diff --git a/src/secret-wrap.ts b/src/secret-wrap.ts
index 3d1c712..e8a2962 100644
--- a/src/secret-wrap.ts
+++ b/src/secret-wrap.ts
@@ -1,16 +1,21 @@
 import logger from 'log'
 import {a2b64, b642a} from 'misc'
-import {BIT_LENGTH, ENCRYPTION_ALGORITHM} from './const'
+import {BIT_LENGTH, ECDH_PARAMETERS, ENCRYPTION_ALGORITHM, SIGNATURE_KEY} from './const'
 
 const log = logger('crypto:secret-wrap')
 
+enum Type {
+  SecretBox, // secret, decrypt, encrypt
+  SecretWrap, // secret, wrap, unwrap
+  Private, // private, deriveBits
+  signature // private, sign
+}
+
 export default class SecretWrap {
   constructor(
     private readonly cipher: Uint8Array,
-    private readonly algorithm: KeyAlgorithm,
-    private readonly usages: KeyUsage[],
-    private readonly type: 'raw' | 'pkcs8',
-    private readonly iv: Uint8Array
+    private readonly iv: Uint8Array,
+    private readonly type: Type
   ) {}
 
   public static async gen(extractable: boolean = true): Promise<CryptoKey> {
@@ -25,25 +30,30 @@ export default class SecretWrap {
     )
   }
 
-  private static format(type: KeyType): 'raw' | 'pkcs8' | null {
-    switch (type) {
-      case 'private':
-        return 'pkcs8'
-      case 'secret':
-        return 'raw'
-      default:
-        log.warn('Trying to wrap public key')
-        return null
+  private static identify(data: CryptoKey): Type {
+    if (data.type === 'secret') {
+      if (data.usages.includes('wrapKey')) return Type.SecretWrap
+      else return Type.SecretBox
+    } else {
+      if (data.usages.includes('sign')) return Type.signature
+      else return Type.Private
+    }
+  }
+  private information(): {format: KeyFormat, algorithm: AesKeyAlgorithm | EcKeyImportParams, usages: KeyUsage[]} {
+    switch (this.type) {
+      case Type.SecretWrap: return {algorithm: {name: ENCRYPTION_ALGORITHM, length: BIT_LENGTH()}, format: 'raw', usages: ['wrapKey', 'unwrapKey']}
+      case Type.SecretBox: return {algorithm: {name: ENCRYPTION_ALGORITHM, length: BIT_LENGTH()}, format: 'raw', usages: ['encrypt', 'decrypt']}
+      case Type.signature: return {algorithm: SIGNATURE_KEY(), format: 'pkcs8', usages: ['sign']}
+      case Type.Private: return {algorithm: ECDH_PARAMETERS(), format: 'pkcs8', usages: ['deriveBits']}
     }
   }
 
   public static async wrap(data: CryptoKey, key: CryptoKey): Promise<SecretWrap | null> {
     log.trace('wrap')
 
-    const format = SecretWrap.format(data.type)
-    if (format === null) return null
-
     const iv = crypto.getRandomValues(new Uint8Array(12))
+    const type = this.identify(data)
+    const format = (type === Type.SecretBox || type === Type.SecretWrap) ? 'raw' : 'pkcs8'
 
     let box: ArrayBuffer
     try {
@@ -54,19 +64,22 @@ export default class SecretWrap {
       return null
     }
 
-    return new SecretWrap(new Uint8Array(box), data.algorithm, data.usages, format, iv)
+    return new SecretWrap(new Uint8Array(box), iv, type)
   }
   public async unwrap(key: CryptoKey): Promise<CryptoKey | null> {
     log.trace('unwrap')
+
+    const info = this.information()
+
     try {
       return await crypto.subtle.unwrapKey(
-        this.type,
+        info.format,
         this.cipher,
         key,
         {name: ENCRYPTION_ALGORITHM, iv: this.iv},
-        this.algorithm,
+        info.algorithm,
         true,
-        this.usages
+        info.usages
       )
     } catch (e) {
       log.warn('Failed to unwrap')
@@ -77,45 +90,39 @@ export default class SecretWrap {
 
   public toString(): string {
     log.trace('toString')
+    const cipher = a2b64(this.cipher)
     const iv = a2b64(this.iv)
-    return JSON.stringify({
-      cipher: a2b64(this.cipher),
-      algorithm: this.algorithm,
-      usages: this.usages,
-      type: this.type,
-      iv
-    })
+    return `${this.type}.${iv}.${cipher}`
   }
   public static fromString(data: string): SecretWrap | null {
     log.trace('fromString')
 
-    const obj = SecretWrap.parseJSON(data)
-    if (obj === null) return null
+    const parts = data.split('.')
+    if (parts.length !== 3) {
+      log.warn(`Part count doesn't match : ${parts.length} != 3`)
+      return null
+    }
 
+    let type: Type
     try {
-      const cipher = b642a(obj.cipher)
-      if (cipher === null) return null
-
-      const iv = b642a(obj.iv)
-      if (iv === null) return null
-
-      return new SecretWrap(cipher, obj.algorithm, obj.usages, obj.type, iv)
+      type = parseInt(parts[0])
     } catch (e) {
-      log.warn('Probable access to undefined field')
+      log.warn('Failed to parse type')
+      log.debug('Type :', parts[0])
       log.debug('Error :', e)
       return null
     }
-  }
-
-  private static parseJSON(
-    data: string
-  ): {cipher: string; iv: string; algorithm: KeyAlgorithm; usages: KeyUsage[]; type: 'raw' | 'pkcs8'} | null {
-    log.trace('parseJSON')
-    try {
-      return JSON.parse(data)
-    } catch (e) {
-      log.warn('Failed to parse JSON')
+    if (!(type in Type)) {
+      log.warn('Unknown type', type)
       return null
     }
+
+    const iv = b642a(parts[1])
+    if (iv === null) return null
+
+    const cipher = b642a(parts[2])
+    if (cipher === null) return null
+
+    return new this(cipher, iv, type)
   }
 }