AWS Cognito user authentication Missing required parameter SRP_A

Question

I am trying to use AWS Cognito services for user authentication through ruby SDK.

I could able to sign_up, confirm sign_up process using the methods

  resp = client.sign_up({ client_id: "ClientIdType",
                          secret_hash: "SecretHashType",
                          username: "UsernameType",
                          password: "PasswordType",
                          user_attributes: [{ name:"AttributeNameType", 
                                    value: "AttributeValueType",
                                     }],
                          validation_data: [{
                          name: "AttributeNameType",
                          value: "AttributeValueType",
                                  }]
                          })

and confirm_sign_up using

  resp = client.confirm_sign_up({client_id: "ClientIdType",
                                 secret_hash: "SecretHashType",
                                 username: "UsernameType",
                                 confirmation_code: "ConfirmationCodeType"
                                })

But while trying to sign in the user through initiate_auth I am getting an error Missing required parameter SRP_A

cog_provider.initiate_auth({client_id: "xxxxxxxxx", auth_parameters: { username: "xxx", password: "xxx"}, auth_flow: "USER_SRP_AUTH"})

What does SRP_A indicate where to find it.

I have searched for this problem and It is suggested to use the admin_initiate_auth method for signing in a user which I don't believe a best practice.

Diminuendo

Answer 1

Yes, SRP_A is a large integer as defined by the Secure Remote Password Protocol. Are you trying to do SRP or just authenticate with username and password. For username/password authentication you should use the AdminInitiateAuth operation.

In our SDKs, you can see the parameters that need to be computed and passed. Take for example the Javascript SDK:

https://github.com/aws/amazon-cognito-identity-js/blob/master/src/CognitoUser.js#L152

Or in the Android SDK:

https://github.com/aws/aws-sdk-android/blob/master/aws-android-sdk-cognitoidentityprovider/src/main/java/com/amazonaws/mobileconnectors/cognitoidentityprovider/CognitoUser.java#L2123

Darla

Is that a best practice to use AdminInitiateAuth ?. Is there any way to get SRP_A value. – Diminuendo

AdminInitiateAuth is only intended for use in a secure server environment. SRP_A is basically a large integer generated client side. For example in Java you can do: a = new BigInteger(EPHEMERAL_KEY_LENGTH, SECURE_RANDOM).mod(N); A = g.modPow(a, N); github.com/aws/aws-sdk-android/blob/master/… Where N is a big prime – Darla

Answer 2

For AWS Java SDK:

here is the class to manage this:

/*
 * To change this license header, choose License Headers in Project Properties.
 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */
package com.math.pro.ak.util.cognito;

import com.amazonaws.AmazonClientException;
import com.amazonaws.util.StringUtils;
import java.math.BigInteger;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

/**
 *
 * @author marcus
 */
public class AuthenticationHelper {

    private BigInteger a;
    private BigInteger A;
    private String poolName;

    public AuthenticationHelper(String userPoolName) {
        do {
            a = new BigInteger(EPHEMERAL_KEY_LENGTH, SECURE_RANDOM).mod(N);
            A = GG.modPow(a, N);
        } while (A.mod(N).equals(BigInteger.ZERO));

        if (userPoolName.contains("_")) {
            poolName = userPoolName.split("_", 2)[1];
        } else {
            poolName = userPoolName;
        }
    }

    public BigInteger geta() {
        return a;
    }

    public BigInteger getA() {
        return A;
    }

    private static final String HEX_N = "FFFFFFFFFFFFFFFFC90FDAA22168C234C4C6628B80DC1CD1"
            + "29024E088A67CC74020BBEA63B139B22514A08798E3404DD"
            + "EF9519B3CD3A431B302B0A6DF25F14374FE1356D6D51C245"
            + "E485B576625E7EC6F44C42E9A637ED6B0BFF5CB6F406B7ED"
            + "EE386BFB5A899FA5AE9F24117C4B1FE649286651ECE45B3D"
            + "C2007CB8A163BF0598DA48361C55D39A69163FA8FD24CF5F"
            + "83655D23DCA3AD961C62F356208552BB9ED529077096966D"
            + "670C354E4ABC9804F1746C08CA18217C32905E462E36CE3B"
            + "E39E772C180E86039B2783A2EC07A28FB5C55DF06F4C52C9"
            + "DE2BCBF6955817183995497CEA956AE515D2261898FA0510"
            + "15728E5A8AAAC42DAD33170D04507A33A85521ABDF1CBA64"
            + "ECFB850458DBEF0A8AEA71575D060C7DB3970F85A6E1E4C7"
            + "ABF5AE8CDB0933D71E8C94E04A25619DCEE3D2261AD2EE6B"
            + "F12FFA06D98A0864D87602733EC86A64521F2B18177B200C"
            + "BBE117577A615D6C770988C0BAD946E208E24FA074E5AB31"
            + "43DB5BFCE0FD108E4B82D120A93AD2CAFFFFFFFFFFFFFFFF";
    private static final BigInteger N = new BigInteger(HEX_N, 16);
    private static final BigInteger GG = BigInteger.valueOf(2);
    private static final BigInteger KK;

    private static final int EPHEMERAL_KEY_LENGTH = 1024;
    private static final int DERIVED_KEY_SIZE = 16;
    private static final String DERIVED_KEY_INFO = "Caldera Derived Key";

    private static final ThreadLocal<MessageDigest> THREAD_MESSAGE_DIGEST = new ThreadLocal<MessageDigest>() {
        @Override
        protected MessageDigest initialValue() {
            try {
                return MessageDigest.getInstance("SHA-256");
            } catch (final NoSuchAlgorithmException e) {
                throw new AmazonClientException("Exception in authentication", e);
            }
        }
    };

    private static final SecureRandom SECURE_RANDOM;

    static {
        try {
            SECURE_RANDOM = SecureRandom.getInstance("SHA1PRNG");

            final MessageDigest messageDigest = THREAD_MESSAGE_DIGEST.get();
            messageDigest.reset();
            messageDigest.update(N.toByteArray());
            final byte[] digest = messageDigest.digest(GG.toByteArray());
            KK = new BigInteger(1, digest);
        } catch (final NoSuchAlgorithmException e) {
            throw new AmazonClientException(e.getMessage(), e);
        }
    }

    public byte[] getPasswordAuthenticationKey(String userId,
            String userPassword,
            BigInteger B,
            BigInteger salt) {
        // Authenticate the password
        // u = H(A, B)
        final MessageDigest messageDigest = THREAD_MESSAGE_DIGEST.get();
        messageDigest.reset();
        messageDigest.update(A.toByteArray());
        final BigInteger u = new BigInteger(1, messageDigest.digest(B.toByteArray()));
        if (u.equals(BigInteger.ZERO)) {
            throw new AmazonClientException("Hash of A and B cannot be zero");
        }

        // x = H(salt | H(poolName | userId | ":" | password))
        messageDigest.reset();
        messageDigest.update(poolName.getBytes(StringUtils.UTF8));
        messageDigest.update(userId.getBytes(StringUtils.UTF8));
        messageDigest.update(":".getBytes(StringUtils.UTF8));
        final byte[] userIdHash = messageDigest.digest(userPassword.getBytes(StringUtils.UTF8));

        messageDigest.reset();
        messageDigest.update(salt.toByteArray());
        final BigInteger x = new BigInteger(1, messageDigest.digest(userIdHash));
        final BigInteger s = (B.subtract(KK.multiply(GG.modPow(x, N)))
                .modPow(a.add(u.multiply(x)), N)).mod(N);

        Hkdf hkdf = null;
        try {
            hkdf = Hkdf.getInstance("HmacSHA256");
        } catch (final NoSuchAlgorithmException e) {
            throw new AmazonClientException(e.getMessage(), e);
        }
        hkdf.init(s.toByteArray(), u.toByteArray());
        final byte[] key = hkdf.deriveKey(DERIVED_KEY_INFO, DERIVED_KEY_SIZE);
        return key;
    }

}

And call this the method: userAuth.put("SRP_A", new AuthenticationHelper(request.getUsername()).getA().toString(16));

Sarabia

Hi Marckaraujo, your code worked like a charm, but as per docs if you send an alias in initiateAuth then its okay - but when I try to do that I get "User does not exists" - I am using a Lambda for signin process. please guide – Cud

alias wasnt working because it was not verified using GetUserAttributeVerificationCode & VerifyUserAttribute api calls.. now it worked – Cud

what is the request atribute u are using for getUsername()? – Abecedarium

what library are you using for Hkdf? – Sloat