Convert XML Dsig format to DER ASN.1 public key

I am working on an iPhone app that retrieves an RSA public key from an ASP.NET web service in the form:

<RSAKeyValue>
  <Modulus>qdd0paiiBJ+xYaN4TKDdbEzrJJw9xlbRAltb5OPdegjLoW60yOjL/sni52WVsGC9QxpNitZR33dnUscmI0cTJoxkXypPjbD94UpH+p4el2tuKBypHlE7bERApuUp55y8BiRkbQNFH8smZFWDwtIc/PsJry开发者_JAVA技巧eGf8fAryel8c5V3PU=</Modulus>
  <Exponent>AQAB</Exponent>
</RSAKeyValue>

I need to then convert this response into an NSData * of the appropriate format (from some intense Googling, most likely 'ASN.1 DER' binary format. I've got code in place to convert both parts from their Base64 representations to the original binary values, but I can't for the life of me figure out a reasonable way to create the one-piece binary key.

The code waiting for the key is the -addPeerPublicKey:(NSString *) keyBits:(NSData *) method of the SecKeyWrapper class from Apple's CryptoExercise example project (Code here).

I would be more than happy to implement this another way--all I need is to encrypt a single string (no decryption required). As far as I can tell, though, the built-in Security framework has what I need, if I could just close this format gap. If there is a way to convert the key and send it Base64-encoded from the webservice, that works for me as well--but I couldn't find any way to ASN.1-encode it there, either.

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]

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)

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;
}