RSA加密算法方法介绍
昨天看到一个Eclipse插件，要注册。就反编译了一把。找到了算注册码的地方。可是还是看不太懂。发现用了BigInteger运算。又用了m.modPow(d, n)这样的代码。没办法，上google搜了一把。终于明白这里是用了RSA 算法。这里是网上的对RSA算法的介绍：
The RSA algorithm was invented in 1978 by Ron Rivest, Adi Shamir, and Leonard Adleman.
Here's the relatively easy to understand math behind RSA public key encryption.
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Find P and Q, two large (e.g., 1024-bit) prime numbers.
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Choose E such that E is greater than 1, E is less than PQ, and E and (P-1)(Q-1) are relatively prime, which means they have no prime factors in common. E does not have to be prime, but it must be odd. (P-1)(Q-1) can't be prime because it's an even number.
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Compute D such that (DE - 1) is evenly divisible by (P-1)(Q-1). Mathematicians write this as DE=1(mod(P-1)(Q-1)), and they call D the multiplicative inverse of E. This is easy to do -- simply find an integer X which causes D = (X(P-1)(Q-1) + 1)/E to be an integer, then use that value of D.
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The encryption function is C=(T^E)modPQ, where C is the ciphertext (a positive integer), T is the plaintext (a positive integer), and ^ indicates exponentiation. The message being encrypted, T, must be less than the modulus, PQ.
- The decryption function is T=(C^D)modPQ, where C is the ciphertext (a positive integer), T is the plaintext (a positive integer), and ^ indicates exponentiation.
Your public key is the pair (PQ,E). Your private key is the number D (reveal it to no one). The product PQ is the modulus (often called N in the literature). E is the public exponent. D is the secret exponent.
You can publish your public key freely, because there are no known easy methods of calculating D, P, or Q given only (PQ, E) (your public key). If P and Q are each 1024 bits long, the sun will burn out before the most powerful computers presently in existence can factor your modulus into P and Q.
An Example of the RSA Algorithm
P = 61 <- first prime number (destroy this after computing E and D) Q = 53 <- second prime number (destroy this after computing E and D) PQ = 3233 <- modulus (give this to others) E = 17 <- public exponent (give this to others) D = 2753 <- private exponent (keep this secret!) Your public key is (E,PQ). Your private key is D. The encryption function is: encrypt(T) = (T^E) mod PQ = (T^17) mod 3233 The decryption function is: decrypt(C) = (C^D) mod PQ = (C^2753) mod 3233 To encrypt the plaintext value 123, do this: encrypt(123) = (123^17) mod 3233 = 337587917446653715596592958817679803 mod 3233 = 855 To decrypt the ciphertext value 855, do this: decrypt(855) = (855^2753) mod 3233 = 123 One way to compute the value of 855^2753 mod 3233 is like this: 2753 = 101011000001 base 2, therefore 2753 = 1 + 2^6 + 2^7 + 2^9 + 2^11 = 1 + 64 + 128 + 512 + 2048 Consider this table of powers of 855: 855^1 = 855 (mod 3233) 855^2 = 367 (mod 3233) 855^4 = 367^2 (mod 3233) = 2136 (mod 3233) 855^8 = 2136^2 (mod 3233) = 733 (mod 3233) 855^16 = 733^2 (mod 3233) = 611 (mod 3233) 855^32 = 611^2 (mod 3233) = 1526 (mod 3233) 855^64 = 1526^2 (mod 3233) = 916 (mod 3233) 855^128 = 916^2 (mod 3233) = 1709 (mod 3233) 855^256 = 1709^2 (mod 3233) = 1282 (mod 3233) 855^512 = 1282^2 (mod 3233) = 1160 (mod 3233) 855^1024 = 1160^2 (mod 3233) = 672 (mod 3233) 855^2048 = 672^2 (mod 3233) = 2197 (mod 3233) Given the above, we know this: 855^2753 (mod 3233) = 855^(1 + 64 + 128 + 512 + 2048) (mod 3233) = 855^1 * 855^64 * 855^128 * 855^512 * 855^2048 (mod 3233) = 855 * 916 * 1709 * 1160 * 2197 (mod 3233) = 794 * 1709 * 1160 * 2197 (mod 3233) = 2319 * 1160 * 2197 (mod 3233) = 184 * 2197 (mod 3233) = 123 (mod 3233) = 123 If you have a computer program (such as the "bc" utility that comes with Linux), you can compute 855^2753 mod 3233 directly, like this: 855^2753 mod 3233 = 50432888958416068734422899127394466631453878360035509315554967564501 05562861208255997874424542811005438349865428933638493024645144150785 17209179665478263530709963803538732650089668607477182974582295034295 04079035818459409563779385865989368838083602840132509768620766977396 67533250542826093475735137988063256482639334453092594385562429233017 51977190016924916912809150596019178760171349725439279215696701789902 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305946326827861270203356980346143245697021484375 mod 3233 = 123 找到程序中的publicKey,用RSATool算P,Q两个factor,但我的机器太慢，算了一会儿没出来我就放弃了。
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