caching_sha2_password: Add scramble generation for fast authentication

auth/caching_sha2.go

@@ -44,6 +44,7 @@ const (
 	MIXCHARS             = 32
 	SALT_LENGTH          = 20
 	ITERATION_MULTIPLIER = 1000
+	MAX_PASSWORD_LENGTH  = 256
 )
 
 func b64From24bit(b []byte, n int) []byte {
@@ -218,3 +219,29 @@ func NewSha2Password(pwd string) string {
 
 	return sha256crypt(pwd, salt, 5*ITERATION_MULTIPLIER)
 }
+
+// GenerateScramble creates a scrambe that can be used for caching_sha2 fast authentication
+// See also: https://dev.mysql.com/doc/dev/mysql-server/latest/page_caching_sha2_authentication_exchanges.html
+func GenerateScramble(password []byte, nonce []byte) ([]byte, error) {
+	if len(password) > MAX_PASSWORD_LENGTH {
+		return nil, errors.New("invalid password length for caching_sha2_password scramble generation")
+	}
+
+	if len(nonce) != SALT_LENGTH {
+		return nil, errors.New("invalid nonce length for caching_sha2_password scramble generation")
+	}
+
+	digestStage1 := sha256.Sum256(password)
+	digestStage2 := sha256.Sum256(digestStage1[:])
+	tempStage3 := sha256.New()
+	tempStage3.Write(digestStage2[:])
+	tempStage3.Write(nonce)
+	digestStage3 := tempStage3.Sum(nil)
+
+	newscramble := digestStage1
+	for i := range digestStage1 {
+		newscramble[i] ^= digestStage3[i]
+	}
+
+	return newscramble[:], nil
+}

auth/caching_sha2_test.go

@@ -15,6 +15,7 @@ package auth
 
 import (
 	"encoding/hex"
+	"strings"
 
 	. "github.com/pingcap/check"
 )
@@ -71,3 +72,21 @@ func (s *testAuthSuite) TestNewSha2Password(c *C) {
 		c.Assert(pwhash[r], Not(Equals), 36) // '$'
 	}
 }
+
+func (s *testAuthSuite) TestGenerateScramble(c *C) {
+	pwd := []byte("abc")
+	nonce, _ := hex.DecodeString("6d642b676464321c561d476e094c316d05180b16")
+	storedScramble, _ := hex.DecodeString("3455aae998ae2959d7170229375c9626735bf545d2a828a7d45f94f9b2cf19f4")
+	scramble, err := GenerateScramble(pwd, nonce)
+	c.Assert(err, IsNil)
+	c.Assert(scramble, BytesEquals, storedScramble)
+
+	_, err = GenerateScramble([]byte(strings.Repeat("x", MAX_PASSWORD_LENGTH+1)), nonce)
+	c.Assert(err, NotNil)
+
+	_, err = GenerateScramble(pwd, append(nonce, byte('A')))
+	c.Assert(err, NotNil)
+
+	_, err = GenerateScramble(pwd, nonce[:SALT_LENGTH-1])
+	c.Assert(err, NotNil)
+}