moderize error handling with errors.As/Is

errors.go

@@ -6,12 +6,17 @@ import (
 
 type CorruptionError struct {
 	msg string
+	err error
 }
 
 func (e CorruptionError) Error() string {
 	return "Triplesec corruption: " + e.msg
 }
 
+func (e CorruptionError) Unwrap() error {
+	return e.err
+}
+
 type VersionError struct {
 	v Version
 }

triplesec.go

@@ -308,7 +308,7 @@ func encryptData(plain, keys []byte, rng RandomnessGenerator, versionParams Vers
 	ivOffset -= len(aesIV)
 
 	if ivOffset != 0 {
-		return nil, CorruptionError{"something went terribly wrong during encryption: ivOffset final value non-zero"}
+		return nil, CorruptionError{msg: "something went terribly wrong during encryption: ivOffset final value non-zero"}
 	}
 
 	return res, nil
@@ -344,20 +344,20 @@ func generateMACs(data, keys []byte, versionParams VersionParams) []byte {
 // authentication fails or on memory failures.
 func (c *Cipher) Decrypt(src []byte) (res []byte, err error) {
 	if len(src) < len(MagicBytes)+VersionBytesLen {
-		err = CorruptionError{"decryption underrun"}
+		err = CorruptionError{msg: "decryption underrun"}
 		return
 	}
 
 	if !bytes.Equal(src[:len(MagicBytes)], MagicBytes[0:]) {
-		err = CorruptionError{"wrong magic bytes"}
+		err = CorruptionError{msg: "wrong magic bytes"}
 		return
 	}
 
 	vB := bytes.NewBuffer(src[len(MagicBytes) : len(MagicBytes)+VersionBytesLen])
 	var version Version
 	err = binary.Read(vB, binary.BigEndian, &version)
 	if err != nil {
-		err = CorruptionError{err.Error()}
+		err = CorruptionError{msg: err.Error(), err: err}
 		return
 	}
 
@@ -442,7 +442,7 @@ func decryptData(dst, data, keys []byte, versionParams VersionParams) error {
 	salsa20.XORKeyStream(dst, buffer[ivOffset:], iv, keyArray)
 
 	if len(buffer[ivOffset:]) != len(data)-versionParams.TotalIVLen {
-		return CorruptionError{"something went terribly wrong during decryption: buffer size is wrong"}
+		return CorruptionError{msg: "something went terribly wrong during decryption: buffer size is wrong"}
 	}
 
 	return nil

triplesec_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"encoding/hex"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"os"
 	"testing"
@@ -201,7 +202,7 @@ func TestBadPwV3(t *testing.T) {
 	_, err := c.Decrypt(ciphertext)
 	if err == nil {
 		t.Error("needed an error on bad PW")
-	} else if _, ok := err.(BadPassphraseError); !ok {
+	} else if !errors.As(err, new(BadPassphraseError)) {
 		t.Error("got wrong type of error")
 	}
 }
@@ -229,7 +230,7 @@ func TestBadPwV4(t *testing.T) {
 	_, err := c.Decrypt(ciphertext)
 	if err == nil {
 		t.Error("needed an error on bad PW")
-	} else if _, ok := err.(BadPassphraseError); !ok {
+	} else if !errors.As(err, new(BadPassphraseError)) {
 		t.Error("got wrong type of error")
 	}
 }