Objective-C Base58 Decoder / Bitcoin Address Validator

decoder.m

#pragma mark Address validation

// C implementation - credits to Gavin Anderseen I believe
-(NSData *)decodeBase58:(const char *)addy bytes:(unsigned char *)bytes {
    static const char *base58 = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz";
    
    int i, j, c;
    const char *p;
    memset(bytes, 0, 25);
    
    for (i = 0; addy[i]; i++) {
        if (!(p = strchr(base58, addy[i]))) return nil;
        
        c = p - base58;
        for (j = 25; j--; ) {
            c += 58 * bytes[j];
            bytes[j] = c % 256;
            c /= 256;
        }
        if (c) return nil;
    }
    return [NSData dataWithBytes:(const void *)bytes length:25];
}

-(BOOL)verifyAddress:(NSString *)addressString{
    unsigned char bytes[32];
    NSData *unbased = [self decodeBase58:[addressString cStringUsingEncoding:NSASCIIStringEncoding] bytes:bytes];
    if (unbased != nil) {
        NSData *doubleSha = [self sha256:[unbased subdataWithRange:NSMakeRange(0, 21)]];
        NSData *first4 = [doubleSha subdataWithRange:NSMakeRange(0, 4)];
        NSData *last4 = [unbased subdataWithRange:NSMakeRange([unbased length]-4,4)];
        return [first4 isEqualToData:last4];
    }
    return false;
}

- (NSData *)sha256:(NSData *)data {
    uint8_t digest[CC_SHA256_DIGEST_LENGTH];
    CC_SHA256(data.bytes, (CC_LONG)data.length, digest);
    
    NSData *firstHash = [NSData dataWithBytes:digest length:CC_SHA256_DIGEST_LENGTH];
    
    uint8_t digest2[CC_SHA256_DIGEST_LENGTH];
    
    // check
    if (CC_SHA256(firstHash.bytes, (CC_LONG)firstHash.length, digest2)) {
        return [NSData dataWithBytes:digest2 length:CC_SHA256_DIGEST_LENGTH];
    }
    return nil;
}