P_SHA-1 in Objective-C for WS-Trust

Question

I'm trying to compute a key for a SOAP signature using P_SHA-1 defined by WS-Trust. The WS-Trust specification says that

key = P_SHA1 (EntropyFromRequest, EntropyFromResponse)

and the TLS spec says P_SHA-1 is

P_SHA-1(secret, seed) =
    HMAC_SHA-1(secret, A(1) + seed) +
    HMAC_SHA-1(secret, A(2) + seed) +
    HMAC_SHA-1(secret, A(3) + seed) + ...
Where + indicates concatenation.

A() is defined as:
A(0) = seed
A(i) = HMAC_SHA-1(secret, A(i-1))

My algorithm looks like so:

- (NSData*) psha1WithSize:(int)bytes;
{
    int numberOfIterations = bytes/16;
    NSData *label;
    NSData *secret;
    NSMutableData *seed;

    NSData *reqEntropy = [NSData dataWithBase64EncodedString:requestEntropy];
    NSData *resEntropy = [NSData dataWithBase64EncodedString:responseEntropy];

    secret = reqEntropy;
    seed = resEntropy;

    NSData *aIMinusOne = seed;      
    NSData *aI = nil;
    NSMutableData *currentData = [[NSMutableData alloc] init];

    for( int i=1; i <= numberOfIterations; i++ )
    {
        aI = [self hmacSha1Data:aIMinusOne withKey:secret];
        NSMutableData *aIPlusSeed = [NSMutableData dataWithData:aI];
        [aIPlusSeed appendData:seed];
        [currentData appendData:[self hmacSha1Data:aIPlusSeed withKey:secret]];
        aIMinusOne = aI;
        aI = nil;
    }
    return currentData;
}

My HMAC looks like these: iPhone and HMAC-SHA-1 encoding

This doesn't seem to be working, and I can't figure out what is wrong with my algorithm. One thought I had was that the service is implemented in .NET WCF, and to get it to accept the hashed password I had to use an NSUTF16LittleEndianStringEncoding, so maybe the response entropy is in the same encoding (prior to base64 encoding)?

Any help is greatly appreciated, thanks.