B4X:
Sub isBase64(text As String) As Boolean
If Regex.IsMatch("^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$", text) And text.Length > 3 Then
Return True
Else
Return False
End If
End Sub
Sub convertStringToBase64(text As String) As String
Private su As StringUtils
Private b() As Byte = text.GetBytes("UTF8")
Return su.EncodeBase64(b)
End Sub
Sub convertBase64ToString(text As String) As String
Private su As StringUtils
Dim b() As Byte = su.DecodeBase64(text)
Return BytesToString(b, 0, b.Length, "UTF8")
End Sub
Credits: @Douglas Farias (code isBase64)
remember: Convert bitmap to base64 use Base64EncodeDecodeImage from @MarcoRome