所以，我使用 SecKeyWrapper 类来生成随机密钥，然后使用 -getPublicKeyBits 获取公钥的二进制表示的方法（内部使用的任何格式）。假设它是DER ASN.1的某种形式，我将它作为十六进制格式化到控制台并将其加载到此程序中。果然，内部表示是DER ASN.1，但它是我通常在RSA密钥表示中找到的非常简化的版本：

So, I used the SecKeyWrapper class to generate a random key, then used the -getPublicKeyBits method to get the binary representation of the public key (in whatever format is used internally). Presuming it is some form of DER ASN.1, I NSLog'd it to the console as hex and loaded it into this program. Sure enough, the internal representation is DER ASN.1, but it is a very simplified version of what I normally found for RSA key representations:

![SEQUENCE { INTEGER, INTEGER }][2]

不应该太强硬从二进制代表动态构建。模数和指数，因为DER编码只是

Shouldn't be too tough to construct on the fly from a binary rep. of the modulus and exponent, since the DER encoding is just

30 (for SEQUENCE) LL (total sequence byte length) 
02 (INTEGER) LL (modulus byte length) XX XX... (modulus data bytes) 
02 LL XX XX XX... (exponent length and bytes)

这是我的代码，为简单起见。它使用了一些用于XML + base64的Google库，只是抬头;还有Apple的演示代码SecKeyWrapper。有关完成此工作的说明，请参阅我的其他问题。另请注意，它与不 ARC兼容;这是留给读者的练习（我今年写的，现在）。

Here's my code, for simplicity. It uses a few Google libs for XML+base64, just heads up; also Apple's demo code SecKeyWrapper. See my other question for a note on making this work. Also, note that it is not ARC-compatible; this is left as an exercise for the reader (I wrote this years ago, now).

#define kTempPublicKey @"tempPayKey"
-(NSData *)encryptedDataWithXMLPublicKey:(NSString *)base64PublicKey data:(NSData *)data {
    if(![data length]){
        @throw [NSException exceptionWithName:@"NSInvalidArgumentException" reason:@"Data not set." userInfo:nil];
    }
    GTMStringEncoding *base64 = [GTMStringEncoding rfc4648Base64StringEncoding];
    NSData *keyData = [base64 decode:base64PublicKey];
    NSError *err = nil;
    GDataXMLDocument *keyDoc = [[GDataXMLDocument alloc] initWithData:keyData options:0 error:&err];
    if(err){
        NSLog(@"Public key parse error: %@",err);
        [keyDoc release];
        return nil;
    }

    NSString *mod64 = [[[[keyDoc rootElement] elementsForName:@"Modulus"] lastObject] stringValue];
    NSString *exp64 = [[[[keyDoc rootElement] elementsForName:@"Exponent"] lastObject] stringValue];
    [keyDoc release];
    if(![mod64 length] || ![exp64 length]){
        @throw [NSException exceptionWithName:@"NSInvalidArgumentException" reason:@"Malformed public key xml." userInfo:nil];
    }

    NSData *modBits = [base64 decode:mod64];
    NSData *expBits = [base64 decode:exp64];

    /* the following is my (bmosher) hack to hand-encode the mod and exp
     * into full DER encoding format, using the following as a guide:
     * http://luca.ntop.org/Teaching/Appunti/asn1.html
     * this is due to the unfortunate fact that the underlying API will
     * only accept this format (not the separate values)
     */

    // 6 extra bytes for tags and lengths
    NSMutableData *fullKey = [[NSMutableData alloc] initWithLength:6+[modBits length]+[expBits length]];
    unsigned char *fullKeyBytes = [fullKey mutableBytes];
    unsigned int bytep = 0; // current byte pointer
    fullKeyBytes[bytep++] = 0x30;
    if(4+[modBits length]+[expBits length] >= 128){
        fullKeyBytes[bytep++] = 0x81;
        [fullKey increaseLengthBy:1];
    }
    unsigned int seqLenLoc = bytep;
    fullKeyBytes[bytep++] = 4+[modBits length]+[expBits length];
    fullKeyBytes[bytep++] = 0x02;
    if([modBits length] >= 128){
        fullKeyBytes[bytep++] = 0x81;
        [fullKey increaseLengthBy:1];
        fullKeyBytes[seqLenLoc]++;
    }
    fullKeyBytes[bytep++] = [modBits length];
    [modBits getBytes:&fullKeyBytes[bytep]];
    bytep += [modBits length];
    fullKeyBytes[bytep++] = 0x02;
    fullKeyBytes[bytep++] = [expBits length];
    [expBits getBytes:&fullKeyBytes[bytep++]];

    SecKeyRef publicKey = [[SecKeyWrapper sharedWrapper] addPeerPublicKey:kTempPublicKey keyBits:fullKey];
    [fullKey release];

    NSData *encrypted = [[SecKeyWrapper sharedWrapper] wrapSymmetricKey:data keyRef:publicKey];
    // remove temporary key from keystore
    [[SecKeyWrapper sharedWrapper] removePeerPublicKey:kTempPublicKey];

    return encrypted;
}