将RSA公钥(2048位)从XML格式转换为iOS版的DER ASN.1公钥

Question

Am正在尝试从C#以XML格式共享的PublicKey（2048位）数据创建SecKeyRef。数据如下：

<Modulus>yLgpOFtg14GjDdle0xha2JCbYrpmKCpXcv+zFx2pi6OlIF8cOSXF6dE19W15+WfuCc2SznVQlVDOLp/NlPGLXKN5L47XYQrPpcls8/xp2PYYW7hjezx7Ig6+WDJnUxSbWgxZQMaiyO1XbCKll5yT2AxUj4od/DfylsVe8ljq154rEb+vCjr/LDpxExijHouJYDNqFV1jglWHWfftBQAzEZADPx7NpHsgXSYrFeVY/WH38GIlyO8FvGWIuDiwyRrHUEXhljmBqAJ3lgULyik3ShfjpN1W4h7BbzFs27mpiAPMtPgToADPNzOadRWFJQjeVVknIq5g6SHHnaZK8wBwrQ==</Modulus><Exponent>AQAB</Exponent>]]>"

我遵循了将XML-Dsig格式转换为DER ASN.1公钥链接，该链接仅适用于1024位密钥，但相同的逻辑无法对上述密钥（2048位）进行DER编码。虽然我可以在评论中看到，转换逻辑需要为256字节的密钥进行修改，但修改后无法成功！

我还尝试过从Modular&Exponent在iOS上使用OpenSSL库，但无法使用此生成RSA对象！以下是示例：

NSString *mod = @"yLgpOFtg14GjDdle0xha2JCbYrpmKCpXcv+zFx2pi6OlIF8cOSXF6dE19W15+WfuCc2SznVQlVDOLp/NlPGLXKN5L47XYQrPpcls8/xp2PYYW7hjezx7Ig6+WDJnUxSbWgxZQMaiyO1XbCKll5yT2AxUj4od/DfylsVe8ljq154rEb+vCjr/LDpxExijHouJYDNqFV1jglWHWfftBQAzEZADPx7NpHsgXSYrFeVY/WH38GIlyO8FvGWIuDiwyRrHUEXhljmBqAJ3lgULyik3ShfjpN1W4h7BbzFs27mpiAPMtPgToADPNzOadRWFJQjeVVknIq5g6SHHnaZK8wBwrQ==";

BIGNUM *modulus = BN_new();
int res = BN_hex2bn(&modulus,[mod cStringUsingEncoding:NSUTF8StringEncoding]);
BIGNUM *exponent = BN_new();
NSString *exp = @"AQAB";
res = BN_hex2bn(&exponent,[exp cStringUsingEncoding:NSUTF8StringEncoding]);
RSA *rsa = RSA_new();
rsa->n = BN_new();
BN_copy(rsa->n,modulus);
rsa->e = BN_new();
BN_copy(rsa->e,exponent);
rsa->iqmp=NULL;
rsa->d=NULL;
rsa->p=NULL;
rsa->q=NULL;
FILE *fp = fopen("/publicKey.pem", "wb");
int suc = PEM_write_RSAPublicKey(fp, rsa, NULL, NULL, 0, 0, NULL);

生成的publicKey.pem文件显然是空的！如果有人能帮助我将XML RSA公钥导入iOS SecKetRef对象，那就太好了。

提前感谢：）

Answer 1

我可以用上面提到的两种方法成功地解决这个问题。这是我的解决方案：

方法#1。将XML密钥转换为ASN.1 DER格式，使用此链接使用256字节密钥（2048位）。https://github.com/meinside/iphonelib/blob/master/security/CryptoUtil.m#L67

方法#2。可以使用以下代码创建的RSA对象（参考：在iOS上使用OpenSSL从模数和指数生成公钥）成功加密：

`NSString*mod=@"yLgpOFtg14GjDdle0xha2JCbYrpmKCpXcv+zFx2pi6OlIF8cOSXF6dE19W15+WfuCc2SnVQlVDOLp/NlPGLXKN5L47XYQrPpcls8/xp2PYYW7hjezx7Ig6+WDJnUxSbWgxZQMaiyO1XbKll5yT2AxUj4od/DfylsVe8ljq154rEb+vCjr/LDpxExijHouJYDNqFV1jglWHWfftBQAZEZ ADPx7NpHsgXSYrFeVY/WH38GIlyO8FvGWIuDiwyRrHUEXhljmBqAJ3lgULyik3ShfjpN1W4h7BbzFs27mpiAPMtPgToADPNzOadRWFJQjeVVknIq5g6SHHnaZK8wBwrQ=="；

NSString*exp=@"AQAB"；

NSData *modBits = [[NSData alloc] initWithBase64EncodedString:mod options:NSDataBase64DecodingIgnoreUnknownCharacters];
NSData *expBits = [[NSData alloc] initWithBase64EncodedString:exp options:NSDataBase64DecodingIgnoreUnknownCharacters];
unsigned char *modBin = (unsigned char *)malloc(modBits.length);
[modBits getBytes:modBin length:modBits.length];
unsigned char *expBin = (unsigned char *)malloc(expBits.length);
[expBits getBytes:expBin length:expBits.length];

BIGNUM *returnValue = BN_new();
BIGNUM *modulus = BN_bin2bn(modBin, modBits.length, returnValue);
BIGNUM *exponent = BN_bin2bn(expBin, expBits.length, NULL);
RSA *rsa = RSA_new();
rsa->n = BN_new();
BN_copy(rsa->n,modulus);
rsa->e = BN_new();
BN_copy(rsa->e,exponent);
rsa->iqmp=NULL;
rsa->d=NULL;
rsa->p=NULL;
rsa->q=NULL;

unsigned char *orgTxt = (unsigned char*)strdup("Hello World");
int lenrsa = RSA_size( rsa ) - 11; // man RSA_public_encrypt for the -11
unsigned char *encrText = (unsigned char *)malloc(lenrsa);
int result = RSA_public_encrypt(strlen((const char*)orgTxt), orgTxt, encrText, rsa,RSA_PKCS1_PADDING);

Be sure of the glen param of RSA_public_encrypt API, it depends on the padding used. man RSA_public_encrypt`了解更多详细信息。