diff --git a/server/domain/user/service/srp/calcSessionKey.ts b/server/domain/user/service/srp/calcSessionKey.ts
index 3c7e263..0afcdff 100644
--- a/server/domain/user/service/srp/calcSessionKey.ts
+++ b/server/domain/user/service/srp/calcSessionKey.ts
@@ -1,4 +1,3 @@
-import assert from 'assert';
 import crypto from 'crypto';
 import { BigInteger } from 'jsbn';
 import { N, Nbytes } from './constants';
@@ -19,15 +18,9 @@ export const calculateSessionKey = (params: {
   v: string;
 }): Buffer => {
   const Aint = new BigInteger(padHex(params.A), 16);
-  const Bint = new BigInteger(padHex(params.B), 16);
   const bInt = new BigInteger(padHex(params.b), 16);
   const vInt = new BigInteger(params.v, 16);
 
-  assert(Aint.compareTo(BigInteger.ZERO) > 0, 'A should be greater than 0');
-  assert(Aint.compareTo(N) < 0, 'A should be less than N');
-  assert(Bint.compareTo(BigInteger.ZERO) > 0, 'B should be greater than 0');
-  assert(Bint.compareTo(N) < 0, 'A should be less than N');
-
   const scramblingParameter = calculateScramblingParameter(params.A, params.B);
 
   // u = H(A,B) % N
diff --git a/server/domain/user/service/srp/calcSrpA.ts b/server/domain/user/service/srp/calcSrpA.ts
new file mode 100644
index 0000000..152627c
--- /dev/null
+++ b/server/domain/user/service/srp/calcSrpA.ts
@@ -0,0 +1,15 @@
+import { Buffer } from 'buffer';
+import crypto from 'crypto';
+import { N, g } from 'domain/user/service/srp/constants';
+import { fromBuffer, toBuffer } from 'domain/user/service/srp/util';
+import { BigInteger } from 'jsbn';
+
+export const calculateSrpA = (): { a: Buffer; A: Buffer } => {
+  let a = Buffer.from([0]);
+  let AInt = BigInteger.ZERO;
+  while (AInt === BigInteger.ZERO) {
+    a = crypto.randomBytes(32);
+    AInt = g.modPow(fromBuffer(a), N);
+  }
+  return { a, A: toBuffer(AInt) };
+};
diff --git a/server/domain/user/service/srp/calcSrpB.ts b/server/domain/user/service/srp/calcSrpB.ts
index d15b313..1bed68f 100644
--- a/server/domain/user/service/srp/calcSrpB.ts
+++ b/server/domain/user/service/srp/calcSrpB.ts
@@ -9,10 +9,17 @@ export const calculateSrpB = (
   b: string;
   B: string;
 } => {
-  const b = crypto.randomBytes(32);
-  const bInt = fromBuffer(b);
+  let BInt = BigInteger.ZERO;
+  let b = Buffer.from([0]);
   const vInt = new BigInteger(v, 16);
+
+  while (BInt === BigInteger.ZERO) {
+    b = crypto.randomBytes(32);
+    const bInt = fromBuffer(b);
+    BInt = multiplierParam.multiply(vInt).add(g.modPow(bInt, N)).mod(N);
+  }
+
   // kv + g^b
-  const B = toBuffer(multiplierParam.multiply(vInt).add(g.modPow(bInt, N)).mod(N));
+  const B = toBuffer(BInt);
   return { b: b.toString('hex'), B: B.toString('hex') };
 };
diff --git a/server/domain/user/service/srp/util.ts b/server/domain/user/service/srp/util.ts
index fec121f..078f691 100644
--- a/server/domain/user/service/srp/util.ts
+++ b/server/domain/user/service/srp/util.ts
@@ -30,7 +30,7 @@ export const padBufferToHex = (buffer: Buffer): string => {
 
 export const toBuffer = (bigInt: BigInteger): Buffer => {
   const str = bigInt.toString(16);
-  return Buffer.from(str, 'hex');
+  return Buffer.from(str.padStart(str.length + (str.length % 2), '0'), 'hex');
 };
 
 export const toBufferWithLength = (bigInt: BigInteger, length: number): Buffer => {
diff --git a/server/tests/api/changePassword.test.ts b/server/tests/api/changePassword.test.ts
index b0e6a29..87d96ec 100644
--- a/server/tests/api/changePassword.test.ts
+++ b/server/tests/api/changePassword.test.ts
@@ -1,8 +1,7 @@
 import assert from 'assert';
-import crypto from 'crypto';
 import { calcClientSignature } from 'domain/user/service/srp/calcClientSignature';
-import { N, g } from 'domain/user/service/srp/constants';
-import { fromBuffer, toBuffer } from 'domain/user/service/srp/util';
+import { calculateSrpA } from 'domain/user/service/srp/calcSrpA';
+import { fromBuffer } from 'domain/user/service/srp/util';
 import { DEFAULT_USER_POOL_CLIENT_ID } from 'service/envValues';
 import { test } from 'vitest';
 import {
@@ -15,8 +14,7 @@ import {
 
 test('changePassword', async () => {
   await createUserClient();
-  const a = crypto.randomBytes(32);
-  const A = toBuffer(g.modPow(fromBuffer(a), N));
+  const { a, A } = calculateSrpA();
   const res1 = await noCookieClient.$post({
     headers: { 'x-amz-target': 'AWSCognitoIdentityProviderService.InitiateAuth' },
     body: {
diff --git a/server/tests/api/signIn.test.ts b/server/tests/api/signIn.test.ts
index 63f362c..ea5fbee 100644
--- a/server/tests/api/signIn.test.ts
+++ b/server/tests/api/signIn.test.ts
@@ -1,8 +1,7 @@
 import assert from 'assert';
-import crypto from 'crypto';
 import { calcClientSignature } from 'domain/user/service/srp/calcClientSignature';
-import { N, g } from 'domain/user/service/srp/constants';
-import { fromBuffer, toBuffer } from 'domain/user/service/srp/util';
+import { calculateSrpA } from 'domain/user/service/srp/calcSrpA';
+import { fromBuffer } from 'domain/user/service/srp/util';
 import { DEFAULT_USER_POOL_CLIENT_ID } from 'service/envValues';
 import { expect, test } from 'vitest';
 import {
@@ -16,8 +15,7 @@ import {
 test('signIn', async () => {
   await createUserClient();
 
-  const a = crypto.randomBytes(32);
-  const A = toBuffer(g.modPow(fromBuffer(a), N));
+  const { a, A } = calculateSrpA();
   const res1 = await noCookieClient.$post({
     headers: { 'x-amz-target': 'AWSCognitoIdentityProviderService.InitiateAuth' },
     body: {