1 files changed, 64 insertions, 0 deletions
diff --git a/dsse/envelope.go b/dsse/envelope.go
new file mode 100644
index 0000000..ed223e9
--- /dev/null
+++ b/dsse/envelope.go
@@ -0,0 +1,64 @@
+package dsse
+
+import (
+	"encoding/base64"
+	"fmt"
+)
+
+/*
+Envelope captures an envelope as described by the DSSE specification. See here:
+https://github.com/secure-systems-lab/dsse/blob/master/envelope.md
+*/
+type Envelope struct {
+	PayloadType string      `json:"payloadType"`
+	Payload     string      `json:"payload"`
+	Signatures  []Signature `json:"signatures"`
+}
+
+/*
+DecodeB64Payload returns the serialized body, decoded from the envelope's
+payload field. A flexible decoder is used, first trying standard base64, then
+URL-encoded base64.
+*/
+func (e *Envelope) DecodeB64Payload() ([]byte, error) {
+	return b64Decode(e.Payload)
+}
+
+/*
+Signature represents a generic in-toto signature that contains the identifier
+of the key which was used to create the signature.
+The used signature scheme has to be agreed upon by the signer and verifer
+out of band.
+The signature is a base64 encoding of the raw bytes from the signature
+algorithm.
+*/
+type Signature struct {
+	KeyID string `json:"keyid"`
+	Sig   string `json:"sig"`
+}
+
+/*
+PAE implementes the DSSE Pre-Authentic Encoding
+https://github.com/secure-systems-lab/dsse/blob/master/protocol.md#signature-definition
+*/
+func PAE(payloadType string, payload []byte) []byte {
+	return []byte(fmt.Sprintf("DSSEv1 %d %s %d %s",
+		len(payloadType), payloadType,
+		len(payload), payload))
+}
+
+/*
+Both standard and url encoding are allowed:
+https://github.com/secure-systems-lab/dsse/blob/master/envelope.md
+*/
+func b64Decode(s string) ([]byte, error) {
+	b, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		b, err = base64.URLEncoding.DecodeString(s)
+		if err != nil {
+			return nil, fmt.Errorf("unable to base64 decode payload (is payload in the right format?)")
+		}
+	}
+
+	return b, nil
+}