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If you can't get access to the key for at least some sample uses, there's no way to be sure. For example, it's impossible to distinguish AES-128 from AES-256 if you don't have access to the key. That's true of any encryption method: without knowing the key, you can't distinguish the ciphertext from random data of the same length.
A professional auditor ...
10
Unless the file has a plaintext header which indicates that it has been encrypted, there is no way to distinguish ciphertext from uniform random data. You can heuristically guess that a file is encrypted if it has absolutely no structure and appears completely random, but you cannot definitively prove it.
Any cipher whose output could be distinguished from ...
7
In addition to what the other answers have stated, "proper" encryption using AES-256 (block mode choice aside) can still allow backdoors, such as by maliciously choosing IVs/nonces. Phil Rogaway and others discuss this in more detail in their paper "Security of Symmetric Encryption against Mass Surveillance" (abstract available here).
4
This question is very easy to answer:
The implementation isn't correct and you absolutely should not use it. Any other attitude towards this black box is hopelessly attackable.
Your stance should be: "I must be able to see the source, audit the source, and build the source myself into a binary". Anything short of that is irresponsible on your part. Do not ...
3
That depends on the encryption. But for all simple monoalphabetic substitutions the answer is yes.
And to don't need a neutral net, but the most simple classifier works. You train it on the letters of the cipher-texts, with the cleartext-letters being the classes. To apply the decryption to an unknown text, just let it classify each letter of the cipher ...
3
Suppose you have two message signature pairs and following values are then public i.e. known to you -
The public keys: $Q_1 (= x_1G)$, $Q_2 (= x_2G)$
The messages and their hashes: $m_1$, $m_2$, $H(m_1)$, $H(m_2)$
The signatures: $(r_1, s_1)$, ($r_2, s_2$)
The following are unknown -
The private keys: $x_1$, $x_2$
The nonce: $k$
The following relations ...
3
If you're wondering about the iPhone's encryption specifically, then this work may have already been done for you. Many Apple/iPhone products have passed formal FIPS 140-2 certification, which does extensive tests on the sorts of things that you're concerned about. If you want to see details about which products have been certified for which algorithms/key ...
1
Many answers have pointed out that what you seek to do is not possible. Proving that something is encrypted with key X is not possible without having key X and a signature for what was encrypted. If it were possible, then the encryption algorithm would be a faulty one. AES does not fit that bill.
However, if you are really in a bind, where management has ...
1
Exceptions to Kerckhoffs's second principle do exist
Background
Kerckhoffs addressed the problem of how to use cryptography in military telegraphy. This was to be a critical issue during World War I (see the German ADFGX and ADFGVX ciphers).
In 1881, Kerckhoffs became professor of German at the Ecole des
Hautes Etudes Commerciales and at the Ecole ...
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