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