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0.1.7 Feb 14, 2022
0.1.3 Nov 14, 2021

#168 in WebAssembly

MIT license

120KB
2K SLoC

GitHub CI docs.rs crates.io

Wasmsign2

A tool and library for signing WebAssembly modules.

WASM signatures

Unlike typical desktop and mobile applications, WebAssembly binaries do not embed any kind of digital signatures to verify that they come from a trusted source, and haven't been tampered with.

Wasmsign2 takes an existing WebAssembly module, computes a signature for its content, and stores the signature in a custom section.

The resulting binary remains a standalone, valid WebAssembly module, but signatures can be verified prior to executing it.

Wasmsign2 is a proof of concept implementation of the WebAssembly modules signatures proposal. The file format is documented in the WebAssembly tool conventions repository.

The proposal, and this implementation, support domain-specific features such as:

  • The ability to have multiple signatures for a single module, with a compact representation
  • The ability to sign a module which was already signed with different keys
  • The ability to extend an existing module with additional custom sections, without invalidating existing signatures
  • The ability to verify multiple subsets of a module's sections with a single signature
  • The ability to turn an embedded signature into a detached one, and the other way round.

Installation

wasmsign2 is a Rust crate, that can be used in other applications.

See the API documentation for details.

It is also a CLI tool to perform common operations, whose usage is summarized below.

The tool requires the Rust compiler, and can be installed with the following command:

cargo install wasmsign2-cli

Usage

USAGE:
    wasmsign2 [FLAGS] [SUBCOMMAND]

FLAGS:
    -d               Print debugging information
    -h, --help       Prints help information
    -V, --version    Prints version information
    -v               Verbose output

SUBCOMMANDS:
    attach           Embed a detach signature into a module
    detach           Detach the signature from a module
    help             Prints this message or the help of the given
                     subcommand(s)
    keygen           Generate a new key pair
    show             Print the structure of a module
    sign             Sign a module
    split            Add cutting points to a module to enable partial
                     verification
    verify           Verify a module's signatures
    verify_matrix    Batch verification against multiple public keys

Inspecting a module

wasmsign2 show --input-file <input_file>

Example:

wasmsign2 show -i z.wasm

The -v switch prints additional details about signature data.

Creating a key pair

wasmsign2 keygen --public-key <public_key_file> --secret-key <secret_key_file>

-K, --public-key <public_key_file>    Public key file
-k, --secret-key <secret_key_file>    Secret key file

Example:

wasmsign2 keygen --public-key key.public --secret-key key.secret

Signing a WebAssembly module

wasmsign2 sign [OPTIONS] --input-file <input_file> --output-file <output_file> --secret-key <secret_key_file>

-i, --input-file <input_file>            Input file
-o, --output-file <output_file>          Output file
-K, --public-key <public_key_file>       Public key file
-k, --secret-key <secret_key_file>       Secret key file
-S, --signature-file <signature_file>    Signature file
-Z, --ssh                                Parse OpenSSH keys

Example:

wasmsign2 sign -i z.wasm -o z2.wasm -k secret.key

The public key is optional. It is only used to include a key identifier into the signature in order to speed up signature verification when a module includes multiple signatures made with different keys.

By default, signatures are assumed to be embedded in modules. Detached signatures can be provided with the optional --signature-file argument.

A module that was already signed can be signed with other keys, and can then be verified by any of the corresponding public keys.

Verifying a WebAssembly module

wasmsign2 verify [FLAGS] [OPTIONS] --input-file <input_file>

-i, --input-file <input_file>            Input file
-K, --public-key <public_key_file>       Public key file
-S, --signature-file <signature_file>    Signature file
-s, --split <regex>                      Custom section names to be verified
-G, --from-github <from_github>          GitHub account to retrieve public keys from
-Z, --ssh                                Parse OpenSSH keys

Example:

wasmsign2 verify -i z2.wasm -K public.key

The optional -s/--split parameter is documented in the "partial verification" section down below.

Verifying a WebAssembly module against multiple public keys

wasmsign2 verify_matrix [FLAGS] [OPTIONS] --input-file <input_file>

-i, --input-file <input_file>              Input file
-K, --public-keys <public_key_files>...    Public key files
-s, --split <regex>                        Custom section names to be verified
-G, --from-github <from_github>            GitHub account to retrieve public keys from
-Z, --ssh                                  Parse OpenSSH keys

The command verifies a module's signatures against multiple keys simultaneously, and reports the set of public keys for which a valid signature was found.

The optional -s/--split parameter is documented in the "partial verification" section down below.

Example:

wasmsign2 verify_matrix -i z2.wasm -K public.key -K public.key2

Detaching a signature from a module

wasmsign2 detach --input-file <input_file> --output-file <output_file> --signature-file <signature_file>

-i, --input-file <input_file>            Input file
-o, --output-file <output_file>          Output file
-S, --signature-file <signature_file>    Signature file

The command extracts and removes the signature from a module, and stores it in a distinct file.

Example:

wasmsign2 detach -i z2.wasm -o z3.wasm -S signature

Embedding a detached signature in a module

wasmsign2 attach --input-file <input_file> --output-file <output_file> --signature-file <signature_file>

-i, --input-file <input_file>            Input file
-o, --output-file <output_file>          Output file
-S, --signature-file <signature_file>    Signature file

The command embeds a detached signature into a module.

Example:

wasmsign2 attach -i z2.wasm -o z3.wasm -S signature

Partial verification

A signature can verify an entire module, but also one or more subsets of it.

This requires "cutting points" to be defined before the signature process. It is impossible to verify a signature beyond cutting point boudaries.

Cutting points can be added to a module with the split command:

wasmsign2 split [OPTIONS] --input-file <input_file> --output-file <output_file>

-i, --input-file <input_file>      Input file
-o, --output-file <output_file>    Output file
-s, --split <regex>                Custom section names to be signed

This adds cutting points so that it is possible to verify only the subset of custom sections whose name matches the regular expression, in addition to standard sections.

This command can be repeated, to add new cutting points to a module that was already prepared for partial verification.

Example:

wasmsign2 split -i z2.wasm -o z3.wasm -s '^.debug_'

The above command makes it possible to verify only the custom sections whose name starts with .debug_, even though the entire module was signed.

In order to do partial verification, the --split parameter is also available in the verification commands:

wasmsign2 verify -i z3.wasm -K public.key -s '^.debug_'
wasmsign2 verify_matrix -i z3.wasm -K public.key -K public.key2 -s '^.debug_'

OpenSSH keys support

In addition to the compact key format documented in the proposal, the API allows loading/saving public and secret keys with DER and PEM encoding.

OpenSSH keys can also be used by adding the --ssh flag to the sign, verify and verify_matrix commands, provided that they are Ed25519 (EdDSA) keys.

Examples:

wasmsign2 sign --ssh -k ~/.ssh/id_ed25519 -i z.wasm -o z2.wasm
wasmsign2 verify --ssh -K ~/.ssh/id_ed25519.pub -i z2.wasm

If a file contains more than a single public key, the verify_matrix command will check the signature against all discovered Ed25519 keys.

Public key sets from GitHub accounts can be downloaded at https://github.com/<account>.keys, replacing <account> with an actual GitHub account name.

Keys downloaded from such URL can be directly used to verify WebAssembly signatures.

GitHub integration

Public keys can also automatically be retrieved from GitHub accounts, using the --from-github parameter.

Examples:

wasmsign2 verify -G example_account -i z2.wasm
wasmsign2 matrix_verify -G example_account -i z2.wasm

Dependencies

~8–21MB
~310K SLoC