Minisign

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

Compilation / installation

Dependencies:

• libsodium
• cmake

Compilation:

$ mkdir build
$ cd build
$ cmake ..
$ make
# make install

Creating a key pair

$ minisign -G

The public key is printed and put into the minisign.pub file. The secret key is encrypted and saved as minisign.key file.

Signing a file

$ minisign -Sm myfile.txt

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

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

The signature is put into myfile.txt.minisig.

Verifying a file

$ minisign -Vm myfile.txt -P RWQf6LRCGA9i53mlYecO4IzT51TGPpvWucNSCh1CBM0QTaLn73Y7GFO3

or

$ minisign -Vm myfile.txt -p signature.pub

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

The public key can either reside in a file (./minisign.pub by default) or be directly specified on the command line.

Usage

$ minisign -G [-p pubkey] [-s seckey]
$ minisign -S [-x sigfile] [-s seckey] [-c untrusted_comment] [-t trusted_comment] -m file
$ minisign -V [-x sigfile] [-p pubkeyfile | -P pubkey] [-q] -m file

-G                generate a new key pair
-S                sign a file
-V                verify that a signature is valid for a given file
-m <file>         file to sign/verify
-p <pubkeyfile>   public key file (default: ./minisign.pub)
-P <pubkey>       public key, as a base64 string
-s <seckey>       secret key file (default: ./minisign.key)
-x <sigfile>      signature file (default: <file>.minisig)
-c <comment>      add a one-line untrusted comment
-t <comment>      add a one-line trusted comment
-q                quiet mode, suppress output
-v                display version number

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 (intended file name, timestamps, resource identifiers, version numbers to prevent downgrade attacks).

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 secret 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.

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(<file data>)
• global_signature: ed25519(<signature> || <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(<signature_algorithm> || <key_id> || <secret_key>), 32 bytes