Minisign

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

It is portable, lightweight, and uses the highly secure Ed25519 public-key signature system.

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 a file named ~/.minisign/minisign.key.

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 [-H] [-x sigfile] [-s seckey] [-c untrusted_comment] [-t trusted_comment] -m file
$ minisign -V [-x sigfile] [-p pubkeyfile | -P pubkey] [-o] [-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
-o                combined with -V, output the file content after verification
-H                combined with -S, pre-hash in order to sign large files
-p <pubkeyfile>   public key file (default: ./minisign.pub)
-P <pubkey>       public key, as a base64 string
-s <seckey>       secret key file (default: ~/.minisign/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
-Q                pretty quiet mode, only print the trusted comment
-f                force. Combined with -G, overwrite a previous key pair
-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).

Compilation / installation

Dependencies:

Compilation:

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

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.

Pre-hashing

By default, signing and verification require as much memory as the size of the file.

Since Minisign 0.6, huge files can be signed and verified with very low memory requirements, by pre-hashing the content.

The -H command-line switch, in combination with -S, generates a pre-hashed signature (HashEdDSA):

$ minisign -SHm myfile.txt

Verification of such a signature doesn't require any specific switch: the appropriate algorithm will automatically be detected.

Signatures generated that way are not compatible with OpenBSD's signify tool and are not compatible with Minisign versions prior to 0.6.

Signature format

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

Public key format

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

Secret key format

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