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// Copyright (c) 2022-2024, Sylabs Inc. All rights reserved.
// This software is licensed under a 3-clause BSD license. Please consult the LICENSE.md file
// distributed with the sources of this project regarding your rights to use or distribute this
// software.
package integrity
import (
"bytes"
"context"
"crypto"
"encoding/base64"
"encoding/json"
"errors"
"fmt"
"io"
"github.com/sigstore/sigstore/pkg/signature"
"github.com/sigstore/sigstore/pkg/signature/dsse"
"github.com/sigstore/sigstore/pkg/signature/options"
)
const metadataMediaType = "application/vnd.sylabs.sif-metadata+json"
type dsseEncoder struct {
ss []signature.Signer
opts []signature.SignOption
}
// newDSSEEncoder returns an encoder that signs messages in DSSE format according to opts, with key
// material from ss. SHA256 is used as the hash algorithm, unless overridden by opts.
func newDSSEEncoder(ss []signature.Signer, opts ...signature.SignOption) *dsseEncoder {
return &dsseEncoder{
ss: ss,
opts: opts,
}
}
// signMessage signs the message from r in DSSE format, and writes the result to w. On success, the
// hash function is returned.
func (en *dsseEncoder) signMessage(ctx context.Context, w io.Writer, r io.Reader) (crypto.Hash, error) {
opts := en.opts
opts = append(opts, options.WithContext(ctx))
var so crypto.SignerOpts
for _, opt := range opts {
opt.ApplyCryptoSignerOpts(&so)
}
// If SignerOpts not explicitly supplied, set default hash algorithm.
if so == nil {
so = crypto.SHA256
opts = append(opts, options.WithCryptoSignerOpts(so))
}
s := dsse.WrapMultiSigner(metadataMediaType, en.ss...)
b, err := s.SignMessage(r, opts...)
if err != nil {
return 0, err
}
_, err = w.Write(b)
return so.HashFunc(), err
}
type dsseDecoder struct {
vs []signature.Verifier
}
// newDSSEDecoder returns a decoder that verifies messages in DSSE format using key material from
// vs.
func newDSSEDecoder(vs ...signature.Verifier) *dsseDecoder {
return &dsseDecoder{
vs: vs,
}
}
var (
errDSSEVerifyEnvelopeFailed = errors.New("dsse: verify envelope failed")
errDSSEUnexpectedPayloadType = errors.New("unexpected DSSE payload type")
)
// verifyMessage reads a message from r, verifies its signature(s), and returns the message
// contents. On success, the accepted public keys are set in vr.
func (de *dsseDecoder) verifyMessage(ctx context.Context, r io.Reader, h crypto.Hash, vr *VerifyResult) ([]byte, error) { //nolint:lll
// Wrap the verifiers so we can accumulate the accepted public keys.
vs := make([]signature.Verifier, 0, len(de.vs))
for _, v := range de.vs {
vs = append(vs, wrappedVerifier{
Verifier: v,
keys: &vr.keys,
})
}
raw, err := io.ReadAll(r)
if err != nil {
return nil, err
}
v := dsse.WrapMultiVerifier(metadataMediaType, 1, vs...)
if err := v.VerifySignature(bytes.NewReader(raw), nil, options.WithContext(ctx), options.WithHash(h)); err != nil {
return nil, fmt.Errorf("%w: %w", errDSSEVerifyEnvelopeFailed, err)
}
var e dsseEnvelope
if err := json.Unmarshal(raw, &e); err != nil {
return nil, err
}
if e.PayloadType != metadataMediaType {
return nil, fmt.Errorf("%w: %v", errDSSEUnexpectedPayloadType, e.PayloadType)
}
return e.DecodedPayload()
}
type wrappedVerifier struct {
signature.Verifier
keys *[]crypto.PublicKey
}
func (wv wrappedVerifier) VerifySignature(signature, message io.Reader, opts ...signature.VerifyOption) error {
err := wv.Verifier.VerifySignature(signature, message, opts...)
if err == nil {
pub, err := wv.Verifier.PublicKey()
if err != nil {
return err
}
*wv.keys = append(*wv.keys, pub)
}
return err
}
// dsseEnvelope describes a DSSE envelope.
type dsseEnvelope struct {
PayloadType string `json:"payloadType"`
Payload string `json:"payload"`
Signatures []struct {
KeyID string `json:"keyid"`
Sig string `json:"sig"`
} `json:"signatures"`
}
// DecodedPayload returns the decoded payload from envelope e.
func (e *dsseEnvelope) DecodedPayload() ([]byte, error) {
b, err := base64.StdEncoding.DecodeString(e.Payload)
if err != nil {
return base64.URLEncoding.DecodeString(e.Payload)
}
return b, nil
}
// isDSSESignature returns true if r contains a signature in a DSSE envelope.
func isDSSESignature(r io.Reader) bool {
var e dsseEnvelope
if err := json.NewDecoder(r).Decode(&e); err != nil {
return false
}
return metadataMediaType == e.PayloadType
}
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