README for sigtool

What is this?

sigtool is an opinionated tool to generate keys, sign, verify, encrypt & decrypt files using Ed25519 signature scheme. In many ways, it is like like OpenBSD's signify -- except written in Golang and definitely easier to use.

It can sign and verify very large files - it prehashes the files with SHA-512 and then signs the SHA-512 checksum. The keys and signatures are YAML files and so, human readable.

It can encrypt files for multiple recipients - each of whom is identified by their Ed25519 public key. The encryption by default generates ephmeral Curve25519 keys and creates pair-wise shared secret for each recipient of the encrypted file. The caller can optionally use a specific secret key during the encryption process - this has the benefit of also authenticating the sender (and the receiver can verify the sender if they possess the corresponding public key).

The sign, verify, encrypt, decrypt operations can use OpenSSH Ed25519 keys or the keys generated by sigtool. This means, you can send encrypted files to any recipient identified by their comment in ~/.ssh/authorized_keys.

How do I build it?

With Go 1.5 and later:

git clone https://github.com/opencoff/sigtool
cd sigtool
make

The binary will be in ./bin/$HOSTOS-$ARCH/sigtool. where $HOSTOS is the host OS where you are building (e.g., openbsd) and $ARCH is the CPU architecture (e.g., amd64).

How do I use it?

Broadly, the tool can:

• generate new key pairs (public key and private key)
• sign a file
• verify a file against its signature
• encrypt a file
• decrypt a file

Generate Key pair

To start with, you generate a new key pair (a public key used for verification and a private key used for signing). e.g.,

sigtool gen /tmp/testkey

The tool then generates /tmp/testkey.pub and /tmp/testkey.key. The secret key (".key") can optionally be encrypted with a user supplied pass phrase - which the user has to enter via interactive prompt:

sigtool gen -p /tmp/testkey

Sign a file

Signing a file requires the user to provide a previously generated Ed25519 private key. The signature (YAML) is written to STDOUT. e.g., to sign archive.tar.gz with private key /tmp/testkey.key:

sigtool sign /tmp/testkey.key archive.tar.gz

If testkey.key was encrypted without a user pass phrase:

sigtool sign --no-password /tmp/testkey.key archive.tar.gz

The signature can also be written directly to a user supplied output file.

sigtool sign -o archive.sig /tmp/testkey.key archive.tar.gz

Verify a signature against a file

Verifying a signature of a file requires the user to supply three pieces of information:

• the Ed25519 public key to be used for verification
• the Ed25519 signature
• the file whose signature must be verified

e.g., to verify the signature of archive.tar.gz against testkey.pub using the signature archive.sig

sigtool verify /tmp/testkey.pub archive.sig archive.tar.gz

Note that signing and verifying can also work with OpenSSH ed25519 keys.

Encrypt a file by authenticating the sender

If the sender wishes to prove to the recipient that they encrypted a file:

sigtool encrypt -s sender.key to.pub -o archive.tar.gz.enc archive.tar.gz

This will create an encrypted file archive.tar.gz.enc such that the recipient in possession of to.key can decrypt it. Furthermore, if the recipient has sender.pub, they can verify that the sender is indeed who they expect.

Decrypt a file and verify the sender

If the receiver has the public key of the sender, they can verify that they indeed sent the file by cryptographically checking the output:

sigtool decrypt -o archive.tar.gz -v sender.pub to.key archive.tar.gz.enc

Note that the verification is optional and if the -v option is not used, then decryption will proceed without verifying the sender.

Encrypt a file without authenticating the sender

sigtool can generate ephemeral keys for encrypting a file such that the receiver doesn't need to authenticate the sender:

sigtool encrypt to.pub -o archive.tar.gz.enc archive.tar.gz

This will create an encrypted file archive.tar.gz.enc such that the recipient in possession of to.key can decrypt it.

Encrypt a file to an OpenSSH recipient without authenticating the sender

Suppose you want to send an encrypted file where the recipient's public key is in ~/.ssh/authorized_keys. Such a recipient is identified by their OpenSSH key comment (typically name@domain):

sigtool encrypt user@domain -o archive.tar.gz.enc archive.tar.gz

If you have their public key in file "name-domain.pub", you can do:

sigtool encrypt name-domain.pub -o archive.tar.gz.enc archive.tar.gz

This will create an encrypted file archive.tar.gz.enc such that the recipient can decrypt using their private key.

Technical Details

How is the private key protected?

The Ed25519 private key is encrypted in AES-GCM-256 mode using a key derived from the user's pass-phrase.

How is the Encryption done?

The file encryption uses AES-GCM-256 in AEAD mode. The encryption uses a random 32-byte AES-256 key. This key is mixed in with the header checksum as a safeguard to protect the header against accidental or malicious corruption. The input is broken into chunks and each chunk is individually AEAD encrypted. The default chunk size is 4MB (4 * 1048576 bytes). Each chunk generates its own nonce from a global salt. The nonce is calculated as a SHA256 hash of the salt, the chunk length and the block number.

What is the public-key cryptography?

sigtool uses Curve25519 ECC to generate shared secrets between pairs of sender & recipients. This pairwise shared secret is expanded using HKDF to generate a key-encryption-key. The file-encryption key is AEAD encrypted with this key-encryption-key. Thus, each recipient has their own individual encrypted key blob.

The Ed25519 keys generated by sigtool are transformed to their corresponding Curve25519 points in order to generate the shared secret. This elliptic co-ordinate transform follows FiloSottile's writeup.

Format of the Encrypted File

Every encrypted file starts with a header and the header-checksum:

• Fixed-size header
• Variable-length header
• SHA256 sum of both of the above

The fixed length header is:

7 byte magic ("SigTool")
1 byte version number
4 byte header length (big endian encoding)

The variable length header has the per-recipient wrapped keys. This is described as a protobuf file (sign/hdr.proto):

    message header {
	uint32 chunk_size	  = 1;
	bytes  salt		  = 2;
	bytes  pk		  = 3;	// sender's ephemeral curve PK
	sender sender_pk	  = 4;   // sender's encrypted ed25519 PK
	repeated wrapped_key keys = 5;
    }

    /*
     * Sender info is wrapped using the data encryption key
     */
    message sender {
	bytes pk = 1;
    }

    /*
     * A file encryption key is wrapped by a recipient specific public
     * key. WrappedKey describes such a wrapped key.
     */
    message wrapped_key {
	bytes key = 2;
    }

The SHA256 sum covers the fixed-length and variable-length headers.

The encrypted data immediately follows the headers above. Each encrypted chunk is encoded the same way:

    4 byte chunk length (big endian encoding)
    encrypted chunk data
    AEAD tag

The chunk length does not include the AEAD tag length; it is implicitly computed.

The chunk data and AEAD tag are treated as an atomic unit for AEAD decryption.

Understanding the Code

The core logic is in src/sign: it is a library that exposes all the functionality: key generation, key parsing, signing, encryption, decryption etc.

• src/encrypt.go contains the core encryption, decryption code
• src/sign.go contains the Ed25519 signing, verification code
• src/keys.go contains key generation, serialization, de-serialization
• src/ssh.go contains code to parse SSH Ed25519 key files
• src/stream.go contains code that provides an io.Reader and io.WriteCloser interface for encryption and decryption.

The generated keys and signatures are proper YAML files and human readable.

The signature file contains a hash of the public key - so that at verification time, the right private key may be used (in situations where there are lots of keys).

Signatures on large files are calculated efficiently by reading them in memory mapped mode (mmap(2)) and hashing the file contents using SHA-512. The Ed25519 signature is calculated on the file-hash.

Example of Keys, Signature

Ed25519 Public Key

A serialized Ed25519 public key looks like so:

pk: uxpDh+gqXojAmxA/6vxZHzA+Uk+8wogUwvEhPBlWgvo=

Ed25519 Private Key

And, a serialized Ed25519 private key looks like so:

    esk: t3vfqHbgUiA733KKPymFjWT8DdnBEkiMfsDHolPUdQWpvVn/F1Z4J6KYV3M5rGO9xgKxh5RAmqt+6LKgOiJAMQ==
    salt: pPHKG55UJYtJ5wU0G9hBvNQJ0DvT0a7T4Fmj4aPB84s=
    algo: scrypt-sha256
    Z: 131072
    r: 16
    p: 1

The Ed25519 private key is encrypted using AES-256-GCM AEAD mode; the encryption key is derived from the user supplied passphrase using scrypt KDF. A user supplied passphrase is first expanded using SHA-512 before being used in scrypt(). In pseudo code, this operation looks like below:

passphrase = get_user_passphrase()
hpass      = SHA512(passphrase)
salt       = randombytes(32)
key        = Scrypt(hpass, salt, N, r, p)
esk        = AES256_GCM(ed25519_private_key, key)

Where, N, r, p are Scrypt parameters. In our implementation:

N = 2^19 (1 << 19)
r = 8
p = 1

Ed25519 Signature

A generated signature looks like below after serialization:

    comment: inpfile=/tmp/file.txt
    pkhash: 36z9tCwTIVNwwDlExrB0SQ==
    signature: ow2oBP+buDbEvlNakOrsxgB5Yc/7PYyPVZCkfyu7oahw8BakF4Qf32uswPaKGZ8RVz4uXboYHdZtfrEjCgP/Cg==

Here, ```pkhash`` is a SHA256 of the public key needed to verify this signature.

Licensing Terms

The tool and code is licensed under the terms of the GNU Public License v2.0 (strictly v2.0). If you need a commercial license or a different license, please get in touch with me.

See the file LICENSE.md for the full terms of the license.

Author

Sudhi Herle sw@herle.net