README for sigtool

What is this?

sigtool is an opinionated tool to generate, sign and verify Ed25519 signatures on files. In many ways, it is like like OpenBSD's signify_ -- except written in Golang and definitely easier to use.

It can sign and verify very large files - it prehashes the files with SHA-512 and then signs the SHA-512 checksum.

All the artifacts produced by this tool are standard YAML files - thus, human readable.

How do I build it?

With Go 1.5 and later:

git clone https://github.com/opencoff/sigtool
cd sigtool
make

The binary will be in ./sigtool.

How do I use it?

Broadly, the tool can:

• generate new key pairs (public key and private key)
• sign a file
• verify a file against its signature

Generate Key pair

To start with, you generate a new key pair (a public key used for verification and a private key used for signing). e.g.,

sigtool gen /tmp/testkey

The tool then generates /tmp/testkey.pub and /tmp/testkey.key. The secret key (".key") can optionally be encrypted with a user supplied pass phrase - which the user has to enter via interactive prompt:

sigtool gen -p /tmp/testkey

Sign a file

Signing a file requires the user to provide a previously generated Ed25519 private key. The signature (YAML) is written to STDOUT. e.g., to sign archive.tar.gz with private key /tmp/testkey.key:

sigtool sign /tmp/testkey.key archive.tar.gz

If testkey.key was encrypted with a user pass phrase:

sigtool sign -p /tmp/testkey.key archive.tar.gz

The signature can also be written directly to a user supplied output file.

sigtool sign -p -o archive.sig /tmp/testkey.key archive.tar.gz

Verify a signature against a file

Verifying a signature of a file requires the user to supply three pieces of information:

• the Ed25519 public key to be used for verification
• the Ed25519 signature
• the file whose signature must be verified

e.g., to verify the signature of archive.tar.gz against testkey.pub using the signature archive.sig

sigtool verify /tmp/testkey.pub archive.sig archive.tar.gz

How is the private key protected?

The Ed25519 private key is encrypted using a key derived from the user supplied pass phrase. This pass phrase is used to derive an encryption key using the Scrypt key derivation algorithm. The resulting derived key is XOR'd with the Ed25519 private key before being committed to disk. To protect the integrity of the process, the essential parameters used for deriving the key, and the derived key are hashed via SHA256 and stored along with the encrypted key.

As an additional security measure, the user supplied pass phrase is hashed with SHA512.

Understanding the Code

src/sign is a library to generate, verify and store Ed25519 keys and signatures. It uses the extended library (golang.org/x/crypto) for the underlying operations.

The generated keys and signatures are proper YAML files and human readable.

The signature file contains a hash of the public key - so that at verification time, the right private key may be used (in situations where there are lots of keys).

Signatures on large files are calculated efficiently by reading them in memory mapped mode (mmap(2)) and hashing the file contents using SHA-512. The Ed25519 signature is calculated on the file-hash.

Example of Keys, Signature

Ed25519 Public Key

A serialized Ed25519 public key looks like so:

pk: uxpDh+gqXojAmxA/6vxZHzA+Uk+8wogUwvEhPBlWgvo=

Ed25519 Private Key

And, a serialized Ed25519 private key looks like so:

esk: t3vfqHbgUiA733KKPymFjWT8DdnBEkiMfsDHolPUdQWpvVn/F1Z4J6KYV3M5rGO9xgKxh5RAmqt+6LKgOiJAMQ==
salt: pPHKG55UJYtJ5wU0G9hBvNQJ0DvT0a7T4Fmj4aPB84s=
algo: scrypt-sha256
verify: JvjRjJMKhJhBmZngC3Pvq7x3KCLKt7gar1AAz7HB4qM=
Z: 131072
r: 16
p: 1

The Ed25519 private key is encrypted using Scrypt password hashing mechanism. A user supplied passphrase to protect the private key is first pre-hashed using SHA-512 before being used in scrypt(). In pseudo code, this operation looks like below:

passphrase = get_user_passphrase()
hpass      = SHA512(passphrase)
salt       = randombytes(32)
xorkey     = Scrypt(hpass, salt, N, r, p)
verify     = SHA256(salt, xorkey)
esk        = ed25519_private_key ^ xorkey

Where, N, r, p are Scrypt parameters. In our implementation:

N = 131072
r = 16
p = 1

verify is used during the decryption of the Ed25519 private key - before actually doing the "xor" operation. This check ensures that the supplied passphrase yields the same value as verify.

Ed25519 Signature

A generated signature looks like below after serialization:

comment: inpfile=/tmp/file.txt
pkhash: 36z9tCwTIVNwwDlExrB0SQ==
signature: ow2oBP+buDbEvlNakOrsxgB5Yc/7PYyPVZCkfyu7oahw8BakF4Qf32uswPaKGZ8RVz4uXboYHdZtfrEjCgP/Cg==

Here, ```pkhash`` is a SHA256 of the public key needed to verify this signature.

Licensing Terms

The tool and code is licensed under the terms of the GNU Public License v2.0 (strictly v2.0). If you need a commercial license or a different license, please get in touch with me.

See the file LICENSE.md for the full terms of the license.

Author

Sudhi Herle sw@herle.net

.. _signify: https://www.openbsd.org/papers/bsdcan-signify.html