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To decrypt at a random spot, you need only know the offset from the beginning and you can perform a single AES encryption call. …

Your permutation idea is leaving the realm of encryption and trying to address a different type of idea: authentication. … Encryption hides the bits, authentication gives confidence that they are being read as intended. But it doesn't do this properly, the attacker might still be able to flip bits. …

Whenever you have information to hide, something related to protecting it needs to itself be hidden. There needs to be some secret involved, and there needs to be some system of using the secret to ac …

The basic principle of encryption (Kerchoff's principle) is that the only thing necessary to decrypt data is the key. … Option (b) would allow for an automated approach that doesn't require you to manually intervene, and it would allow you to not know the encryption key either. …

Also any twin-encryption algo-s around? …

Short story: Ultimately, your question comes down to a question of the quality of the Windows RNG. Your approach is fine if your RNG is good, but you're trusting the largely opaque Windows RNG to give …

That should OK. PBKDF2 has sufficient collision resistance and pre-image resistance for password authentication, and the fact that the passwords to both are the same shouldn't cause any problems if th …

Split the encryption and decryption sides evenly. You need $(2^8)^4 \times 2 = 2^{33}$ block cipher calls, because you need 4 layers of $2^8$ encryption calls and 4 layers of $2^8$ decryption calls. … That uses just one encryption/decryption split, also known as one-dimensional MITM. …

Instead, the IV should be generated separately from the key and should be unique for each encryption with the key. Perhaps start with a random IV and increment by one for each encrypted message. …

Back in the day (up through the 90s), U.S. government internal encryption standards was not closely aligned with public sector cryptography, and we largely had to speculate as to whether public crypto …

It sounds like you want Format Preserving Encryption. FPE works by encrypting from an arbitrary domain $X$ onto $X$. … FPE addresses that issue by only allowing encryption and decryption from and to domains that are valid plaintexts. …

OFB is a mode of operation to ensure confidentiality of messages a) longer than the block size of the encryption algorithm, and b) that can be re-broadcast. … OFB mode prevents this from happening by using randomized encryption to ensure that repeated plaintext within the message does not cause repeated ciphertext. …

There is a stream cipher called the Solitaire Cipher that is designed to be implemented by a human using only a deck of playing cards. It is very simple to memorize, use, and implement in code.

Based on the rest of your statement, it sounds like you want to implement a real-world encryption/signing scheme using RSA, but not some higher protocol that uses it. … (RSA-OAEP is probably the most recommended padding scheme for encryption if that's what you want.) …

am making an app where both the sender and receiver share a key and there is no way to create an initialization vector for each encryption. … If transporting the IV is a problem, the unique number could even be a counter that both sides maintain and use to derive the IV at encryption/decryption time. …