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import ( "crypto/hkdf" "crypto/hmac" "crypto/rand" "crypto/sha256" "encoding/binary" "fmt"
"github.com/bluesky-social/indigo/atproto/atcrypto" "github.com/bluesky-social/indigo/atproto/syntax" "tangled.org/bnewbold.net/cobalt/atspace/xsyntax")
// TODO(review): the current structure of the constructor here constructs the context string twice when generating and signing a space commit. Also need to pass space and author args repeatedly. Maybe a wrapper helper would be good? or different API shape?// TODO(review): is it good to used fixed-size byte arrays like [32]byte? I assume these can be stack-allocated nicely, but maybe this is resulting in copies// TODO(review): should we use atdata.Bytes instead of [32]byte or []byte, so that JSON representation is mashalled as valid atproto data?// TODO(review): the stdlib version of hkdf.Expand expects 'info' to be an actual 'string' as opposed to '[]byte'. but we are putting big-endian length prefixes in there, which might not be valid UTF-8. probably "fine" depending on how string([]byte{}) works, but feels a bit off.
const ATPROTO_SPACE_COMMIT_VERSION int64 = 1const ATPROTO_SPACE_PROTOCOL_TAG string = "atproto-space-v1"
type SpaceCommit struct { // commit format version (ATPROTO_SPACE_COMMIT_VERSION) Version int64 `json:"ver" cborgen:"ver"` // SHA-256 of the LtHash state (32 bytes) Hash [32]byte `json:"hash" cborgen:"hash"` // Per-signature nonce (32 random bytes) IKM [32]byte `json:"ikm" cborgen:"ikm"` // Signature of context string bytes Signature []byte `json:"sig" cborgen:"sig"` // HMAC over context string, ikm, and hash: HMAC-SHA256(HKDF-Expand(ikm, ctx, 32), hash) MAC []byte `json:"mac" cborgen:"mac"` // Commit revision (TID), also bound into context string Revision syntax.TID `json:"rev" cborgen:"rev"`}
// Constructs an unsigned space commit struct with 'mac' and secure random 'ikm'func NewSpaceCommit(hash [32]byte, rev syntax.TID, space xsyntax.SpaceRef, author syntax.DID) SpaceCommit { ikm := new([32]byte) rand.Read(ikm[:])
ctxstr := BuildContextString(space, author, rev, *ikm) mac := ComputeMAC(*ikm, ctxstr, hash)
return SpaceCommit{ Version: ATPROTO_SPACE_COMMIT_VERSION, Hash: hash, IKM: *ikm, MAC: mac, Revision: rev, }}
func (c *SpaceCommit) Sign(space xsyntax.SpaceRef, author syntax.DID, privkey atcrypto.PrivateKey) error { ctxstr := BuildContextString(space, author, c.Revision, c.IKM) sig, err := privkey.HashAndSign(ctxstr) if err != nil { return err } c.Signature = sig return nil}
func (c *SpaceCommit) VerifySignature(space xsyntax.SpaceRef, author syntax.DID, pubkey atcrypto.PublicKey) error { if len(c.Signature) == 0 { return fmt.Errorf("can not verify unsigned space commit") } ctxstr := BuildContextString(space, author, c.Revision, c.IKM) return pubkey.HashAndVerify(ctxstr, c.Signature)}
func (c *SpaceCommit) VerifyMAC(space xsyntax.SpaceRef, author syntax.DID) error { ctxstr := BuildContextString(space, author, c.Revision, c.IKM) mac := ComputeMAC(c.IKM, ctxstr, c.Hash) if !hmac.Equal(c.MAC, mac) { return fmt.Errorf("space commit MAC validation mismatch") } return nil}
func BuildContextString(space xsyntax.SpaceRef, author syntax.DID, rev syntax.TID, ikm [32]byte) []byte { /* "atproto-space-v1" // fixed protocol tag || uint16be(len(space)) || space // space URI (at://authority/space/type/skey) || uint16be(len(author)) || author // author DID of the repo || uint16be(len(rev)) || rev // commit revision (TID) || uint16be(len(ikm)) || ikm // per-signature nonce (below) */
// pre-allocate buffer out := make([]byte, 0, len(ATPROTO_SPACE_PROTOCOL_TAG)+len(space)+len(author)+len(rev)+len(ikm)+2*4) out = append(out, []byte(ATPROTO_SPACE_PROTOCOL_TAG)...) out = binary.BigEndian.AppendUint16(out, uint16(len(space))) out = append(out, []byte(space)...) out = binary.BigEndian.AppendUint16(out, uint16(len(author))) out = append(out, []byte(author)...) out = binary.BigEndian.AppendUint16(out, uint16(len(rev))) out = append(out, []byte(rev)...) out = binary.BigEndian.AppendUint16(out, uint16(len(ikm))) out = append(out, ikm[:]...) return out}
func ComputeMAC(ikm [32]byte, ctxstr []byte, hash [32]byte) []byte {
key, err := hkdf.Expand(sha256.New, ikm[:], string(ctxstr), 32) if err != nil || len(key) != 32 { panic("unexpected HKDF expansion error") } hmac := hmac.New(sha256.New, key) return hmac.Sum(hash[:])}