According to The Intercept:
"...According to a document obtained by The Intercept, your blue-bubbled texts do leave behind a log of which phone numbers you are poised to contact and shares this (and other potentially sensitive metadata) with law enforcement when compelled by court order.
Every time you type a number into your iPhone for a text conversation, the Messages app contacts Apple servers to determine whether to route a given message over the ubiquitous SMS system, represented in the app by those déclassé green text bubbles, or over Apple’s proprietary and more secure messaging network, represented by pleasant blue bubbles, according to the document. Apple records each query in which your phone calls home to see who’s in the iMessage system and who’s not.
This log also includes the date and time when you entered a number, along with your IP address — which could, contrary to a 2013 Apple claim that “we do not store data related to customers’ location,” identify a customer’s location. Apple is compelled to turn over such information via court orders for systems known as “pen registers” or “trap and trace devices,” orders that are not particularly onerous to obtain, requiring only that government lawyers represent they are “likely” to obtain information whose “use is relevant to an ongoing criminal investigation.” Apple confirmed to The Intercept that it only retains these logs for a period of 30 days, though court orders of this kind can typically be extended in additional 30-day periods, meaning a series of monthlong log snapshots from Apple could be strung together by police to create a longer list of whose numbers someone has been entering."
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