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Testing for Weak Encryption (OTG-CRYPST-004)

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Incorrect uses of encryption algorithm may result in sensitive data exposure, key leakage, broken authentication, insecure session and spoofing attack. There are some encryption or hash algorithm is known to be weak and not suggested to be used anymore such MD5 and RC4.

In addition to the right choices of secure encryption or hash algorithm, the right uses of parameters also mater the security level. For example, ECB (Electronic Code Book) mode is not suggested to be used in symmetric encryption.

The testing guide is trying to provide a guideline how to identify the weak encryption and hash.

How to Test

Basic Security Checklist

  • When the uses of AES128 and AES256, The IV (Initialization vector) must be random and unpredictable. Refer to FIPS 140-2, Security Requirements for Cryptographic Modules, section 4.9.1. random number generator tests. For example, in Java, "java.util.Random" is considered a weak random number generator. "" should be used instead of "java.util.Random".
  • When uses of RSA in encryption, Optimal Asymmetric Encryption Padding (OAEP) mode is recommended.
  • When uses of RSA in signature, PSS padding is recommended.
  • Weak hash/encryption algorithms should not be used such MD5, RC4, DES, Blowfish, SHA1. 1024-bit RSA or DSA, 160-bit ECDSA (elliptic curves), 80/112-bit 2TDEA (two key triple DES)
  • Minimum Key length requirement:
Key exchange: Diffie–Hellman key exchange with minimum 2048 bits
Message Integrity: HMAC-SHA2
Message Hash: SHA2 256 bits
Assymetric encryption: RSA 2048 bits
Symmetric-key algorithm: AES 128 bits
Password Hashing: PBKDF2, Scrypt, Bcrypt
ECDH、ECDSA: 256 bits
  • Uses of SSH, CBC mode should not be used.
  • When symmetric encryption algorithm is used, ECB (Electronic Code Book) mode should not be used.
  • When PBKDF2 is used to hash password, the parameter of iteration is recommended to be over 10000. NIST also suggests at least 10,000 iterations of the hash function. In addition, MD5 hash function is forbidden to be used with PBKDF2 such as PBKDF2WithHmacMD5.

Source Code Review

  • Search for the following keyword to check if any weak encryption algorithm is used.
MD4, MD5, RC4, RC2, DES, Blowfish, SHA-1, ECB
  • For Java implementation, the following API is related to encyprtion. Review the parameters of the encryption implementation. The following example uses of DES is not recommended since DES is considered a weak encryption algorithm.
SecretKeyFactory(SecretKeyFactorySpi keyFacSpi, Provider provider, String algorithm)
SecretKeySpec(byte[] key, int offset, int len, String algorithm)
Cipher c = Cipher.getInstance("DES/CBC/PKCS5Padding");
  • For the padding mode in RSA Asymmetric encryption, the uses of padding modes :

PKCS#1 v2.1 (PSS, OAEP) -- Recommended

PKCS#1 v1.5 (RSAES, RSASSA) -- Legency uses only.

  • Search for "ECB" or "Cipher.getInstance" , the ECB should not be used in symmetric encryption.
The mode of operation needs to be specified since the default will be "ECB" if not specified.
Cipher c = Cipher.getInstance("AES/CBC/PKCS5Padding") ;
The following implementation will result in using the default "ECB" mode, and should be avoided.
Cipher c = Cipher.getInstance("AES");
  • Review if different IV (initial Vector) is used.
// Use a different IV value for every encryption
byte[] newIv = ...;
s = new GCMParameterSpec(s.getTLen(), newIv);
cipher.init(..., s);
  • Search for "IvParameterSpec", check if the IV value is generated differently and randomly.
 IvParameterSpec iv = new IvParameterSpec(randBytes);
 SecretKeySpec skey = new SecretKeySpec(key.getBytes(), "AES");
 Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
 cipher.init(Cipher.ENCRYPT_MODE, skey, iv);
  • In Java, search for MessageDigest to check if weak hash althorithm (MD5 or CRC) is used. For example:
MessageDigest md5 = MessageDigest.getInstance("MD5");

  • For signature, SHA1 and MD5 should not be used. For example:
Signature sig = Signature.getInstance("SHA1withRSA");
  • Search for "PBKDF2". To generate the hash value of password, PBKDF2 is suggested to be used. Review the parameters to generate the PBKDF2 has value.

The iterations should be over 10000, and the salt value should be generated as random value.

private static byte[] pbkdf2(char[] password, byte[] salt, int iterations, int bytes)
    throws NoSuchAlgorithmException, InvalidKeySpecException
       PBEKeySpec spec = new PBEKeySpec(password, salt, iterations, bytes * 8);
       SecretKeyFactory skf = SecretKeyFactory.getInstance(PBKDF2_ALGORITHM);
       return skf.generateSecret(spec).getEncoded();
  • Hard-coded senstive information
User related keywords: name, root, su, superuser, login, username, uid
Key related keywords: public key, AK, SK, secret Key,  private key, passwd, password, pwd, share key, cryto, base64
Other common senstive keywords: sysadmin, root, privilege, pass, key, code, master, admin, uname, session, joken, Oauth, priviatekey


  • Mozilla TLS observatory
  • Vulnerability scanner such as Nessus to scan weak encryption used in protocol such as SNMP, TLS,SSH
  • Use static code analysis tool to do source code review such as klocwork, Fortify, Coverity, CheckMark for the following cases.
CWE-261: Weak Cryptography for Passwords
CWE-323: Reusing a Nonce, Key Pair in Encryption.
CWE-326: Inadequate Encryption Strength
CWE-327: Use of a Broken or Risky Cryptographic Algorithm
CWE-328: Reversible One-Way Hash
CWE-329: Not Using a Random IV with CBC Mode
CWE-330: Use of Insufficiently Random Values
CWE-347: Improper Verification of Cryptographic Signature
CWE-354: Improper Validation of Integrity Check Value
CWE-547: Use of Hard-coded, Security-relevant Constants
CWE-780 Use of RSA Algorithm without OAEP


Authors and Primary Editors

Tony Hsu - hsiang_chih[at]