Add digital signature verification #12

Summary

Add digital signatures to Clavimit messages so that recipients can verify that an encrypted message was produced by the holder of the expected sender private key.

Motivation

Clavimit currently provides confidentiality but does not authenticate the sender of an encrypted message.

Digital signatures would allow the recipient to verify that the encrypted message was signed by the holder of a specific private key and that the signed content has not been modified.

The recipient remains responsible for verifying that the public key used for signature verification actually belongs to the expected sender.

This issue depends on support for multiple Clavimit message format versions, since signed messages will require additional information in the encrypted message format.

Proposed behavior

Encryption

When signature support is enabled:

• allow the sender to provide their private key for signing;
• generate a digital signature for the message;
• include the signature in the appropriate Clavimit message format version;
• keep the existing encryption process unchanged where possible;
• clear the signing private key from the UI after the operation completes, whether signing/encryption succeeds or fails.

Decryption and signature verification

When decrypting a signed Clavimit message:

• allow the recipient to provide the sender's public key;
• verify the signature using that public key;
• clearly display whether signature verification succeeded or failed;

The recipient is responsible for obtaining and verifying the sender's public key through an external trusted channel.

Message format

Signed messages should use a dedicated message format version capable of carrying the signature and any information required for verification.

The new message format version should include a field for the digital signature.

For signed messages, the field contains the generated signature.

For unsigned messages, the field is set to null.

signature: <signature>

or:

signature: null

The other fields remain unchanged

{
    "version": 2,
    "encryptedKey": "...",
    "iv": "...",
    "ciphertext": "...",
    "signature": <ignature>
}

or

{
    "version": 2,
    "encryptedKey": "...",
    "iv": "...",
    "ciphertext": "...",
    "signature": null
}

This keeps the structure of the message format consistent regardless of whether the sender chooses to sign the message.

Existing message format versions should remain supported.

Out of scope

This issue does not introduce automatic identity verification, public-key directories, certificate authorities, or key-management services.

Clavimit verifies the cryptographic signature against the public key supplied by the user, but does not determine whether that public key belongs to a particular person.