Probabilistic encryption

Security

Probabilistic encryption is particularly important when using public key cryptography. Suppose that the adversary observes a ciphertext, and suspects that the plaintext is either "YES" or "NO", or has a hunch that the plaintext might be "ATTACK AT CALAIS". When a deterministic encryption algorithm is used, the adversary can simply try encrypting each of their guesses under the recipient's public key, and compare each result to the target ciphertext. To combat this attack, public key encryption schemes must incorporate an element of randomness, ensuring that each plaintext maps into one of a large number of possible ciphertexts.

An intuitive approach to converting a deterministic encryption scheme into a probabilistic one is to simply pad the plaintext with a random string before encrypting with the deterministic algorithm. Conversely, decryption involves applying a deterministic algorithm and ignoring the random padding. However, early schemes which applied this naive approach were broken due to limitations in some deterministic encryption schemes. Techniques such as optimal asymmetric encryption padding (OAEP) integrate random padding in a manner that is secure using any trapdoor permutation.

Examples

Example of probabilistic encryption using any trapdoor permutation:

• x - single bit plaintext
• f - trapdoor permutation (deterministic encryption algorithm)
• b - hard core predicate of f
• r - random string

${\displaystyle \mathrm{E}\mathrm{n}\mathrm{c}(x)=(f(r),x\oplus b(r))}$

${\displaystyle \mathrm{D}\mathrm{e}\mathrm{c}(y,z)=b(f^{-1}(y))\oplus z}$

This is inefficient because only a single bit is encrypted. In other words, the message expansion factor is equal to the public key size.

Example of probabilistic encryption in the random oracle model:

• x - plaintext
• f - trapdoor permutation (deterministic encryption algorithm)
• h - random oracle (typically implemented using a publicly specified hash function)
• r - random string

${\displaystyle \mathrm{E}\mathrm{n}\mathrm{c}(x)=(f(r),x\oplus h(r))}$

${\displaystyle \mathrm{D}\mathrm{e}\mathrm{c}(y,z)=h(f^{-1}(y))\oplus z}$