa

@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIIEowIBAAKCAQEAzrru6v5tfwQl6L+rOUjtLo8kbhMUlCLXP7TYngSGrkzPMWe+
+0gB04UAmiPZXfBmvj5fPqYiFjIaEDHE/SD41vJB/RJKKtId2gCAIHhBLkbr+4+60
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+16oMF+spe+impEyeo1CsreJFghBQcb9o8qFjUPBiKvROBP0hLcscZ4BYy29HSBgo
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+ltjo9qE6kPXWcTBVckTKGFlntyCT283FC0/vMmHo8dTdtxF4/wSbkqs3ORuJ3p5J
+cNtLYGD3vgwLmg6tTur4U60XN+tYDzWGteez8J9GwTMfKJmuS9af
+-----END RSA PRIVATE KEY-----

pkgs/top-level/all-packages.nix

@@ -13592,6 +13592,8 @@ in
 
   remake = callPackage ../development/tools/build-managers/remake { };
 
+  replace-secret = callPackage ../build-support/replace-secret/replace-secret.nix { };
+
   replacement = callPackage ../development/tools/misc/replacement { };
 
   retdec = callPackage ../development/tools/analysis/retdec {