[feature] add TOTP two-factor authentication (2FA) (#3960)

* [feature] add TOTP two-factor authentication (2FA)

* use byteutil.S2B to avoid allocations when comparing + generating password hashes

* don't bother with string conversion  for consts

* use io.ReadFull

* use MustGenerateSecret for backup codes

* rename util functions

vendor/github.com/boombuler/barcode/qr/blocks.go

@@ -0,0 +1,59 @@
+package qr
+
+type block struct {
+	data []byte
+	ecc  []byte
+}
+type blockList []*block
+
+func splitToBlocks(data <-chan byte, vi *versionInfo) blockList {
+	result := make(blockList, vi.NumberOfBlocksInGroup1+vi.NumberOfBlocksInGroup2)
+
+	for b := 0; b < int(vi.NumberOfBlocksInGroup1); b++ {
+		blk := new(block)
+		blk.data = make([]byte, vi.DataCodeWordsPerBlockInGroup1)
+		for cw := 0; cw < int(vi.DataCodeWordsPerBlockInGroup1); cw++ {
+			blk.data[cw] = <-data
+		}
+		blk.ecc = ec.calcECC(blk.data, vi.ErrorCorrectionCodewordsPerBlock)
+		result[b] = blk
+	}
+
+	for b := 0; b < int(vi.NumberOfBlocksInGroup2); b++ {
+		blk := new(block)
+		blk.data = make([]byte, vi.DataCodeWordsPerBlockInGroup2)
+		for cw := 0; cw < int(vi.DataCodeWordsPerBlockInGroup2); cw++ {
+			blk.data[cw] = <-data
+		}
+		blk.ecc = ec.calcECC(blk.data, vi.ErrorCorrectionCodewordsPerBlock)
+		result[int(vi.NumberOfBlocksInGroup1)+b] = blk
+	}
+
+	return result
+}
+
+func (bl blockList) interleave(vi *versionInfo) []byte {
+	var maxCodewordCount int
+	if vi.DataCodeWordsPerBlockInGroup1 > vi.DataCodeWordsPerBlockInGroup2 {
+		maxCodewordCount = int(vi.DataCodeWordsPerBlockInGroup1)
+	} else {
+		maxCodewordCount = int(vi.DataCodeWordsPerBlockInGroup2)
+	}
+	resultLen := (vi.DataCodeWordsPerBlockInGroup1+vi.ErrorCorrectionCodewordsPerBlock)*vi.NumberOfBlocksInGroup1 +
+		(vi.DataCodeWordsPerBlockInGroup2+vi.ErrorCorrectionCodewordsPerBlock)*vi.NumberOfBlocksInGroup2
+
+	result := make([]byte, 0, resultLen)
+	for i := 0; i < maxCodewordCount; i++ {
+		for b := 0; b < len(bl); b++ {
+			if len(bl[b].data) > i {
+				result = append(result, bl[b].data[i])
+			}
+		}
+	}
+	for i := 0; i < int(vi.ErrorCorrectionCodewordsPerBlock); i++ {
+		for b := 0; b < len(bl); b++ {
+			result = append(result, bl[b].ecc[i])
+		}
+	}
+	return result
+}