使用公钥的RSA加密.基于密钥的数据大小 [英] RSA encryption using public key. Data size based on key

问题描述

我正在使用OpenSSL RSA API通过服务器的公钥加密数据.

I'm using the OpenSSL RSA API to encrypt data with a server's public key.

uint32_t rsasize = RSA_size(keypair); // -- RSA key size == 256

uint32_t dl = 255
uint8_t *d; // points to 255 bytes of data

unsigned char *encrypt = (unsigned char*)malloc(rsasize);

auto num = RSA_public_encrypt(dl, d, encrypt, keypair, RSA_NO_PADDING);
if ( num == -1 )
{
    auto err = malloc(130);
    ERR_load_crypto_strings();
    ERR_error_string(ERR_get_error(), (char*)err);
    printf("Error encrypting message: %s\n", err);
    return nullptr;
}

我正在使用RSA_NO_PADDING，因此RSA应该使用256字节的公钥轻松加密255字节.但我收到了这个:

I'm using RSA_NO_PADDING so RSA should crypt 255 bytes with 256 byte public key easily. But I'm receiving this:

Error encrypting message: error:0406B07A:rsa routines:RSA_padding_add_none:data too small for key size

我将dl(data_lenght)更改为256(仅+1)，我明白了:

I changed dl(data_lenght) to 256 (just +1) and I got this:

Error encrypting message: error:04068084:rsa routines:RSA_EAY_PUBLIC_ENCRYPT:data too large for modulus

我知道RSA可以使用256密钥编码255个字节.有什么问题吗?

I know that RSA can encode 255 bytes with 256-key. What's the problem?

推荐答案

您在错误的一端进行填充.如果您有255个字节的数据，并且想将其填充到256个字节，则应在高阶末尾添加一个零字节 .换句话说，您应该在d [0]之前插入0.

You are padding at the wrong end. If you have 255 bytes of data and you want to pad it out to 256 you should add a zero byte at the high-order end. In other words, you should insert a 0 just before d[0].

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