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对称密码的类型可分为序列密码和分组密码。序列密码每次加密一位或一个字节的明文。具体而言，首先选择一个随机串作为密钥，然后将明文转变成一个位串，比如使用明文的ASCII表示法。最后，逐位计算这两个串的异或值，结果得到的密文不可能破解，因为即使有了足够数量的密文样本，每个字符的出现概率都是相等的，每任意个字母组合出现的概率也是相等的。这种方法被称为“一次一密”，信息论的创始人香农论证了只有这一种密码算法是理论上不可解的。

Symmetric ciphers can be divided into stream ciphers and block ciphers . Stream ciphers encrypt one or each time a byte plaintext. Specifically, first choose a random string as the key , and then the plaintext into a bit string , such as the use of plaintext ASCII representation . Finally, calculate bitwise XOR of two strings value , the result of ciphertext impossible to crack , because even with a sufficient number of samples of ciphertext , each character is equal to the probability of occurrence of each combination of any letters probability is equal. This method is called " one-time pad " , the founder of Shannon information theory proves only that a cryptographic algorithm is theoretically unsolvable .

分组密码是将明文消息编码而成的数字（简称明文数字）序列，划分成长度为n的组（可看成长度为n的失量），每组分别在密钥的控制下变换成等长的输出数字（简称密文数字）序列。

Plaintext message block cipher is encoded in a digital ( abbreviated plain numbers ) sequences of length n into a group (which can be considered as loss of the amount of length n ) , respectively, each under the control of the key length is converted into other output digital ( referred to ciphertext digit ) sequence.

In order to guarantee the security of block cipher strength of encryption algorithm requires the following:

分组长度足够大。当分组长度较小时，分组密码类似于古典的代替密码，它仍然保留了明文的统计作息，这种统计信息将给攻击者留下可乘之机，攻击者可以有效地穷举明文空间，得到密码变换本身。
Packet length is sufficiently large . When the packet length is small, similar to the classical block cipher instead of the password , it still retains a clear statistical routines , this statistical information will be exploited by an attacker , the attacker can effectively exhaustive plaintext space , get password transform itself .

Key is large enough . Identified key block cipher cryptographic transformation is just very small part in all permutations . If this part is small enough , an attacker can be effectively determined by the exhaustive permutations plaintext space . At this time, an attacker can decrypt the ciphertext to obtain meaningful plaintext.
Password transform complex enough . Allow an attacker to brute-force method in addition to outside , no other efficient way to decipher . Currently used in the block cipher overall structure can be divided into Feistel structure (such DEAL/E2 etc. ), SP network (such as Safer +, Serpent ) and other cipher structure ( eg Frog and HPC). Since the announcement of Feistel structure known as DES , has been adopted by many of the block cipher . Feistel structure, it is easy to ensure the greatest advantage similar decryption , which is particularly important in reality .

Symmetric key algorithms are used : Data Encryption Standard (Data Encryption Standard, referred to as DES), IDEA, 3DES, Advanced Encryption Standard (Advanced Encryption Standard, referred to as AES) , etc. Older Data Encryption Standard DES is currently the highest encryption strength symmetric encryption algorithm is Advanced Encryption Standard AES.

Asymmetric key encryption and decryption algorithms that use different keys , one can本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。