zendo data size restrictions zendo is a new mobile phone app which encrypts messages using a one-time pad:

http://techcrunch.com/2015/03/24/one-time-pads-ride-again/

users have to meet in person and exchange a key via a qr code. the one-time pad data is then initialized on both phones using this key to make a secure transfer. however, according to the article, only 0.5MB of pad data is exchanged:


  The process takes a few seconds for 0.5MB of pad to be exchanged —
  which is enough for “thousands” of messages between the two users.
  More than 3,000 based on a conservative estimate of average text
  message length, according to Newbold. You can also exchange multiple
  pads to stockpile more megabytes for even more future missives. (To be
  clear, if users run out of pad, the app defaults to AES encryption so
  messages are never sent unencrypted — it just steps down to industry
  standard levels of security.)
  
  Pictures can also be sent via Zendo, and those are encrypted with a
  single use AES 256bit key and an HMAC key (for authentication), which
  are then sent using One-time pad encryption, so the photos are secured
  via OTP without needing to use up too much of the pad to send them.
  The same method will be used to encrypt and send videos, audio and
  documents/attachments — file transfer features which will be coming in
  future updates to the app.


it was my understanding that a one-time pad must be as long as the plaintext being transferred, so 0.5MB of pad would enable no more than 0.5MB of plaintext to be transmitted with perfect secrecy.

however the above excerpt makes it sound like the pad can be shorter than the plaintext while still maintaining the complete security of a one-time pad:


  so the photos are secured via OTP without needing to use up too much of the pad to send them


i'm just wondering if this is some cryptographic technique that i have not heard of, or if the article is wrong to imply that the cyphertext will have the same security (entropy) as the pad, for data that is longer than the 0.5MB pad (as most photos and videos are)?



edit

my problem was actually with the wording of the article. i was reading the above quote as:


  Pictures can also be sent via Zendo, and those are encrypted with a
  single use AES 256bit key and an HMAC key (for authentication), [the
  pictures] are then sent using One-time pad encryption, so the photos are
  secured via OTP without needing to use up too much of the pad to send them.


but actually i think the article means to say:


  Pictures can also be sent via Zendo, and those are encrypted with a
  single use AES 256bit key and an HMAC key (for authentication), [the aes
  key and hmac key, but not the pictures] are then sent using One-time pad
  encryption, so the photos are secured via OTP without needing to use up too
  much of the pad to send them.


and this makes a lot more sense.

sorry this turned out to be more of a reading comprehension question than a cryptography question in the end :p

i don't want to add another question now as that would be unfair to those who have already given answers addressing my first misunderstanding-question. thanks.
The system described in the quoted article depends on the security of AES with random keys (not only on the theoretical unbreakability of the OTP) for at least two things:


the encryption of large files, as apparent in the quotes of the question;
the initial establishment of the OTP, as shown by this other quote


  The first step is always optical, and that is an exchange of an AES 256bit key, plus an authentication key, and so those are the keys to encrypt the One-time pad as it’s being transferred wirelessly via Multipeer [or Wi-Fi Direct]… with a symmetrical AES key that was exchanged optically.



Claims that a practical system base its security directly on the OTP are typically misleading, on purpose as a marketing point or by incompetence, often both.



Addition per comment: the technique used for photos seems to be using 256 bits from the pad as an AES key enciphering the photo. This is not as secure as the OTP from an information-theoretic standpoint, yet this is believed practically secure when done properly.