A tool to add and verify digital signatures to/from WASM binaries.
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 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 additional custom sections, without breaking 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.
It is also a CLI tool to perform common operations.
The tool requires the Rust compiler, and can be installed with the following command:
Usage
USAGE:
wasmsign2 [FLAGS] [SUBCOMMAND]
FLAGS:
-d Debug information
-h, --help Prints help information
-V, --version Prints version information
-v Verbose output
SUBCOMMANDS:
attach
detach
help Prints this message or the help of the given subcommand(s)
keygen
show
sign
split
verify
verify_matrix
Inspecting a module
wasmsign2 show --input-file <input_file>
Example:
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:
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
Example:
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 [OPTIONS] --input-file <input_file> --public-key <public_key_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
Example:
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 --input-file <input_file> --public-keys <public_key_files>...
-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
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:
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:
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:
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:
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: