Merge pull request #1 from juanli16/new-sealer-opener

add stateless opening

hpke/aead.go

@@ -2,6 +2,7 @@ package hpke
 
 import (
 	"crypto/cipher"
+	"encoding/base64"
 	"fmt"
 )
 
@@ -101,3 +102,54 @@ func (c *openContext) Open(ct, aad []byte) ([]byte, error) {
 	}
 	return pt, nil
 }
+
+// SealWithNonce takes a plaintext pt and associated data aad, and returns a ciphertext
+// that is base64-encoded along with the nonce used for encryption. The nonce is auto-incremented
+// and is prepended to the ciphertext before base64-encoding.
+func (c *sealContext) SealWithNonce(pt, aad []byte) ([]byte, error) {
+	nonce := c.calcNonce()
+	ct := c.AEAD.Seal(nil, nonce, pt, aad)
+	err := c.increment()
+	if err != nil {
+		for i := range ct {
+			ct[i] = 0
+		}
+		return nil, err
+	}
+
+	// prepended the nonce to the ciphertext.
+	ct = append(nonce, ct...)
+
+	// base64-encoded the ciphertext.
+	encodedCt := make([]byte, base64.RawStdEncoding.EncodedLen(len(ct)))
+	base64.RawStdEncoding.Encode(encodedCt, ct)
+	return encodedCt, nil
+}
+
+// OpenWithNonce takes a base64-encoded ciphertext ct and associated data aad,
+// and returns the corresponding plaintext. It assumes that the nonce is prepended
+// to the ciphertext before base64-encoding.
+func (c *openContext) OpenWithNonce(encodedCt, aad []byte) ([]byte, error) {
+	// base64-decodes the ciphertext.
+	decodedCt := make([]byte, base64.RawStdEncoding.DecodedLen(len(encodedCt)))
+	n, err := base64.RawStdEncoding.Decode(decodedCt, encodedCt)
+	if err != nil {
+		return nil, err
+	}
+
+	decodedCt = decodedCt[:n]
+
+	// The nonce is extracted from the ciphertext.
+	Nn := c.AEAD.NonceSize()
+	if len(decodedCt) < Nn {
+		return nil, fmt.Errorf("invalid ciphertext")
+	}
+
+	// decrypts the ciphertext.
+	pt, err := c.AEAD.Open(nil, decodedCt[:Nn], decodedCt[Nn:], aad)
+	if err != nil {
+		return nil, err
+	}
+
+	return pt, nil
+}