Generating SRP_A for AWS Cognito user login for a performance test script using JMeter

You can generate this SRP_A value using suitable JSR223 Test Element and Groovy language.

Here is example code you can use as it is or as a reference:

import com.amazonaws.AmazonClientException
import com.amazonaws.util.StringUtils

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;
    }

}

More information:

• User Pool Authentication Flow
• AWS SDK for Java (you need to have it under JMeter Classpath in order to be able to use the above code)
• AWS Cognito user authentication Missing required parameter SRP_A