Abstract: A circuit and method for sharing a key among n individuals by distributing a secondary key to each of the n individuals such that only k of the n secondary keys are required in order to reconstruct the key, where k may be less than n. The key is encoded into a code vector which is corrupted by combining it with an error vector to yield a corrupted code vector. The secondary keys are chosen so that each one is a mutually exclusive part of the error vector. Reconstructing the key, which is the objective of the process and is analagous to opening a lock, requires correcting the corrupted code vector and transforming the code vector into the key. The corrupted code vector is corrected by a combination of an error correcting code and use of at least k of the secondary keys.

Abstract: An encryption and decryption circuit and method using subkeys which is therefore particularly useful for the encryption and decryption of database information. Each data field has a corresponding write key and a different read key. A database record is encrypted by multiplying each field by its corresponding write key and adding up all such products modulo the product of all read keys. The read keys must be prime numbers and the write keys must satisfy a certain relationship with respect to the read keys. A database record may have a single field decrypted by dividing the encrypted record by the read key for that field and taking the remainder. The security of each field may thereby be individually controlled without compromising the security of the entire record.

Abstract: A complete substitution permutation enciphering and deciphering circuit and method for transforming a plaintext signal into an enciphered signal which is secure during transmission. After receipt, the enciphered signal is transformed back into the original plaintext signal by use of a similar circuit and method. A complete substitution permutation enciphering and deciphering circuit is constructed by interconnecting a number of substitution boxes. Each substitution box is complete, that is each binary bit of the substitution box output is dependent on all binary bits of the substitution box input for all possible combinations of input signals. When such substitution boxes are connected according to the invention, the substitution permutation enciphering and deciphering circuit formed thereby is complete in the sense that every output signal of the circuit is dependent on all input signals of the circuit.