[][src]Module sequoia_guide::chapter_01

Describes key creation, signing and verification.

In this chapter, we will see how to use Sequoia's low-level API to generate an OpenPGP key, and use it to sign and verify some data. We will construct this program from top to bottom, concatenating the fragments yields the openpgp/examples/generate-sign-verify.rs.

use std::io::{self, Write};
 
extern crate failure;
extern crate sequoia_openpgp as openpgp;
use openpgp::serialize::stream::*;
use openpgp::packet::prelude::*;
use openpgp::parse::stream::*;
 
const MESSAGE: &'static str = "дружба";
 
fn main() {
    // Generate a key.
    let key = generate().unwrap();
 
    // Sign the message.
    let mut signed_message = Vec::new();
    sign(&mut signed_message, MESSAGE, &key).unwrap();
 
    // Verify the message.
    let mut plaintext = Vec::new();
    verify(&mut plaintext, &signed_message, &key).unwrap();
 
    assert_eq!(MESSAGE.as_bytes(), &plaintext[..]);
}

Key generation

First, we need to generate a new key. This key shall have one user id, and one signing-capable subkey. We use the TPKBuilder to create it:

/// Generates an signing-capable key.
fn generate() -> openpgp::Result<openpgp::TPK> {
    let (tpk, _revocation) = openpgp::tpk::TPKBuilder::new()
        .add_userid("someone@example.org")
        .add_signing_subkey()
        .generate()?;
 
    // Save the revocation certificate somewhere.
 
    Ok(tpk)
}

Signing

To sign a message, we first compose a writer stack corresponding to the desired output format and packet structure. The resulting object implements io::Write, and we simply write the plaintext to it.

/// Signs the given message.
fn sign(sink: &mut Write, plaintext: &str, tsk: &openpgp::TPK)
           -> openpgp::Result<()> {
    // Get the keypair to do the signing from the TPK.
    let key : key::UnspecifiedSecret
        = tsk.keys_valid().signing_capable().nth(0).unwrap().2.clone().into();
    let mut keypair = key.into_keypair()?;
 
    // Start streaming an OpenPGP message.
    let message = Message::new(sink);
 
    // We want to sign a literal data packet.
    let signer = Signer::new(message, vec![&mut keypair], None)?;
 
    // Emit a literal data packet.
    let mut literal_writer = LiteralWriter::new(signer, None, None, None)?;
 
    // Sign the data.
    literal_writer.write_all(plaintext.as_bytes())?;
 
    // Finalize the OpenPGP message to make sure that all data is
    // written.
    literal_writer.finalize()?;
 
    Ok(())
}

Verification

Verification is more difficult than signing. When we sign, we control the packet structure being generated. However, when we verify, the control flow is determined by the message being processed.

To use Sequoia's low-level streaming verifier, we need to provide an object that implements VerificationHelper. This object provides public and for the signature verification, and implements the signature verification policy.

To decrypt messages, we create a Verifier with our helper. Verified data can be read from this using io::Read.

/// Verifies the given message.
fn verify(sink: &mut Write, signed_message: &[u8], sender: &openpgp::TPK)
          -> openpgp::Result<()> {
    // Make a helper that that feeds the sender's public key to the
    // verifier.
    let helper = Helper {
        tpk: sender,
    };
 
    // Now, create a verifier with a helper using the given TPKs.
    let mut verifier = Verifier::from_bytes(signed_message, helper, None)?;
 
    // Verify the data.
    io::copy(&mut verifier, sink)?;
 
    Ok(())
}
 
struct Helper<'a> {
    tpk: &'a openpgp::TPK,
}
 
impl<'a> VerificationHelper for Helper<'a> {
    fn get_public_keys(&mut self, _ids: &[openpgp::KeyID])
                       -> openpgp::Result<Vec<openpgp::TPK>> {
        // Return public keys for signature verification here.
        Ok(vec![self.tpk.clone()])
    }
 
    fn check(&mut self, structure: &MessageStructure)
             -> openpgp::Result<()> {
        // In this function, we implement our signature verification
        // policy.
 
        let mut good = false;
        for (i, layer) in structure.iter().enumerate() {
            match (i, layer) {
                // First, we are interested in signatures over the
                // data, i.e. level 0 signatures.
                (0, MessageLayer::SignatureGroup { ref results }) => {
                    // Finally, given a VerificationResult, which only says
                    // whether the signature checks out mathematically, we apply
                    // our policy.
                    match results.get(0) {
                        Some(VerificationResult::GoodChecksum(..)) =>
                            good = true,
                        Some(VerificationResult::MissingKey(_)) =>
                            return Err(failure::err_msg(
                                "Missing key to verify signature")),
                        Some(VerificationResult::BadChecksum(_)) =>
                            return Err(failure::err_msg("Bad signature")),
                        None =>
                            return Err(failure::err_msg("No signature")),
                    }
                },
                _ => return Err(failure::err_msg(
                    "Unexpected message structure")),
            }
        }
 
        if good {
            Ok(()) // Good signature.
        } else {
            Err(failure::err_msg("Signature verification failed"))
        }
    }
}

Further reading

For more examples on how to read a key from a file, and then either create a signed message, or a detached signature, see openpgp/examples/sign.rs and openpgp/examples/sign-detached.rs.