Minisign

Minisign is a dead simple tool to sign files and verify signatures.

Compilation / installation

Dependencies:

• libsodium
• cmake

Compilation:

$ cmake .
$ make
# make install

Creating a key pair

$ minisign -G

The public key is put into the minisign.pub file, and the secret key into the minisign.key file.

Signing a file

$ minisign -S -m myfile.txt

Or to include a comment in the signature, that will be verified and displayed when verifying the file:

$ minisign -S -m myfile.txt -t 'This comment will be signed as well'

The signature is put into myfile.txt.minisig.

Verifying a file

$ minisign -V -m myfile.txt

This requires the signature myfile.txt.minisig to be present in the same directory.

Options list

$ minisign -G -p pubkey -s seckey

$ minisign -S -s seckey -m file [-x sigfile] [-c untrusted_comment] [-t trusted_comment]

$ minisign -V -p pubkey -m file [-x sigfile] [-q]

Trusted comments

Signature files include an untrusted comment line that can be freely modified, even after signature creation.

They also include a second comment line, that cannot be modified without the secret key.

Trusted comments can be used to add instructions or application-specific metadata (timestamps, version numbers, resource identifiers).

Compatibility with OpenBSD signify

Signature written by minisign can be verified using OpenBSD's signify tool: public key files and signature files are compatible.

However, minisign uses a slightly different format to store private keys.

minisign signatures include trusted comments in addition to untrusted comments. Trusted comments are signed, thus verified, before being displayed. This adds two lines to the signature files, that signify silently ignores. minisign has to be used in order to verify trusted comments.

Signature format

untrusted comment: <arbitrary text>
base64(<signature_algorithm> || <key_id> || <signature>)
trusted_comment: <arbitrary text>
base64(<global_signature>)

• signature_algorithm: Ed
• key_id: 8 random bytes, matching the public key
• signature: ed25519()
• global_signature: ed25519( || <trusted_comment>)

Public key format

untrusted comment: <arbitrary text>
base64(<signature_algorithm> || <key_id> || <public_key>)

• signature_algorithm: Ed
• key_id: 8 random bytes
• public_key: Ed25519 public key

Secret key format

untrusted comment: <arbitrary text>
base64(<signature_algorithm> || <kdf_algorithm> || <cksum_algorithm> ||
       <kdf_salt> || <kdf_opslimit> || <kdf_memlimit> || <keynum_sk>)

• signature_algorithm: Ed
• kdf_algorithm: Sc
• cksum_algorithm: B2
• kdf_salt: 32 random bytes
• kdf_opslimit: crypto_pwhash_scryptsalsa208sha256_OPSLIMIT_SENSITIVE
• kdf_memlimit: crypto_pwhash_scryptsalsa208sha256_MEMLIMIT_SENSITIVE
• keynum_sk: <kdf_output> ^ (<key_id> || secret_key> || <checksum>)
• key_id: 8 random bytes
• secret_key: Ed25519 secret key
• checksum: Blake2b(<key_id> || <secret_key>), 32 bytes