If anyone is interested in a more detailed security breakdown of Decoy, here it is:

The core goal is protecting the original image. The secondary goal is preventing the identification of the Decoy image as s decoy. The original inage goes through the following steps:

1. The picture is taken by the device's camera, stored in temporary memory.
2. The picture data is converted to a base64 string and the original image is deleted.
3. The base64 string is encrypted using AES along with a passcode chosen by the user.
4. The encrypted ciphertext is encoded into the decoy image, which is sent via SMS to the recipient.
5. The passcode is exchanged outside the context of the app with the recipient.
6. The recipient enters the passcode, and the original ciphertext is decrypted, and the base64 string is rendered by the browser.
7. The string is cleared from memory after closing the app.

The presents the following possible vulnerabilities. (Vulnerabilties which assume malicious intent on the part of the app are ignored.)

1. If the user's phone is compromised or Apple turns evil, the image file in memory can be relayed elsewhere.
2. If Apple's file deletion processes are flawed, an attacker with physical access to the phone could recover the deleted file.
3. If the user's phone is compromised, the passcode can be leaked. Or, an attacker could gain access to the image data, and if any of the following flaws exist, those could be exploited: the AES protocol, the SJCL library used to implement AES, the native window.Crypto javascript object.
4. If the user's phone is compromised, the encoding can leak details about the passcode.
5. If the passcode is exchanged using insecure means, it can be eavesdropped. A weak password can be trivially brute-forced. A strong password can be socially engineered.
6. If the recipient's phone is compromised, the passcode can be relayed elsewhere, or the original image data.
7. If Apple's deletion processes are insecure, the image data string or passcode could be recovered by an attacker with physical access to the phone.

Thus, any attack from someone other than the recipient requires either, 1. Physical access to the victim's phone, which is then compromised, and returned, undetected to the victim. Or 2. Access to the decoy image and passcode. As the first attack would represent a catastrophic breach of security far beyond a simple lost phone, which is not unique to this app, I won't spend a ton of time discussing it.

Thus, the primary attack type will involve an attacker firstly obtaining a set of images from the victim, secondly identifying a picture from that set as a Decoy, and thirdly, some means of obtaining the passcode. Means of acquiring a set of images include:

1. Scraping images from publicly available sources. (no victim information or contact needed)
2. Gaining permanent physical access to a victim's phone (e.g. if it is lost or stolen). (requires victim contact)
3. Accessing a victim's photo storage service (DropBox, iCloud, etc.) (requires significant amount of victim information or contact)

Once access to an imageset is acquired, the attacker must decide the most efficient way to narrow down which images are potential Decoys. Potential detection methods include:

1. Attempting to decrypt all images saved/created before either the release date of Decoy or the download date of Decoy (needs zero time, and zero information about victim. produces 0% false negative rate and 100% false positive rate)
2. Using statistical methods to examine noise distribution of suspect images (needs small amount of time, and zero information about victim. produces both a sizable false negative and false positive rate.)
3. Using context such as text messages, or multiple similar images implying non-Decoys, or images that appear out of place (needs possibly significant amount of time and significant information about victim, produces minimal false positives and minimal false negatives).

Potential password attack methods include:

1. Using a password that you know the victim has used previously, e.g. their iCloud/DropBox password, or socially engineering the password. (small amount of time, large chance of success, significant victim information needed)
2. Performing a superficial brute-force attack. (small amount of time, low chance of success, no victim information needed)
3. Performing a full brute-force attack. (large amount of time, large chance of success, no victim information needed)

Thus the most attractive attack types will be: Fusking Attacks: This involves scraping a large number of images from publicly available sources with little regard for victim identity. Images are run through cursory detection, and superficial brute force attacks are applied to all images that pass the cursory detection. This will yield an extremely low success rate but requires very little time or effort. You can protect against this attack by using a strong password, which will resist superficial brute-force attempts.

Stolen-Phone Attacks: If a phone is lost or stolen, this significantly narrows down the size of the imageset. If Decoy is present on the phone in question, it significantly raises the probability that any given image is a decoy. Because of the small size of the imageset, detection will likely be unnecessary. The phone can be examined for clues to possible passwords (by looking through text message history, possible password managers on the phone, passwords sent in plaintext via email from -insert service here-, etc.) These passwords will be tried first on all images, then a cursory brute force will be attempted on all images. If an image can be conclusively determined as a decoy, (for example, if you leave a copy of it in your text message log), a fullscale brute force attack can be attempted. (This can be protected against by using a strong password, and not reusing passwords across multiple services)

Targeted Attacks: If you're a celebrity or public figure or someone that people have a reason to specifically single you out, the attacker will likely attempt to access some form of cloud-based image storage, such as your iCloud account. Once they gain access to this, they will use that same password on all images regardless of whether they are suspected Decoys or not, as the time it would take to do so is trivial. After that, a similar brute force using common variations on that password will be attempted. After that, a cursory brute force will be attempted. Finally, a fullscale brute-force will be launched on all images. (This can be protected against by using a strong password, and not reusing passwords across multiple services)