The Paillier cryptosystem, named after and invented by Pascal Paillier in 1999, is a probabilistic asymmetric algorithm for public key cryptography. The problem of computing nth residue classes is believed to be computationally difficult. The decisional composite residuosity assumption is the intractability hypothesis upon which this cryptosystem is based.
The scheme is an additive homomorphic cryptosystem; this means that, given only the publickey and the encryption of and , one can compute the encryption of .
Algorithm[]
The scheme works as follows:
Key generation[]
 Choose two large prime numbers p and q randomly and independently of each other such that . This property is assured if both primes are of equivalent length, i.e., for security parameter .^{[1]}
 Compute and .
 Select random integer where
 Ensure divides the order of by checking the existence of the following modular multiplicative inverse: ,
 where function is defined as .
 Note that the notation does not denote the modular multiplication of times the modular multiplicative inverse of but rather the quotient of divided by , i.e., the largest integer value to satisfy the relation .
 The public (encryption) key is .
 The private (decryption) key is
If using p,q of equivalent length, a simpler variant of the above key generation steps would be to set and , where .^{[1]}
Encryption[]
 Let be a message to be encrypted where
 Select random where
 Compute ciphertext as:
Decryption[]
 Ciphertext
 Compute message:
As the original paper points out, decryption is "essentially one exponentiation modulo ."
Homomorphic properties[]
A notable feature of the Paillier cryptosystem is its homomorphic properties. As the encryption function is additively homomorphic, the following identities can be described:
 Homomorphic addition of plaintexts
 The product of two ciphertexts will decrypt to the sum of their corresponding plaintexts,
 The product of a ciphertext with a plaintext raising g will decrypt to the sum of the corresponding plaintexts,
 Homomorphic multiplication of plaintexts
 An encrypted plaintext raised to the power of another plaintext will decrypt to the product of the two plaintexts,
 More generally, an encrypted plaintext raised to a constant k will decrypt to the product of the plaintext and the constant,
However, given the Paillier encryptions of two messages there is no known way to compute an encryption of the product of these messages without knowing the private key.
Semantic Security[]
The original cryptosystem as shown above does provide semantic security against chosenplaintext attacks (INDCPA). The ability to successfully distinguish the challenge ciphertext essentially amounts to the ability to decide composite residuosity. The socalled decisional composite residuosity assumption (DCRA) is believed to be intractable.
Because of the aforementioned homomorphic properties however, the system is malleable, and therefore does not enjoy the highest echelon of semantic security that protects against adaptive chosenciphertext attacks (INDCCA2). Usually in cryptography the notion of malleability is not seen as an "advantage," but under certain applications such as secure electronic voting and threshold cryptosystems, this property may indeed be necessary.
Paillier and Pointcheval however went on to propose an improved cryptosystem that incorporates the combined hashing of message m with random r. Similar in intent to the CramerShoup cryptosystem, the hashing prevents an attacker, given only c, from being able to change m in a meaningful way. Through this adaptation the improved scheme can be shown to be INDCCA2 secure in the random oracle model.
Applications[]
 Electronic voting
Semantic security is not the only consideration. There are situations under which malleability may be desirable. The above homomorphic properties can be utilized by secure electronic voting systems. Consider a simple binary ("for" or "against") vote. Let m voters cast a vote of either 1 (for) or 0 (against). Each voter encrypts their choice before casting their vote. The election official takes the product of the m encrypted votes and then decrypts the result and obtains the value n, which is the sum of all the votes. The election official then knows that n people voted for and mn people voted against. The role of the random r ensures that two equivalent votes will encrypt to the same value only with negligible likelihood, hence ensuring voter privacy.
 Electronic cash
Another feature named in paper is the notion of selfblinding. This is the ability to change one ciphertext into another without changing the content of its decryption. This has application to the development of electronic cash, an effort originally spearheaded by David Chaum. Imagine paying for an item online without the vendor needing to know your credit card number, and hence your identity. The goal in both electronic cash and electronic voting, is to ensure the ecoin (likewise evote) is valid, while at the same time not disclosing the identity of the person with whom it is currently associated.
See also[]
 The OkamotoUchiyama cryptosystem as a historical antecedent of Paillier.
 The DamgårdJurik cryptosystem is a generalization of Paillier.
 The Paillier cryptosystem interactive simulator demonstrates a voting application.
 An interactive demo of the Paillier cryptosystem.
 A googletechtalk video on voting using cryptographic methods.
References[]
 Pascal Paillier, PublicKey Cryptosystems Based on Composite Degree Residuosity Classes, EUROCRYPT 1999, pp223238.
 Pascal Paillier, David Pointcheval, Efficient PublicKey Cryptosystems Provably Secure Against Active Adversaries, ASIACRYPT 1999
 Pascal Paillier, PhD Thesis, 1999
 Pascal Paillier, CompositeResiduosity Based Cryptography: An Overview, CryptoBytes Vol. 5 No. 1, 2002
Notes[]
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External links[]
 The Homomorphic Encryption Project implements the Paillier cryptosystem along with its homomorphic operations.

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 ↑ ^{1.0} ^{1.1} Jonathan Katz, Yehuda Lindell, "Introduction to Modern Cryptography: Principles and Protocols," Chapman & Hall/CRC, 2007