Signify - Ed25519 signatures for your files (in Rust)

(by )

From time to time I try to write a piece of code or port some existing library or application just for fun. So a while back in June I had some free time again and I came across signify. I ported it to rust: signify-rs

signify is a small command line utility to create Ed25519 signatures of files. It was developed to cryptographically sign and verify OpenBSD releases. Read “Securing OpenBSD From Us To You” for more details.

Now all you need to create signatures and verify them is a private & a public key. Both parts are super short, so it is no problem to embed or print them. This is what a public key would look like:

RWR/JMX+u3pyzGSyTSnOFINVcHJycZE/o6UTUshxvpcp6S4annmJK4DB

The corresponding private key is this:

RWRCSwAAAAASfh0v1XJS4FYg59ntf4d7zYS9GR848h1/JMX+u3pyzIFNjvCMxLUgurelzAWTCSd9y6ghUcbcPVMuwnlDJy4WZLJNKc4Ug1VwcnJxkT+jpRNSyHG+lynpLhqeeYkrgME=

(Warning: Do NOT use above keys for anything! I included both for demonstration only)

signify will add an additional comment to the key files, but they are not used for any verification (besides making sure they are actually in the file).

Whereas pure Ed25519 keys are just 32 bytes, above keys already include some additional information used by signify to create and verify signatures. The private key can also be protected by a passphrase.

When I started porting this small application I knew nothing about Ed25519 or signify. I more or less translated the existing C code into Rust. Back then, I used rust-crypto, a pure Rust implementation various common cryptographic algorithms. It provided all I needed: Ed25519 key generation, signing & verification and bcrypt for the passphrase handling. In just one day I had a working application and less than 2 weeks later I also implemented proper passphrase-protection.

In August then the most promising Rust crypto library, *ring*, was released on crates.io as well (before that it could only be used as a git dependency). I used *ring* before for nobsign and Brian Smith, the author of *ring*, already helped out with some code review & helping me use the API properly.
So I was not too surprised when Brian reached out to me asking if I would be willing to port signify-rs over to *ring* as well.

I was (however, it was shortly before RustFest, so I couldn’t dedicate much time to it).

I took a look at the *ring* documentation and immediately realised I had a completely wrong understanding of Ed25519. Whereas in signify the public key is stored in 32 byte and the private key is stored in 64 byte, *ring* had both keys as only 32 byte long. With feedback from Brian I realised that the longer private key as used in signify is actually just the 32-byte public key concatenated to the (real) 32-byte private key, resulting in 64 byte total.

Equipped with this information (and after the first RustFest day was over), it was easy to port over signify-rs to *ring*.

signify-rs still depends on rust-crypto though, as it provides the necessary bcrypt_pbkdf for encrypting the private key with a user-chosen passphrase. *ring* does not provide bcrypt and probably won’t do so anytime soon. If anyone wants to implement a really good bcrypt crate, please contact me or Brian for feedback.

Use signify-rs

First you need to install it, do so with cargo:

cargo install signify

Generate a key pair:

signify -G -p public-key -s secret-key

This will ask you for a passphrase to protect the secret key. Remember that. Now you can sign a file:

signify -S -s secret-key -m README.md

This will create README.md.sig containing the signature. To verify it:

signify -V -p public-key -m README.md

If it prints Signature Verified it went well. Otherwise it will show an error.

A signature

This is the signature using above private key on the README.md:

untrusted comment: signature from signify secret key RWR/JMX+u3pyzIDv+Gt4JwMbWsb+dt0R/9tYDEjVw7zmBQoQR06Pcd2yr03XqvTSqaJBTbUhm74iUxB98BQVAZemq692g5Xv0gs=

If you put that signature in a file, put the public key from above into another, you can verify it!

Future plans

I still have todos left for this project. First I want this to be fully compatbile with the original implemenation. The original implementation can embed signatures into the signed file and also verify an embedded signature. I want to add that. I also want to distribute pre-compiled binaries for various platforms (hello, rust-everywhere) and provide proper Ed25519 signatures on all those releases.

If you want to help with any of that, jump over to the GitHub repository and start hacking.