diff --git a/packages/core/src/base-client.ts b/packages/core/src/base-client.ts
index dec5e955..e9d84a34 100644
--- a/packages/core/src/base-client.ts
+++ b/packages/core/src/base-client.ts
@@ -38,6 +38,7 @@ import { ITelegramStorage, MemoryStorage } from './storage'
 
 import defaultReaderMap from '@mtcute/tl/binary/reader'
 import defaultWriterMap from '@mtcute/tl/binary/writer'
+import { readStringSession, writeStringSession } from "./utils/string-session";
 
 export namespace BaseTelegramClient {
     export interface Options {
@@ -444,41 +445,23 @@ export class BaseTelegramClient extends EventEmitter {
         )
 
         if ((this._importForce || !this.primaryConnection.getAuthKey()) && this._importFrom) {
-            const buf = parseUrlSafeBase64(this._importFrom)
-            if (buf[0] !== 1)
-                throw new Error(`Invalid session string (version = ${buf[0]})`)
+            const data = readStringSession(this._readerMap, this._importFrom)
 
-            const reader = new TlBinaryReader(this._readerMap, buf, 1)
-
-            const flags = reader.int()
-            const hasSelf = flags & 1
-
-            if (!(flags & 2) !== !this._testMode) {
+            if (data.testMode !== !this._testMode) {
                 throw new Error(
                     'This session string is not for the current backend'
                 )
             }
 
-            const primaryDc = reader.object()
-            if (primaryDc._ !== 'dcOption') {
-                throw new Error(
-                    `Invalid session string (dc._ = ${primaryDc._})`
-                )
+            this._primaryDc = this.primaryConnection.params.dc = data.primaryDc
+            await this.storage.setDefaultDc(data.primaryDc)
+
+            if (data.self) {
+                await this.storage.setSelf(data.self)
             }
 
-            this._primaryDc = this.primaryConnection.params.dc = primaryDc
-            await this.storage.setDefaultDc(primaryDc)
-
-            if (hasSelf) {
-                const selfId = reader.int53()
-                const selfBot = reader.boolean()
-
-                await this.storage.setSelf({ userId: selfId, isBot: selfBot })
-            }
-
-            const key = reader.bytes()
-            await this.primaryConnection.setupKeys(key)
-            await this.storage.setAuthKeyFor(primaryDc.id, key)
+            await this.primaryConnection.setupKeys(data.authKey)
+            await this.storage.setAuthKeyFor(data.primaryDc.id, data.authKey)
 
             await this._saveStorage(true)
         }
@@ -968,35 +951,13 @@ export class BaseTelegramClient extends EventEmitter {
         if (!this.primaryConnection.getAuthKey())
             throw new Error('Auth key is not generated yet')
 
-        const writer = TlBinaryWriter.alloc(this._writerMap, 512)
-
-        const self = await this.storage.getSelf()
-
-        const version = 1
-        let flags = 0
-
-        if (self) {
-            flags |= 1
-        }
-
-        if (this._testMode) {
-            flags |= 2
-        }
-
-        writer.buffer[0] = version
-        writer.pos += 1
-
-        writer.int(flags)
-        writer.object(this._primaryDc)
-
-        if (self) {
-            writer.int53(self.userId)
-            writer.boolean(self.isBot)
-        }
-
-        writer.bytes(this.primaryConnection.getAuthKey()!)
-
-        return encodeUrlSafeBase64(writer.result())
+        return writeStringSession(this._writerMap, {
+            version: 1,
+            self: await this.storage.getSelf(),
+            testMode: this._testMode,
+            primaryDc: this._primaryDc,
+            authKey: this.primaryConnection.getAuthKey()!,
+        })
     }
 
     /**
diff --git a/packages/core/src/utils/index.ts b/packages/core/src/utils/index.ts
index 7dcd11ce..ca014169 100644
--- a/packages/core/src/utils/index.ts
+++ b/packages/core/src/utils/index.ts
@@ -19,3 +19,4 @@ export * from './misc-utils'
 export * from './peer-utils'
 export * from './sorted-array'
 export * from './tl-json'
+export * from './string-session'
diff --git a/packages/core/src/utils/string-session.ts b/packages/core/src/utils/string-session.ts
new file mode 100644
index 00000000..3d492c88
--- /dev/null
+++ b/packages/core/src/utils/string-session.ts
@@ -0,0 +1,89 @@
+import { tl } from '@mtcute/tl'
+import { TlBinaryReader, TlBinaryWriter, TlReaderMap, TlWriterMap } from "@mtcute/tl-runtime/dist";
+import { encodeUrlSafeBase64, parseUrlSafeBase64 } from "./buffer-utils";
+import { ITelegramStorage } from '../storage'
+
+export interface StringSessionData {
+    version: number
+    testMode: boolean
+    primaryDc: tl.TypeDcOption
+    self?: ITelegramStorage.SelfInfo | null
+    authKey: Buffer
+}
+
+export function writeStringSession(
+    writerMap: TlWriterMap,
+    data: StringSessionData
+): string {
+    const writer = TlBinaryWriter.alloc(writerMap, 512)
+
+    const version = data.version
+    if (version !== 1) {
+        throw new Error(`Unsupported string session version: ${version}`)
+    }
+
+    let flags = 0
+
+    if (data.self) {
+        flags |= 1
+    }
+
+    if (data.testMode) {
+        flags |= 2
+    }
+
+    writer.buffer[0] = version
+    writer.pos += 1
+
+    writer.int(flags)
+    writer.object(data.primaryDc)
+
+    if (data.self) {
+        writer.int53(data.self.userId)
+        writer.boolean(data.self.isBot)
+    }
+
+    writer.bytes(data.authKey)
+
+    return encodeUrlSafeBase64(writer.result())
+}
+
+export function readStringSession(readerMap: TlReaderMap, data: string): StringSessionData {
+    const buf = parseUrlSafeBase64(data)
+    if (buf[0] !== 1)
+        throw new Error(`Invalid session string (version = ${buf[0]})`)
+
+    const reader = new TlBinaryReader(readerMap, buf, 1)
+
+    const flags = reader.int()
+    const hasSelf = flags & 1
+    const testMode = !!(flags & 2)
+
+    const primaryDc = reader.object()
+    if (primaryDc._ !== 'dcOption') {
+        throw new Error(
+            `Invalid session string (dc._ = ${primaryDc._})`
+        )
+    }
+
+    let self: ITelegramStorage.SelfInfo | null = null
+    if (hasSelf) {
+        const selfId = reader.int53()
+        const selfBot = reader.boolean()
+
+        self = {
+            userId: selfId,
+            isBot: selfBot,
+        }
+    }
+
+    const key = reader.bytes()
+
+    return {
+        version: 1,
+        testMode,
+        primaryDc,
+        self,
+        authKey: key,
+    }
+}