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Studying for Cryptology and came across a presentation regarding on "Integrity vs Authenticity" where the discussion briefly mentions how Encryption "does not provide integrity or authenticity"?

Why is this the case with Symmetric Encryption for example? Or is it with Cryptology in general? Not much details are mentioned on my presentation.

From looking around the internet (and my own general understanding), a basic example of this case in terms with Symmetric Encryption is how a key is used to encrypt and decipher a plaintext which ensures authenticity for the message between two individuals.

However, the integrity aspect is not present as the encrypted text can still be messed with, causing the decipher plaintext to appear rubbish due to the modifications done to the cipher text in order to figure out what the message says without the utilization of a key.

Is this the reason for integrity and authenticity to be present? What is general idea to learn when reading through this material?

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Confidentiality vs authenticity

Encryption aims at transmitting a secret message, maintaining it's confidentiality. It does not necessarily provide integrity or authenticity. That it to say, even with proper encryption, it might be possible to alter the encrypted message without that alteration being detected by the receiver.

That matters in many practical situations because adversaries (including Murphy's law) can alter encrypted messages:

  • Even if the decrypted plaintext is nonsense, that could still be an issue, e.g. crash a computer system, which might be enough to cause harm, perhaps serve the interest of an attacker.
  • Knowing that Alice sent to her Banker the order Transfer $10 to Carol using a stream cipher like AES-CTR, or the One Time Pad (which perform secure encryption), it is easy to alter the encrypted message so that it deciphers to Transfer $90 to Carol (a 1-bit change with common encoding of text) or even Transfer $960 to John. All that's required is knowing the position, encoding, original and new value of what's changed, and that the new message is no longer than the original.
  • If it's used public-key encryption, it's trivial to create from the public key a ciphertext that deciphers to anything an adversary wants.

Fortunately, cryptography also has solutions to that, with authentication (or about equivalently in a cryptographic context, integrity protection). The methods for that just are not named encryption. The proper names are

In symmetric cryptography, sender and receiver share a secret key (or passphrase) assumed not known to adversaries. Leaving aside confidentiality and encryption, for authentication using a MAC, a MAC is produced by the sender from message and key, and the MAC sent with the message (e.g. at end ). The receiver separates alleged message and alleged MAC, recomputes the MAC from alleged message and key, and tests if that's the alleged MAC. There's a match if there was no alteration, no match for most random alterations. It's practically impossible to make an alteration that's undetected without knowledge of (or derived from) the key.

In asymmetric cryptography also know as public key cryptography), the key becomes a public/private key pair. The private key is a secret known to the key pair's owner. The public key is assumed known to all (including the receiver). For authentication, a signature is produced by the sender from message and the sender's private key, and sent with the message. The receiver separates alleged message and alleged signature, feeds the sender's public key and alleged signature to a verification procedure, that outputs true if there was no alteration, or false for most random alterations. It's practically impossible to make an alteration that's undetected without knowledge of (or derived from) the private key.


Getting both confidentiality and authenticity

In symmetric cryptography, we can build authenticated encryption from normal encryption, and a MAC (e.g. over the result of the encryption). Increasingly, the two are integrated within the same primitive (e.g. AES-GCM).

In asymmetric cryptography, the sender encrypts with the receiver's public key, and signs with the sender's private key. The receiver checks the signature with the sender's public key, and deciphers with the receiver's private key.

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