RSA签名消息的长度限制

Question

我正在创建签名，并且在签名的消息长度方面遇到了麻烦。通过遵循Apple iOS（http://apple.co/2ucaOTB）的编码指南，以下代码适用于任何短于223长度的消息，不能超过222.您能告诉我为什么它严格限制在222？< / p>

NSString *dataToSign = @"eyJhbGciOiJSUzUxMiIsImtpZCI6IkFTLVVXUC1DZXJ0LTAxIiwieDVIjoiSmpBZHB3TGt4YVNvWWF0Z2NnaElpZlJ4c3A0In0śhgrphgrpehgersfnrwuiefbgerwighberuyabfgeruykbverkhgerbiuseyJzdWIiOiI2ZjAzNGViZS101MDc4ZDBjZTI0ZjIiLCJlbfasdfdfdfdfdfdqweja";//222 chars.

NSData* data2sign = [dataToSign dataUsingEncoding:NSUTF8StringEncoding];

NSLog(@"Data is %u", data2sign.length);// %u will be 222.

SecKeyAlgorithm algorithm = kSecKeyAlgorithmRSASignatureDigestPSSSHA256;

// privateKey is 2048 bits asymmetric RSA.
BOOL canSign = SecKeyIsAlgorithmSupported(privateKey, kSecKeyOperationTypeSign, algorithm);

if (canSign) {// It is true, and it tries to create signature.
    CFErrorRef error = NULL;
    signature = (NSData *)CFBridgingRelease(
            SecKeyCreateSignature(
                privateKey,
                algorithm,
                (__bridge CFDataRef)data2Sign,
                &error));

    if (!signature) {
        NSError *err = CFBridgingRelease(error);
        NSLog(@"The error is: %@", err);
    }
    NSLog(@"The signature is %@", signature);
}