Add SetContext on Builder interface and GetContext on Biscuit struct.

biscuit.go

@@ -386,6 +386,14 @@ func (b *Biscuit) Checks() [][]datalog.Check {
 	return result
 }
 
+func (b *Biscuit) GetContext() string {
+	if b == nil || b.authority == nil {
+		return ""
+	}
+
+	return b.authority.context
+}
+
 func (b *Biscuit) Serialize() ([]byte, error) {
 	return proto.Marshal(b.container)
 }

biscuit_test.go

@@ -13,6 +13,7 @@ import (
 func TestBiscuit(t *testing.T) {
 	rng := rand.Reader
 	const rootKeyID = 123
+	const contextText = "current_context"
 	publicRoot, privateRoot, _ := ed25519.GenerateKey(rng)
 
 	builder := NewBuilder(
@@ -30,8 +31,11 @@ func TestBiscuit(t *testing.T) {
 		Predicate: Predicate{Name: "right", IDs: []Term{String("/a/file2"), String("read")}},
 	})
 
+	builder.SetContext(contextText)
+
 	b1, err := builder.Build()
 	require.NoError(t, err)
+	require.EqualValues(t, contextText, b1.GetContext(), "context authority")
 	{
 		keyID := b1.RootKeyID()
 		require.NotNil(t, keyID, "root key ID present")

builder.go

@@ -22,6 +22,7 @@ type Builder interface {
 	AddAuthorityFact(fact Fact) error
 	AddAuthorityRule(rule Rule) error
 	AddAuthorityCheck(check Check) error
+	SetContext(string)
 	Build() (*Biscuit, error)
 }
 
@@ -113,6 +114,10 @@ func (b *builderOptions) AddAuthorityCheck(check Check) error {
 	return nil
 }
 
+func (b *builderOptions) SetContext(context string) {
+	b.context = context
+}
+
 func (b *builderOptions) Build() (*Biscuit, error) {
 	opts := make([]biscuitOption, 0, 2)
 	if v := b.rng; v != nil {